Practical case: Automatic power switching

Automatic power switching prototype (Maker Style)

Level: Medium – Implement an SPDT relay to automatically alternate between a main power supply and a backup battery.

Objective and use case

In this practical case, you will build a power management circuit using a Single Pole Double Throw (SPDT) relay. The circuit will automatically switch a connected load to a backup battery whenever the main power supply fails.

This automated switching topology is highly useful in several real-world applications:
* Uninterruptible Power Supplies (UPS) for routers and critical network equipment.
* Alarm and security systems that require continuous operation during grid power outages.
* Medical monitoring devices that must remain functional during patient transport.
* Solar power systems that automatically switch to battery power after sunset.

Expected outcome:
* When the main supply (V_MAIN) is active, the relay coil energizes, and the load connects to the Normally Open (NO) contact powered by the main supply.
* When the main supply fails (drops to 0 V), the relay de-energizes, seamlessly transferring the load to the Normally Closed (NC) contact powered by the backup battery.
* The load voltage (V_LOAD_OUT) remains continuous, save for a minor mechanical switching delay.
* A visual LED indicator successfully reports the presence of the main power supply.

Target audience: Hobbyists and intermediate electronics students learning about electromechanical relays and power redundancy.

Materials

  • V1: 12 V DC supply, function: main power source
  • V2: 9 V DC supply, function: backup battery source
  • K1: 12 V SPDT relay, function: automatic power switch
  • D1: 1N4007 diode, function: flyback diode to protect against relay coil voltage spikes
  • D2: Red LED, function: main power indicator
  • R1: 1 kΩ resistor, function: LED current limiting
  • R2: 100 Ω resistor, function: simulated system load

Wiring guide

  • V1: Connect positive terminal to V_MAIN and negative terminal to 0 (GND).
  • V2: Connect positive terminal to V_BACKUP and negative terminal to 0 (GND).
  • K1 (Coil): Connect one side to V_MAIN and the other side to 0 (GND).
  • D1: Connect parallel to the K1 coil. Connect the cathode to V_MAIN and the anode to 0 (GND).
  • K1 (NO Contact): Connect the Normally Open terminal to V_MAIN.
  • K1 (NC Contact): Connect the Normally Closed terminal to V_BACKUP.
  • K1 (COM Contact): Connect the Common terminal to V_LOAD_OUT.
  • R2: Connect between V_LOAD_OUT and 0 (GND).
  • R1: Connect between V_MAIN and NODE_LED.
  • D2: Connect the anode to NODE_LED and the cathode to 0 (GND).

Conceptual block diagram

Conceptual block diagram — Relay Power Switch
Quick read: inputs → main block → output (actuator or measurement). This summarizes the ASCII schematic below.

Schematic

[ V1: 12 V Main ] --(V_MAIN)--> [ K1 Coil || D1 (Rev) ] --> GND
                                       |
                                 (Magnetic Link)
                                       v
[ V1: 12 V Main ] --(V_MAIN)--> [ K1: NO Contact ] --+
                                                    |
                                                  (COM)--> [ V_LOAD_OUT ] --> [ R2: 100 Ω Load ] --> GND
                                                    |
[ V2: 9 V Backup ] -(V_BACKUP)-> [ K1: NC Contact ] -+

[ V1: 12 V Main ] --(V_MAIN)--> [ R1: 1 kΩ ] --(NODE_LED)--> [ D2: Red LED ] --> GND
Electrical Schematic

Electrical diagram

Electrical diagram for case: Automatic power switching
Generated from the validated SPICE netlist for this case.

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Measurements and tests

  1. Connect and activate both V1 (12 V) and V2 (9 V).
  2. Measure the voltage at V_MAIN and V_BACKUP with a multimeter to verify both sources are stable.
  3. Measure the voltage at V_LOAD_OUT. It should read approximately 12 V. The relay should emit an audible «click» upon power-up, and the indicator LED (D2) should be brightly lit.
  4. Disconnect V1 to simulate a power outage (V_MAIN drops to 0 V).
  5. Measure the voltage at V_LOAD_OUT again. It should now read approximately 9 V, confirming the load has successfully transferred to the backup battery. The LED should turn off.
  6. Reconnect V1. Observe the relay clicking again as V_LOAD_OUT returns to 12 V.

SPICE netlist and simulation

Reference SPICE Netlist (ngspice) — excerptFull SPICE netlist (ngspice)

* Automatic power switching
.width out=256

* Power Sources
V1 V_MAIN 0 PULSE(12 0 200u 1u 1u 200u 500u)
V2 V_BACKUP 0 DC 9

* K1 Relay Coil (modeled as series inductor and resistor)
L_K1 V_MAIN K1_COIL_INT 1m
R_K1 K1_COIL_INT 0 400

* K1 Relay Contacts (modeled as voltage-controlled switches)
* Normally Open (NO) contact between V_MAIN and V_LOAD_OUT, controlled by V_MAIN
S_K1_NO V_MAIN V_LOAD_OUT V_MAIN 0 Relay_NO
* Normally Closed (NC) contact between V_BACKUP and V_LOAD_OUT, controlled by inverted V_MAIN
S_K1_NC V_BACKUP V_LOAD_OUT 0 V_MAIN Relay_NC

* D1 Flyback Diode (Anode to 0, Cathode to V_MAIN)
D1 0 V_MAIN 1N4007

* ... (truncated in public view) ...

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* Automatic power switching
.width out=256

* Power Sources
V1 V_MAIN 0 PULSE(12 0 200u 1u 1u 200u 500u)
V2 V_BACKUP 0 DC 9

* K1 Relay Coil (modeled as series inductor and resistor)
L_K1 V_MAIN K1_COIL_INT 1m
R_K1 K1_COIL_INT 0 400

* K1 Relay Contacts (modeled as voltage-controlled switches)
* Normally Open (NO) contact between V_MAIN and V_LOAD_OUT, controlled by V_MAIN
S_K1_NO V_MAIN V_LOAD_OUT V_MAIN 0 Relay_NO
* Normally Closed (NC) contact between V_BACKUP and V_LOAD_OUT, controlled by inverted V_MAIN
S_K1_NC V_BACKUP V_LOAD_OUT 0 V_MAIN Relay_NC

* D1 Flyback Diode (Anode to 0, Cathode to V_MAIN)
D1 0 V_MAIN 1N4007

* Main Power Indicator
R1 V_MAIN NODE_LED 1k
D2 NODE_LED 0 DLED

* Simulated System Load
R2 V_LOAD_OUT 0 100

* Component Models
.model 1N4007 D(IS=7.02767n RS=0.0341512 N=1.80803 EG=1.05743 XTI=5 BV=1000 IBV=5e-08 CJO=1e-11 VJ=0.7 M=0.5 FC=0.5 TT=1e-07)
.model DLED D(IS=1e-15 RS=10 N=2.0)
.model Relay_NO SW(vt=6 vh=0.5 ron=0.05 roff=10Meg)
.model Relay_NC SW(vt=-6 vh=0.5 ron=0.05 roff=10Meg)

* Analysis Directives
.op
.tran 1u 500u
.print tran V(V_MAIN) V(V_LOAD_OUT) V(V_BACKUP) V(NODE_LED) I(L_K1)

.end

Simulation Results (Transient Analysis)

Simulation Results (Transient Analysis)
Analysis: The simulation shows V_MAIN starting at 12V, during which V_LOAD_OUT is approximately 12V. At t=200us, V_MAIN drops to 0V, and V_LOAD_OUT seamlessly switches to the 9V backup supply. When V_MAIN recovers at t=400us, V_LOAD_OUT returns to 12V.
Show raw data table (557 rows) 
Index   time            v(v_main)       v(v_load_out)   v(v_backup)     v(node_led)     l_k1#branch
0	0.000000e+00	1.200000e+01	1.199400e+01	9.000000e+00	1.653685e+00	3.000000e-02
1	1.000000e-08	1.200000e+01	1.199400e+01	9.000000e+00	1.653685e+00	3.000000e-02
2	2.000000e-08	1.200000e+01	1.199400e+01	9.000000e+00	1.653685e+00	3.000000e-02
3	4.000000e-08	1.200000e+01	1.199400e+01	9.000000e+00	1.653685e+00	3.000000e-02
4	8.000000e-08	1.200000e+01	1.199400e+01	9.000000e+00	1.653685e+00	3.000000e-02
5	1.600000e-07	1.200000e+01	1.199400e+01	9.000000e+00	1.653685e+00	3.000000e-02
6	3.200000e-07	1.200000e+01	1.199400e+01	9.000000e+00	1.653685e+00	3.000000e-02
7	6.400000e-07	1.200000e+01	1.199400e+01	9.000000e+00	1.653685e+00	3.000000e-02
8	1.280000e-06	1.200000e+01	1.199400e+01	9.000000e+00	1.653685e+00	3.000000e-02
9	2.280000e-06	1.200000e+01	1.199400e+01	9.000000e+00	1.653685e+00	3.000000e-02
10	3.280000e-06	1.200000e+01	1.199400e+01	9.000000e+00	1.653685e+00	3.000000e-02
11	4.280000e-06	1.200000e+01	1.199400e+01	9.000000e+00	1.653685e+00	3.000000e-02
12	5.280000e-06	1.200000e+01	1.199400e+01	9.000000e+00	1.653685e+00	3.000000e-02
13	6.280000e-06	1.200000e+01	1.199400e+01	9.000000e+00	1.653685e+00	3.000000e-02
14	7.280000e-06	1.200000e+01	1.199400e+01	9.000000e+00	1.653685e+00	3.000000e-02
15	8.280000e-06	1.200000e+01	1.199400e+01	9.000000e+00	1.653685e+00	3.000000e-02
16	9.280000e-06	1.200000e+01	1.199400e+01	9.000000e+00	1.653685e+00	3.000000e-02
17	1.028000e-05	1.200000e+01	1.199400e+01	9.000000e+00	1.653685e+00	3.000000e-02
18	1.128000e-05	1.200000e+01	1.199400e+01	9.000000e+00	1.653685e+00	3.000000e-02
19	1.228000e-05	1.200000e+01	1.199400e+01	9.000000e+00	1.653685e+00	3.000000e-02
20	1.328000e-05	1.200000e+01	1.199400e+01	9.000000e+00	1.653685e+00	3.000000e-02
21	1.428000e-05	1.200000e+01	1.199400e+01	9.000000e+00	1.653685e+00	3.000000e-02
22	1.528000e-05	1.200000e+01	1.199400e+01	9.000000e+00	1.653685e+00	3.000000e-02
23	1.628000e-05	1.200000e+01	1.199400e+01	9.000000e+00	1.653685e+00	3.000000e-02
... (533 more rows) ...

Common mistakes and how to avoid them

  • Relay coil voltage mismatch: Using a 5 V relay on a 12 V line will cause the coil to overheat and fail quickly. Always ensure the relay’s rated coil voltage matches the main supply voltage exactly.
  • Omitting the flyback diode: Failing to install the reverse-biased diode across the relay coil can result in high-voltage spikes when the main power is abruptly disconnected, potentially damaging parallel components on the main power bus.
  • Reversing NO and NC contacts: Wiring the backup battery to the NO contact and the main supply to the NC contact will result in a dead system when the main power fails. Verify the relay pinout before soldering or powering the circuit.

Troubleshooting

  • Symptom: The load completely loses power when the main supply drops.
    • Cause: The backup battery is either dead or connected to the Normally Open (NO) terminal instead of the Normally Closed (NC) terminal.
    • Fix: Measure the battery voltage independently, then verify its connection to the NC terminal of the relay.
  • Symptom: The relay chatters or buzzes continuously instead of switching cleanly.
    • Cause: The main power supply cannot provide enough current for both the relay coil and the load, causing the voltage to repeatedly dip below the relay’s hold threshold.
    • Fix: Upgrade the main power supply to a higher current rating, or add a large smoothing capacitor across the V_MAIN line.
  • Symptom: The indicator LED does not light up, but the switching works.
    • Cause: The LED is inserted with reverse polarity, or the current limiting resistor is disconnected.
    • Fix: Verify that the flat side (cathode) of the LED is connected to ground.

Possible improvements and extensions

  • Add a large electrolytic capacitor (e.g., 1000 µF) in parallel with the load (R2) to smooth out the brief power interruption (brownout) caused by the mechanical switching time of the relay contacts.
  • Replace the mechanical relay with a solid-state diode OR-ing circuit (using Schottky diodes) for completely seamless, zero-delay switching without any moving parts.

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Quick Quiz

Question 1: What is the primary function of the SPDT relay in this circuit?




Question 2: What does SPDT stand for in the context of relays?




Question 3: Which contact does the load connect to when the main supply (V_MAIN) is active?




Question 4: What happens to the relay coil when the main power supply fails?




Question 5: Which contact is powered by the backup battery in this circuit?




Question 6: Which of the following is a real-world application for this automated switching topology?




Question 7: What is the expected behavior of the load voltage (V_LOAD_OUT) during the switch?




Question 8: Why is this circuit useful for alarm and security systems?




Question 9: In a solar power system using this topology, when does the system automatically switch to battery power?




Question 10: What triggers the relay to switch the load back to the main power supply?




Carlos Núñez Zorrilla
Carlos Núñez Zorrilla
Electronics & Computer Engineer

Telecommunications Electronics Engineer and Computer Engineer (official degrees in Spain).

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Practical case: DC motor control with a transistor

DC motor control with a transistor prototype (Maker Style)

Level: Basic – Learn to use an NPN transistor as a switch to drive a DC motor, including the use of a flyback diode.

Objective and use case

In this practical case, you will build a low-side switch circuit using an NPN transistor to safely control a high-current DC motor from a low-power control signal.

This topology is highly useful in the real world for several reasons:
* Interfacing low-voltage microcontrollers (like an Arduino or Raspberry Pi) with higher power loads that require external power supplies.
* Automating small cooling fans in temperature-controlled systems.
* Building basic drive systems for small hobbyist robotics.
* Protecting delicate control logic from the damaging voltage spikes generated by inductive loads.

Expected outcome:
* Applying a 5 V control signal to the base circuit will saturate the transistor.
* The DC motor will spin as the transistor bridges its connection to ground.
* The flyback diode will safely dissipate the motor’s inductive kickback when the control signal is turned off.
* Measurable base voltage (VBE) around 0.7 V, near-zero collector-emitter voltage (VCE) indicating saturation, and clearly observable base current (IB) and collector current (IC).

Target audience and level: Beginners in electronics and hobbyists looking to control mechanical loads safely.

Materials

  • V1: 9 V DC supply, function: main power source for the DC motor
  • V2: 5 V DC supply, function: simulated control signal source
  • SW1: SPST switch, function: manual control of the base signal
  • Q1: 2N2222 NPN transistor, function: low-side switch to drive the motor
  • M1: 9 V DC motor, function: inductive mechanical load
  • D1: 1N4007 diode, function: flyback diode to suppress inductive spikes
  • R1: 1 kΩ resistor, function: base current limiting resistor
  • R2: 10 kΩ resistor, function: pull-down resistor for the control signal

Wiring guide

  • V1: connects between nodes 9 V_PWR and 0
  • V2: connects between nodes 5 V_CTRL and 0
  • SW1: connects between nodes 5 V_CTRL and CTRL_IN
  • R2: connects between nodes CTRL_IN and 0
  • R1: connects between nodes CTRL_IN and BASE
  • Q1: Collector connects to node COLLECTOR, Base connects to node BASE, Emitter connects to node 0
  • M1: connects between nodes 9 V_PWR and COLLECTOR
  • D1: Anode connects to node COLLECTOR, Cathode connects to node 9 V_PWR

Conceptual block diagram

Conceptual block diagram — Transistor Motor Control
Quick read: inputs → main block → output (actuator or measurement). This summarizes the ASCII schematic below.

Schematic

[ 5 V_CTRL ] --> [ SW1 ] --(CTRL_IN)--+--> [ R1: 1 kΩ ] --(BASE)--> [ Q1:Base ]
                                           |                                |
                                       [ R2: 10 kΩ ]                         |
                                           |                                |
                                          GND                               |
                                                                            |
      [ 9 V_PWR ] --+--> [ M1: 9 V Motor ] -----------------+--(COLLECTOR)--> [ Q1:Collector ] --( )-- [ Q1:Emitter ] --> GND
                   |                                      |
                   +--> [ D1: 1N4007 (Cath->Anode) ] -----+
Electrical Schematic

Electrical diagram

Electrical diagram for case: DC motor control with a transistor
Generated from the validated SPICE netlist for this case.

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Measurements and tests

  1. Verify Control Signal: Close SW1. Measure the voltage at node CTRL_IN with respect to node 0. It should read 5 V. When open, it should read 0 V due to the pull-down resistor R2.
  2. Measure Base-Emitter Voltage (VBE): With SW1 closed, place your multimeter probes across node BASE and node 0. You should measure approximately 0.7 V, confirming the transistor’s base-emitter junction is forward-biased.
  3. Measure Collector-Emitter Voltage (VCE): With the motor running (SW1 closed), measure the voltage between node COLLECTOR and node 0. A reading of around 0.2 V indicates the transistor is correctly operating in the saturation region. When SW1 is open, this voltage should rise to 9 V.
  4. Measure Base Current (IB): Set your multimeter to measure current (mA range) and place it in series between R1 and node BASE. You should measure a small current (around 4.3 mA).
  5. Measure Collector Current (IC): Place your ammeter in series between M1 and node COLLECTOR. You will measure the actual current drawn by the motor (which could range from tens to hundreds of mA depending on the specific motor).

SPICE netlist and simulation

Reference SPICE Netlist (ngspice) — excerptFull SPICE netlist (ngspice)

* DC Motor Control with a Transistor
.width out=256

* Power Supplies
V1 9V_PWR 0 DC 9
V2 5V_CTRL 0 DC 5

* Switch SW1 modeled as a voltage-controlled switch to simulate user interaction
S1 5V_CTRL CTRL_IN SW_CTRL 0 mySW
.model mySW SW(Vt=2.5 Vh=0.5 Ron=0.1 Roff=100MEG)

* Control signal to simulate the user pressing the switch
V_SW_CTRL SW_CTRL 0 PULSE(0 5 10m 1u 1u 245m 1s)

* Resistors
R2 CTRL_IN 0 10k
R1 CTRL_IN BASE 1k

* Transistor Q1 (Low-side switch)
Q1 COLLECTOR BASE 0 2N2222MOD
* ... (truncated in public view) ...

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* DC Motor Control with a Transistor
.width out=256

* Power Supplies
V1 9V_PWR 0 DC 9
V2 5V_CTRL 0 DC 5

* Switch SW1 modeled as a voltage-controlled switch to simulate user interaction
S1 5V_CTRL CTRL_IN SW_CTRL 0 mySW
.model mySW SW(Vt=2.5 Vh=0.5 Ron=0.1 Roff=100MEG)

* Control signal to simulate the user pressing the switch
V_SW_CTRL SW_CTRL 0 PULSE(0 5 10m 1u 1u 245m 1s)

* Resistors
R2 CTRL_IN 0 10k
R1 CTRL_IN BASE 1k

* Transistor Q1 (Low-side switch)
Q1 COLLECTOR BASE 0 2N2222MOD

* Motor M1 modeled as a series inductor and resistor representing the inductive mechanical load
LM1 9V_PWR M1_INT 1mH
RM1 M1_INT COLLECTOR 20

* Flyback diode D1
D1 COLLECTOR 9V_PWR 1N4007MOD

* Component Models
.model 2N2222MOD NPN(IS=1E-14 VAF=100 BF=200 IKF=0.3 XTB=1.5 BR=3 CJC=8E-12 CJE=25E-12 TR=100E-9 TF=400E-12 ITF=1 VTF=2 XTF=3 RB=10 RC=0.3 RE=0.2)
.model 1N4007MOD D(IS=7.02767n RS=0.0341512 N=1.80803 EG=1.11 XTI=3.0 BV=1000 IBV=5e-08 CJO=1e-11 VJ=0.7 M=0.5 FC=0.5 TT=1e-07)

* Simulation Commands
.op
.tran 0.1m 250m
.print tran V(CTRL_IN) V(COLLECTOR) V(BASE) I(LM1)
.end

Simulation Results (Transient Analysis)

Simulation Results (Transient Analysis)
Analysis: The simulation shows that when the control signal (v(ctrl_in)) goes high to ~5V at t=10ms, the transistor turns on, pulling the collector voltage down from 9V to ~1.64V. The base voltage rises to ~0.94V, and the motor current (lm1#branch) ramps up to ~368mA, indicating successful motor activation.
Show raw data table (2541 rows) 
Index   time            v(ctrl_in)      v(collector)    v(base)         lm1#branch
0	0.000000e+00	5.000400e-04	9.000000e+00	5.000490e-04	1.799750e-11
1	1.000000e-06	5.000400e-04	9.000000e+00	5.000490e-04	1.800624e-11
2	2.000000e-06	5.000400e-04	9.000000e+00	5.000490e-04	1.800815e-11
3	4.000000e-06	5.000400e-04	9.000000e+00	5.000490e-04	1.800528e-11
4	8.000000e-06	5.000400e-04	9.000000e+00	5.000490e-04	1.799050e-11
5	1.600000e-05	5.000400e-04	9.000000e+00	5.000490e-04	1.798412e-11
6	3.200000e-05	5.000400e-04	9.000000e+00	5.000490e-04	1.797999e-11
7	6.400000e-05	5.000400e-04	9.000000e+00	5.000490e-04	1.798801e-11
8	1.280000e-04	5.000400e-04	9.000000e+00	5.000490e-04	1.797977e-11
9	2.280000e-04	5.000400e-04	9.000000e+00	5.000490e-04	1.799637e-11
10	3.280000e-04	5.000400e-04	9.000000e+00	5.000490e-04	1.799685e-11
11	4.280000e-04	5.000400e-04	9.000000e+00	5.000490e-04	1.799640e-11
12	5.280000e-04	5.000400e-04	9.000000e+00	5.000490e-04	1.799689e-11
13	6.280000e-04	5.000400e-04	9.000000e+00	5.000490e-04	1.799636e-11
14	7.280000e-04	5.000400e-04	9.000000e+00	5.000490e-04	1.799685e-11
15	8.280000e-04	5.000400e-04	9.000000e+00	5.000490e-04	1.799639e-11
16	9.280000e-04	5.000400e-04	9.000000e+00	5.000490e-04	1.799690e-11
17	1.028000e-03	5.000400e-04	9.000000e+00	5.000490e-04	1.799645e-11
18	1.128000e-03	5.000400e-04	9.000000e+00	5.000490e-04	1.799690e-11
19	1.228000e-03	5.000400e-04	9.000000e+00	5.000490e-04	1.799640e-11
20	1.328000e-03	5.000400e-04	9.000000e+00	5.000490e-04	1.799689e-11
21	1.428000e-03	5.000400e-04	9.000000e+00	5.000490e-04	1.799641e-11
22	1.528000e-03	5.000400e-04	9.000000e+00	5.000490e-04	1.799690e-11
23	1.628000e-03	5.000400e-04	9.000000e+00	5.000490e-04	1.799640e-11
... (2517 more rows) ...

Common mistakes and how to avoid them

  • Omitting the flyback diode (D1): A DC motor is an inductive load. When the transistor turns off, the collapsing magnetic field creates a massive voltage spike. Without the diode, this spike will instantly destroy the transistor. Always place a diode in parallel with the motor, reverse-biased relative to the normal current flow.
  • Forgetting the base resistor (R1): Connecting a 5 V control signal directly to the transistor’s base will draw excessive current, immediately destroying the control source (e.g., your microcontroller) or the transistor. Always use a current-limiting resistor.
  • Swapping the Collector and Emitter pins: Inserting the NPN transistor backward will result in very poor current gain (hFE). The motor may barely turn, and the transistor will heat up significantly because it cannot fully saturate. Double-check the datasheet for your specific transistor’s pinout.

Troubleshooting

  • Symptom: The motor does not spin when the switch is closed.
    • Cause: The transistor is not turning on, or the motor lacks power.
    • Fix: Measure the voltage at node BASE. If it is 0 V, check your switch SW1 and resistor R1. Measure node 9 V_PWR to ensure the main power supply is active.
  • Symptom: The transistor becomes extremely hot very quickly.
    • Cause: The transistor is operating in the active/linear region instead of fully saturating, usually because the base current (IB) is too low for the required collector current (IC).
    • Fix: Calculate the required base current (IC / hFE). If the current is too low, reduce the value of R1 (e.g., to 470 Ω or 330 Ω) to allow more base current, ensuring saturation.
  • Symptom: The microcontroller resets or behaves erratically when the motor turns on/off.
    • Cause: Electrical noise from the motor brushes or voltage drops on the power line.
    • Fix: Ensure the motor power supply (V1) is completely separate from the control logic supply (V2), sharing only the ground (0) connection. Add a 100 nF ceramic capacitor across the motor terminals to suppress brush noise.

Possible improvements and extensions

  • PWM Speed Control: Replace the manual switch (SW1) with a Pulse Width Modulation (PWM) signal from a microcontroller. By rapidly turning the transistor on and off, you can smoothly control the rotational speed of the motor rather than just having it on or off.
  • Optoisolation for superior safety: Introduce an optocoupler between the control signal and the transistor base. This physically separates the low-voltage control circuit from the higher-voltage motor circuit using light, providing total electrical isolation and preventing catastrophic failures from reaching your logic board.

More Practical Cases on Prometeo.blog

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Quick Quiz

Question 1: What is the primary objective of the circuit described in the context?




Question 2: What type of switch circuit is built using the NPN transistor in this practical case?




Question 3: Why is this topology useful for microcontrollers like Arduino or Raspberry Pi?




Question 4: What is the purpose of the flyback diode in this circuit?




Question 5: What happens when a 5 V control signal is applied to the base circuit?




Question 6: How does the DC motor spin in this circuit configuration?




Question 7: What is the expected measurable base-emitter voltage (V_BE) when the transistor is saturated?




Question 8: What collector-emitter voltage (V_CE) indicates that the transistor is in saturation?




Question 9: Which of the following is a real-world application mentioned for this circuit?




Question 10: What type of load is a DC motor considered in the context of voltage spikes?




Carlos Núñez Zorrilla
Carlos Núñez Zorrilla
Electronics & Computer Engineer

Telecommunications Electronics Engineer and Computer Engineer (official degrees in Spain).

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Practical case: Noise suppression with RF choke

Level: Medium – Demonstrate the high impedance of the inductor at high frequencies to block noise in power lines.

Objective and use case

You will construct an LR low-pass filter using an RF choke to isolate a DC power line from high-frequency AC noise. By superimposing an AC signal onto a DC voltage supply, you will observe how the inductor’s frequency-dependent reactance permits DC to pass while heavily attenuating high-frequency noise before it reaches the load.

This circuit concept is highly useful in the real world for:
* Preventing high-frequency switching noise from entering sensitive analog sensor circuits.
* Filtering out radio frequency interference (RFI) from long power supply lines.
* Isolating different functional blocks that share a common power rail on a PCB.
* Protecting automotive audio and communication electronics from alternator whine.

Expected outcome:
* The mixed input signal (V_IN_MIX) will display a steady DC offset combined with significant high-frequency ripples.
* The output voltage (V_OUT_CLEAN) across the load will show a stable DC level with the AC noise vastly reduced.
* An FFT (Fast Fourier Transform) analysis of the input will reveal a large 0 Hz (DC) component and a prominent high-frequency peak.
* An FFT analysis of the output will show the high-frequency peak almost completely suppressed, confirming the choke’s blocking action.

Target audience: Intermediate electronics students learning about reactive components and AC/DC superimposition.

Materials

  • V1: 5 V DC source, function: main DC power supply
  • V2: 500 mV peak sine wave AC source at 100 kHz, function: high-frequency noise simulator
  • L1: 1 mH inductor, function: RF choke to block high-frequency noise
  • R1: 100 Ω resistor, function: load simulation

Wiring guide

  • V1: connects between V_DC and 0
  • V2: connects between V_IN_MIX and V_DC
  • L1: connects between V_IN_MIX and V_OUT_CLEAN
  • R1: connects between V_OUT_CLEAN and 0

Conceptual block diagram

Conceptual block diagram — 1mH RF Choke
Quick read: inputs → main block → output (actuator or measurement). This summarizes the ASCII schematic below.

Schematic

[ V1: 5 V DC Source ] --(V_DC)--> [ V2: AC Noise Simulator ] --(V_IN_MIX)--> [ L1: 1mH RF Choke ] --(V_OUT_CLEAN)--> [ R1: 100 Ω Load ] --> GND
Electrical Schematic

Electrical diagram

Electrical diagram for case: Practical case: Noise suppression with RF choke
Generated from the validated SPICE netlist for this case.

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Measurements and tests

  1. Connect an oscilloscope probe to V_IN_MIX with the ground clip attached to node 0. Set the channel coupling to DC. You should observe a 5 V DC baseline with a 1 V peak-to-peak 100 kHz sine wave riding on top of it.
  2. Connect a second oscilloscope probe to V_OUT_CLEAN. Observe that the DC voltage remains at approximately 5 V, but the high-frequency 100 kHz ripple is drastically attenuated due to the high inductive reactance (XL = 2\pi fL) of the choke.
  3. Activate the FFT (Fast Fourier Transform) math function on the oscilloscope for the V_IN_MIX channel. Note the massive spike at 0 Hz (representing the 5 V DC component) and the distinct noise spike at 100 kHz.
  4. Apply the FFT function to the V_OUT_CLEAN channel. Compare the magnitude of the 100 kHz spike against the input measurement; it should be significantly reduced, successfully proving the inductor’s high-frequency blocking capabilities.

SPICE netlist and simulation

Reference SPICE Netlist (ngspice) — excerptFull SPICE netlist (ngspice)

* Noise suppression with RF choke
.width out=256

* Main DC power supply (5V)
V1 V_DC 0 DC 5

* High-frequency noise simulator (500mV peak, 100kHz sine wave superimposed on DC)
V2 V_IN_MIX V_DC SINE(0 500m 100k)

* RF choke to block high-frequency noise (1mH)
L1 V_IN_MIX V_OUT_CLEAN 1m

* Load simulation (100 ohms)
* ... (truncated in public view) ...

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🔓 See premium access plans 

* Noise suppression with RF choke
.width out=256

* Main DC power supply (5V)
V1 V_DC 0 DC 5

* High-frequency noise simulator (500mV peak, 100kHz sine wave superimposed on DC)
V2 V_IN_MIX V_DC SINE(0 500m 100k)

* RF choke to block high-frequency noise (1mH)
L1 V_IN_MIX V_OUT_CLEAN 1m

* Load simulation (100 ohms)
R1 V_OUT_CLEAN 0 100

* Analysis directives
.op
* Simulate for 100us to capture 10 full cycles of the 100kHz noise
.tran 0.1u 100u
.print tran V(V_IN_MIX) V(V_OUT_CLEAN) V(V_DC) I(L1)

.end

Simulation Results (Transient Analysis)

Simulation Results (Transient Analysis)
Analysis: The simulation shows a 5V DC signal with a superimposed 500mV peak 100kHz sine wave at the input (V_IN_MIX ranges from 4.5V to 5.5V). At the output (V_OUT_CLEAN), the voltage ranges from 4.92V to 5.12V, indicating that the 1mH RF choke significantly attenuates the high-frequency noise while passing the DC component to the 100-ohm load.
Show raw data table (1008 rows) 
Index   time            v(v_in_mix)     v(v_out_clean)  v(v_dc)         l1#branch
0	0.000000e+00	5.000000e+00	5.000000e+00	5.000000e+00	5.000000e-02
1	1.000000e-09	5.000314e+00	5.000000e+00	5.000000e+00	5.000000e-02
2	2.000000e-09	5.000628e+00	5.000000e+00	5.000000e+00	5.000000e-02
3	4.000000e-09	5.001257e+00	5.000000e+00	5.000000e+00	5.000000e-02
4	8.000000e-09	5.002513e+00	5.000001e+00	5.000000e+00	5.000001e-02
5	1.600000e-08	5.005026e+00	5.000004e+00	5.000000e+00	5.000004e-02
6	3.200000e-08	5.010052e+00	5.000016e+00	5.000000e+00	5.000016e-02
7	6.400000e-08	5.020101e+00	5.000064e+00	5.000000e+00	5.000064e-02
8	1.280000e-07	5.040169e+00	5.000256e+00	5.000000e+00	5.000256e-02
9	2.280000e-07	5.071384e+00	5.000808e+00	5.000000e+00	5.000808e-02
10	3.280000e-07	5.102316e+00	5.001665e+00	5.000000e+00	5.001665e-02
11	4.280000e-07	5.132845e+00	5.002818e+00	5.000000e+00	5.002818e-02
12	5.280000e-07	5.162850e+00	5.004261e+00	5.000000e+00	5.004261e-02
13	6.280000e-07	5.192212e+00	5.005985e+00	5.000000e+00	5.005985e-02
14	7.280000e-07	5.220816e+00	5.007980e+00	5.000000e+00	5.007980e-02
15	8.280000e-07	5.248548e+00	5.010236e+00	5.000000e+00	5.010236e-02
16	9.280000e-07	5.275299e+00	5.012741e+00	5.000000e+00	5.012741e-02
17	1.028000e-06	5.300963e+00	5.015481e+00	5.000000e+00	5.015481e-02
18	1.128000e-06	5.325440e+00	5.018443e+00	5.000000e+00	5.018443e-02
19	1.228000e-06	5.348633e+00	5.021613e+00	5.000000e+00	5.021613e-02
20	1.328000e-06	5.370449e+00	5.024976e+00	5.000000e+00	5.024976e-02
21	1.428000e-06	5.390804e+00	5.028515e+00	5.000000e+00	5.028515e-02
22	1.528000e-06	5.409616e+00	5.032213e+00	5.000000e+00	5.032213e-02
23	1.628000e-06	5.426812e+00	5.036054e+00	5.000000e+00	5.036054e-02
... (984 more rows) ...

Common mistakes and how to avoid them

  • Using an inductor with a low self-resonant frequency (SRF): All inductors have parasitic winding capacitance. If the noise frequency exceeds the inductor’s SRF, the component behaves like a capacitor and allows high-frequency noise to pass straight through. Always verify the SRF is well above your target noise frequency.
  • Neglecting the inductor’s DC resistance (DCR): Inductors are made of coiled wire which naturally possesses resistance. High load currents passing through an inductor with high DCR will cause an unacceptable DC voltage drop. Choose a choke with an appropriately low DCR for your load.
  • Core saturation due to high DC current: If the load draws more continuous current than the inductor’s saturation rating (Isat), the core’s magnetic flux saturates. This causes the inductance to drop sharply, destroying its filtering capability. Always check the saturation current rating.

Troubleshooting

  • Symptom: High-frequency noise is still heavily present at V_OUT_CLEAN.
  • Cause: The inductor value is too low to provide significant reactance at the simulated noise frequency, or its SRF has been exceeded.
  • Fix: Increase the inductance value (e.g., scale from 10 µH to 1 mH) or verify the frequency limits of the specific choke being used.
  • Symptom: Significant DC voltage drop at V_OUT_CLEAN under load (e.g., reading 4 V instead of 5 V).
  • Cause: The inductor’s internal DC resistance (DCR) is too high relative to the load resistor R1.
  • Fix: Replace the inductor with a physically larger one that uses thicker wire, which lowers the DCR, or increase the load resistance if it’s drawing more current than intended.
  • Symptom: The choke gets excessively hot during operation.
  • Cause: The DC current drawn by the load exceeds the continuous thermal current rating (Irms) of the inductor.
  • Fix: Select a higher-rated power inductor capable of safely handling the steady-state load current.

Possible improvements and extensions

  • Form an LC Low-Pass Filter: Add a decoupling capacitor (e.g., 100 nF or 1 µF) parallel to the load (between V_OUT_CLEAN and 0). This creates a second-order filter, providing a much steeper roll-off and vastly superior noise attenuation compared to the simple LR configuration.
  • Implement a Pi-Filter: Use a Capacitor-Inductor-Capacitor (C-L-C) arrangement to provide bidirectional noise suppression. This not only cleans the power entering the load but also prevents any switching noise generated by the load from polluting the main DC supply line.

More Practical Cases on Prometeo.blog

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Quick Quiz

Question 1: What is the primary purpose of the inductor in the described circuit?




Question 2: What type of filter is constructed in this experiment?




Question 3: How does the inductor react to the DC portion of the signal?




Question 4: Which of the following is a real-world application of this circuit concept mentioned in the text?




Question 5: What will the mixed input signal (V_IN_MIX) display according to the expected outcomes?




Question 6: Based on the expected outcomes, what is the effect of the circuit on the output voltage (V_OUT_CLEAN)?




Question 7: What is the stated difficulty level of demonstrating the high impedance of the inductor in this context?




Question 8: What specific type of interference can this circuit filter out from long power supply lines?




Question 9: How does the circuit help different functional blocks that share a common power rail on a PCB?




Question 10: What component is specifically used as an RF choke in this low-pass filter circuit?




Carlos Núñez Zorrilla
Carlos Núñez Zorrilla
Electronics & Computer Engineer

Telecommunications Electronics Engineer and Computer Engineer (official degrees in Spain).

Follow me:

Practical case: Resonance in LC tank circuit

Level: Medium | Analyze the energy exchange and determine the resonant frequency of an AC-driven LC tank.

Objective and use case

In this practical case, you will build a parallel LC tank circuit driven by an AC sine wave source through a series resistor. By sweeping the input frequency, you will observe the precise point where inductive and capacitive reactances cancel out, maximizing the circuit’s impedance.

Understanding LC resonance is essential in modern electronics because these circuits are the fundamental building blocks of frequency selection. Real-world applications include:
* Radio frequency (RF) tuning: Selecting a specific station’s frequency while rejecting others.
* Audio and signal filtering: Creating band-pass or band-stop (notch) filters to eliminate noise.
* Wireless power transfer: Maximizing the efficiency of inductive coupling between transmitter and receiver coils.
* Oscillator circuits: Generating stable clock signals for microcontrollers and transceivers.

Expected outcome:
* You will calculate the theoretical resonant frequency based on the chosen $L$ and $C$ values.
* The total current drawn from the source (Itotal) will drop to its minimum value at resonance.
* The voltage across the LC tank (VLC) will peak at the resonant frequency.
* You will observe how energy continuously sloshes back and forth between the capacitor’s electric field and the inductor’s magnetic field.

Target audience: Intermediate electronics students transitioning from DC basics to AC reactive circuits.

Materials

  • V1: 5 V peak-to-peak AC voltage source, function: sine wave generator for frequency sweep
  • R1: 1 kΩ resistor, function: source impedance to allow voltage variations across the tank
  • L1: 10 mH inductor, function: magnetic energy storage
  • C1: 100 nF ceramic or film capacitor, function: electric energy storage

Wiring guide

  • V1: Connect the positive terminal to node IN and the negative terminal to node 0 (GND).
  • R1: Connect one pin to node IN and the other pin to node TANK.
  • L1: Connect one pin to node TANK and the other pin to node 0 (GND).
  • C1: Connect one pin to node TANK and the other pin to node 0 (GND).

Conceptual block diagram

Conceptual block diagram — LC Tank Circuit
Quick read: inputs → main block → output (actuator or measurement). This summarizes the ASCII schematic below.

Schematic

[ V1: 5 V AC ] --(IN)--> [ R1: 1k ohm ] --(Node TANK)--+--> [ L1: 10mH ] --> GND
                                                      |
                                                      +--> [ C1: 100nF ] --> GND
Electrical Schematic

Electrical diagram

Electrical diagram for case: Practical case: Resonance in LC tank circuit
Generated from the validated SPICE netlist for this case.

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Measurements and tests

  1. Calculate the theoretical resonant frequency (fr):
    Use the formula fr = (1 / 2\pi\sqrtLC). With L = 10 mH and C = 100 nF, the expected resonant frequency is approximately 5032 Hz.
  2. Set up the frequency sweep:
    Configure V1 to output a 5 V peak-to-peak sine wave. Begin with a frequency of 1 kHz and gradually increase it up to 10 kHz.
  3. Measure VLC (Tank Voltage):
    Monitor the voltage amplitude at node TANK relative to node 0 (GND) using an oscilloscope or an AC voltmeter. As you approach 5 kHz, the voltage amplitude will rise steadily, hitting a sharp maximum exactly at resonance, and then fall as the frequency increases further.
  4. Measure Itotal (Source Current):
    Measure the current flowing through R1 (this can be done by observing the voltage difference between IN and TANK and applying Ohm’s law: Itotal = ((VIN – VTANK) / R1)). Note that at resonance, the parallel LC tank exhibits maximum impedance, meaning Itotal will drop to its minimum.
  5. Calculate the circuit’s Q-factor:
    Identify the -3dB (half-power) frequencies above and below the resonance peak to find the bandwidth ($BW$). The Quality Factor is Q = (fr / BW).

SPICE netlist and simulation

Reference SPICE Netlist (ngspice) — excerptFull SPICE netlist (ngspice)

* Practical case: Resonance in LC tank circuit
.width out=256

* 5V peak-to-peak implies an amplitude of 2.5V. 
* The resonant frequency of 10mH and 100nF is approximately 5033 Hz.
* We configure V1 with both a transient sine wave at resonance and an AC magnitude for optional AC analysis.
V1 IN 0 DC 0 AC 2.5 SIN(0 2.5 5033)

* Source impedance
R1 IN TANK 1k

* LC Tank circuit components
L1 TANK 0 10mH
* ... (truncated in public view) ...

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🔓 See premium access plans 

* Practical case: Resonance in LC tank circuit
.width out=256

* 5V peak-to-peak implies an amplitude of 2.5V. 
* The resonant frequency of 10mH and 100nF is approximately 5033 Hz.
* We configure V1 with both a transient sine wave at resonance and an AC magnitude for optional AC analysis.
V1 IN 0 DC 0 AC 2.5 SIN(0 2.5 5033)

* Source impedance
R1 IN TANK 1k

* LC Tank circuit components
L1 TANK 0 10mH
C1 TANK 0 100nF

* Operating point and Transient analysis
.op
.tran 1u 2m

* Print directives for logging the input and output (resonance) nodes
.print tran V(IN) V(TANK) I(L1)

.end

Simulation Results (Transient Analysis)

Simulation Results (Transient Analysis)
Analysis: The transient simulation shows the input voltage V(IN) oscillating as a sine wave with a 2.5V amplitude (5V peak-to-peak). The voltage at the tank node V(TANK) closely follows V(IN) with nearly the same amplitude, and the inductor current oscillates, confirming the resonant behavior of the LC tank circuit at the specified frequency.
Show raw data table (2015 rows) 
Index   time            v(in)           v(tank)         l1#branch
0	0.000000e+00	0.000000e+00	0.000000e+00	0.000000e+00
1	1.000000e-08	7.905818e-04	7.905026e-08	7.905026e-14
2	1.084006e-08	8.569951e-04	8.624878e-08	8.629565e-14
3	1.252017e-08	9.898217e-04	1.017615e-07	1.020896e-13
4	1.588039e-08	1.255475e-03	1.394809e-07	1.426210e-13
5	2.260084e-08	1.786781e-03	2.416948e-07	2.707046e-13
6	3.604174e-08	2.849394e-03	5.532131e-07	8.049184e-13
7	5.708432e-08	4.512980e-03	1.327631e-06	2.783809e-12
8	8.603868e-08	6.802053e-03	2.965106e-06	8.998482e-12
9	1.305078e-07	1.031768e-02	6.769425e-06	3.064276e-11
10	1.955195e-07	1.545732e-02	1.514065e-05	1.018634e-10
11	2.946313e-07	2.329267e-02	3.431881e-05	3.469641e-10
12	4.417944e-07	3.492633e-02	7.707420e-05	1.166612e-09
13	6.644501e-07	5.252635e-02	1.741480e-04	3.963414e-09
14	9.972436e-07	7.882720e-02	3.917455e-04	1.337970e-08
15	1.499113e-06	1.184727e-01	8.834917e-04	4.537981e-08
16	2.252017e-06	1.778899e-01	1.987598e-03	1.534626e-07
17	3.252017e-06	2.566456e-01	4.126641e-03	4.591745e-07
18	4.252017e-06	3.351447e-01	7.022468e-03	1.016630e-06
19	5.252017e-06	4.133086e-01	1.066173e-02	1.900840e-06
20	6.252017e-06	4.910592e-01	1.502968e-02	3.185410e-06
21	7.252017e-06	5.683189e-01	2.011023e-02	4.942405e-06
22	8.252017e-06	6.450102e-01	2.588597e-02	7.242215e-06
23	9.252017e-06	7.210565e-01	3.233820e-02	1.015342e-05
... (1991 more rows) ...

Common mistakes and how to avoid them

  • Using a polarized capacitor in an AC circuit: Electrolytic capacitors are generally polarized and can fail or explode if subjected to reversing AC voltages. Always use non-polarized capacitors (like ceramic or film) for an LC tank.
  • Ignoring the inductor’s Equivalent Series Resistance (ESR): Real inductors consist of long coils of wire, adding parasitic DC resistance to the tank. If the measured Q-factor is much lower than expected (resulting in a wider, flatter peak), inductor ESR is usually the culprit.
  • Confusing angular frequency (\omega) with standard frequency ($f$): Remember that \omega = (1 / \sqrtLC) yields results in radians per second. You must divide by 2\pi to get the frequency in Hertz.

Troubleshooting

  • Symptom: The measured resonant frequency is significantly higher or lower than the calculated 5032 Hz.
    • Cause: Component tolerances. Standard ceramic capacitors can have a ±20% tolerance, and inductors often have ±10%.
    • Fix: Measure the exact values of L1 and C1 using an LCR meter and recalculate the expected frequency.
  • Symptom: VLC shows no noticeable peak during the sweep; the voltage remains relatively flat.
    • Cause: The chosen frequency sweep range does not cover the resonant point, or R1 is too small, effectively shorting the tank to the rigid voltage source.
    • Fix: Double-check the math for your specific $L$ and $C$ values to ensure the sweep range encompasses fr. Ensure R1 is adequately sized (1 kΩ is a good starting point).
  • Symptom: Signal distortion or clipping is observed at node TANK.
    • Cause: The AC source might be overdriving the circuit, or core saturation is occurring in the inductor (if using a very small ferrite core at high currents).
    • Fix: Reduce the amplitude of V1 from 5 V to 1 V peak-to-peak and check if the sine wave becomes clean again.

Possible improvements and extensions

  • Vary the damping resistor: Swap R1 for different values (e.g., 470 Ω, 10 kΩ) or add a resistor directly in parallel with the LC tank. Observe and chart how this affects the sharpness of the resonance peak (the Q-factor).
  • Build an active oscillator: Remove the AC source and connect the LC tank to a transistor or an op-amp with positive feedback (such as a Colpitts or Hartley configuration) to create a standalone circuit that generates its own continuous sine wave at the resonant frequency.

More Practical Cases on Prometeo.blog

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Quick Quiz

Question 1: What is the primary objective of sweeping the input frequency in the described LC tank circuit?




Question 2: In a parallel LC tank circuit at resonance, what happens to the total current drawn from the source?




Question 3: What happens to the voltage across the LC tank at the resonant frequency?




Question 4: How does energy behave in an LC tank circuit at resonance?




Question 5: Which of the following is a real-world application of LC resonance mentioned in the text?




Question 6: What is the function of the series resistor (R1) in this practical case?




Question 7: What type of source is used to drive the LC tank circuit in this practical case?




Question 8: In the context of audio and signal filtering, what can LC circuits be used to create?




Question 9: Why is understanding LC resonance essential for wireless power transfer?




Question 10: Who is the target audience for this practical case?




Carlos Núñez Zorrilla
Carlos Núñez Zorrilla
Electronics & Computer Engineer

Telecommunications Electronics Engineer and Computer Engineer (official degrees in Spain).

Follow me:

Practical case: Boost converter storage

Level: Medium | Understand magnetic energy storage to boost voltage.

Objective and use case

In this practical case, you will build a basic open-loop Boost converter to demonstrate how an inductor stores and releases magnetic energy to step up a DC voltage.

Why it is useful:
* Allows battery-powered devices to operate at higher voltages (e.g., generating 5 V from a single 3.7 V Li-ion cell).
* Drives strings of LEDs that require a constant, high forward voltage.
* Captures and steps up voltage in energy harvesting and regenerative braking systems.
* Provides versatile power rails in compact portable electronics without requiring multiple batteries.

Expected outcome:
* You will observe the inductor current (I_inductor) ramping up when the switch is closed and ramping down when it opens.
* The output voltage (V_out) will be demonstrably higher than the input voltage source.
* You will record the direct relationship between the switch’s Duty Cycle and the resulting V_out magnitude.

Target audience and level:
Intermediate electronics students learning the fundamentals of switch-mode power supplies.

Materials

  • V1: 5 V DC source, function: main power input
  • V2: Pulse voltage source (0-5 V, 100kHz, 50% duty cycle), function: PWM signal for the switch
  • L1: 100 µH inductor, function: magnetic energy storage
  • M1: N-channel MOSFET (e.g., IRLZ44N), function: main switching element
  • D1: Schottky diode (e.g., 1N5819), function: prevents reverse current from capacitor
  • C1: 47 µF capacitor, function: output voltage smoothing
  • R1: 100 Ω resistor, function: basic load to discharge capacitor

Wiring guide

  • V1: connects between VIN and 0 (GND).
  • V2: connects between GATE_PWM and 0 (GND).
  • L1: connects between VIN and SW_NODE.
  • M1: Drain connects to SW_NODE, Gate connects to GATE_PWM, Source connects to 0 (GND).
  • D1: Anode connects to SW_NODE, Cathode connects to VOUT.
  • C1: connects between VOUT and 0 (GND).
  • R1: connects between VOUT and 0 (GND).

Conceptual block diagram

Conceptual block diagram — Boost Converter
Quick read: inputs → main block → output (actuator or measurement). This summarizes the ASCII schematic below.

Schematic

Control Signal:
[ V2: PWM (0-5 V) ] --(GATE_PWM)--> [ M1:Gate ]

Power & Switching Path:
[ V1: 5 V DC ] --(VIN)--> [ L1: 100µH ] --(SW_NODE)--> [ M1:Drain ] --(Switch)--> [ M1:Source ] --> GND
                                             |
Boost Output & Load:                         |
                                             +--> [ D1: Schottky ] --(VOUT)--> [ R1: 100 Ω ] --> GND
                                                                       |
                                                                       +--> [ C1: 47µF ] --> GND
Electrical Schematic

Electrical diagram

Electrical diagram for case: Practical case: Boost converter storage
Generated from the validated SPICE netlist for this case.

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Measurements and tests

  1. Initial state check: Apply V1 (5 V) with V2 turned off (0% duty cycle). Measure VOUT. The voltage should be roughly 4.7 V (the 5 V input minus the forward voltage drop of the Schottky diode).
  2. Switching activation: Activate V2 to supply a 100kHz square wave at a 50% duty cycle. Measure VOUT across R1. The voltage should rise to approximately 9 V-10 V, demonstrating the step-up action.
  3. Inductor current observation: Probe the current flowing through L1 (I_inductor). You will observe a triangular waveform. The upward slope occurs while M1 is ON (energy storage), and the downward slope occurs while M1 is OFF (energy release to VOUT).
  4. Duty Cycle mapping: Adjust the Duty Cycle of V2 from 30% to 70% in 10% increments. Record VOUT at each step to verify that a higher duty cycle yields a higher output voltage.

SPICE netlist and simulation

Reference SPICE Netlist (ngspice) — excerptFull SPICE netlist (ngspice)

* Boost converter storage

* Main power input
V1 VIN 0 DC 5

* PWM signal for the switch (100kHz, 50% duty cycle)
V2 GATE_PWM 0 PULSE(0 5 0 10n 10n 5u 10u)

* Magnetic energy storage
L1 VIN SW_NODE 100u

* Main switching element (N-channel MOSFET)
* Drain: SW_NODE, Gate: GATE_PWM, Source: 0, Bulk: 0
M1 SW_NODE GATE_PWM 0 0 IRLZ44N

* Prevents reverse current from capacitor
* Anode: SW_NODE, Cathode: VOUT
D1 SW_NODE VOUT 1N5819

* Output voltage smoothing
* ... (truncated in public view) ...

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* Boost converter storage

* Main power input
V1 VIN 0 DC 5

* PWM signal for the switch (100kHz, 50% duty cycle)
V2 GATE_PWM 0 PULSE(0 5 0 10n 10n 5u 10u)

* Magnetic energy storage
L1 VIN SW_NODE 100u

* Main switching element (N-channel MOSFET)
* Drain: SW_NODE, Gate: GATE_PWM, Source: 0, Bulk: 0
M1 SW_NODE GATE_PWM 0 0 IRLZ44N

* Prevents reverse current from capacitor
* Anode: SW_NODE, Cathode: VOUT
D1 SW_NODE VOUT 1N5819

* Output voltage smoothing
C1 VOUT 0 47u

* Basic load to discharge capacitor
R1 VOUT 0 100

* Models
.model IRLZ44N NMOS(Level=1 VTO=2.0 KP=10.0 RS=0.05 RD=0.05)
.model 1N5819 D(IS=1e-6 RS=0.1 N=1.05 EG=0.69 XTI=2)

* Output Directives
* VOUT is the main output, GATE_PWM is the input stimulus
.print tran V(VOUT) V(GATE_PWM) V(SW_NODE) V(VIN) I(L1)

* Analysis
* Time constant is R*C = 4.7ms. Simulating for 10ms to observe steady-state boost voltage.
.op
.tran 0.1u 10m

.end

Simulation Results (Transient Analysis)

Simulation Results (Transient Analysis)
Analysis: The simulation shows the boost converter operating correctly. The output voltage (VOUT) starts near 5V and rises to a steady-state value of approximately 9.6V, with the switch node (SW_NODE) switching between ~0V and ~10V as driven by the 100kHz PWM signal.
Show raw data table (119800 rows) 
Index   time            v(vout)         v(gate_pwm)     v(sw_node)      v(vin)          l1#branch
0	0.000000e+00	4.702912e+00	0.000000e+00	5.000000e+00	5.000000e+00	4.702912e-02
1	1.000000e-10	4.702912e+00	5.000000e-02	4.999798e+00	5.000000e+00	4.702912e-02
2	2.000000e-10	4.702912e+00	1.000000e-01	4.999798e+00	5.000000e+00	4.702912e-02
3	4.000000e-10	4.702912e+00	2.000000e-01	4.999797e+00	5.000000e+00	4.702912e-02
4	8.000000e-10	4.702912e+00	4.000000e-01	4.999797e+00	5.000000e+00	4.702912e-02
5	1.600000e-09	4.702912e+00	8.000000e-01	4.999797e+00	5.000000e+00	4.702912e-02
6	3.200000e-09	4.702912e+00	1.600000e+00	4.999797e+00	5.000000e+00	4.702913e-02
7	6.400000e-09	4.702910e+00	3.200000e+00	8.651034e-03	5.000000e+00	4.710899e-02
8	1.000000e-08	4.702907e+00	5.000000e+00	6.306948e-03	5.000000e+00	4.728872e-02
9	1.064000e-08	4.702906e+00	5.000000e+00	6.311218e-03	5.000000e+00	4.732068e-02
10	1.192000e-08	4.702905e+00	5.000000e+00	6.319746e-03	5.000000e+00	4.738460e-02
11	1.448000e-08	4.702902e+00	5.000000e+00	6.336800e-03	5.000000e+00	4.751244e-02
12	1.960000e-08	4.702897e+00	5.000000e+00	6.370908e-03	5.000000e+00	4.776811e-02
13	2.984000e-08	4.702887e+00	5.000000e+00	6.439123e-03	5.000000e+00	4.827946e-02
14	5.032000e-08	4.702866e+00	5.000000e+00	6.575553e-03	5.000000e+00	4.930212e-02
15	9.128000e-08	4.702825e+00	5.000000e+00	6.848406e-03	5.000000e+00	5.134738e-02
16	1.732000e-07	4.702743e+00	5.000000e+00	7.394086e-03	5.000000e+00	5.543754e-02
17	2.732000e-07	4.702643e+00	5.000000e+00	8.060152e-03	5.000000e+00	6.042981e-02
18	3.732000e-07	4.702543e+00	5.000000e+00	8.726166e-03	5.000000e+00	6.542142e-02
19	4.732000e-07	4.702443e+00	5.000000e+00	9.392128e-03	5.000000e+00	7.041236e-02
20	5.732000e-07	4.702343e+00	5.000000e+00	1.005804e-02	5.000000e+00	7.540264e-02
21	6.732000e-07	4.702243e+00	5.000000e+00	1.072390e-02	5.000000e+00	8.039225e-02
22	7.732000e-07	4.702143e+00	5.000000e+00	1.138970e-02	5.000000e+00	8.538119e-02
23	8.732000e-07	4.702043e+00	5.000000e+00	1.205546e-02	5.000000e+00	9.036947e-02
... (119776 more rows) ...

Common mistakes and how to avoid them

  • Using a standard rectifier diode (e.g., 1N4007): Standard diodes are too slow to turn off at 100kHz, leading to massive switching losses and poor voltage conversion. Always use a fast-recovery or Schottky diode like the 1N5819.
  • Inductor core saturation: If the inductor’s maximum current rating is lower than the peak switching current, the magnetic core will saturate. The inductor will then act as a short circuit, potentially destroying the MOSFET. Always verify the inductor’s saturation current rating.
  • Operating without a load: Running a boost converter with no load resistor (R1) can cause the output voltage to continuously rise with every switching cycle, theoretically reaching infinity and destroying the output capacitor or MOSFET. Always include a minimum load.

Troubleshooting

  • Symptom: Output voltage equals the input voltage (minus diode drop).
  • Cause: The MOSFET is not switching. V2 might be disconnected or the voltage level is too low to surpass the MOSFET’s gate threshold.
  • Fix: Check the GATE_PWM signal with an oscilloscope. Use a logic-level MOSFET if your PWM signal is limited to 3.3 V or 5 V.
  • Symptom: MOSFET becomes extremely hot very quickly.
  • Cause: The inductor is saturating, or the MOSFET has a high ON-resistance (RDS(on)) and is experiencing high conduction losses.
  • Fix: Swap the inductor for one with a higher current rating. Ensure the gate drive voltage is sufficient to turn the MOSFET completely ON.
  • Symptom: Unstable or highly rippled output voltage.
  • Cause: The output capacitor C1 is too small for the load or has a high Equivalent Series Resistance (ESR).
  • Fix: Increase the capacitance of C1 or place a ceramic capacitor in parallel with the electrolytic capacitor to lower the overall ESR.

Possible improvements and extensions

  • Closed-loop control: Add a voltage divider at the output connected to an error amplifier or microcontroller analog input. Dynamically adjust the PWM duty cycle to maintain a constant VOUT regardless of changes in R1 (the load).
  • Synchronous rectification: Replace the Schottky diode D1 with a second P-channel or driven N-channel MOSFET. Switching this second MOSFET synchronously (inversely to M1) reduces the voltage drop typical of a diode, significantly improving overall converter efficiency.

More Practical Cases on Prometeo.blog

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Quick Quiz

Question 1: What is the primary purpose of the Boost converter in this practical case?




Question 2: Which component is responsible for storing and releasing magnetic energy?




Question 3: What happens to the inductor current when the switch is closed?




Question 4: Which of the following is a mentioned use case for a Boost converter?




Question 5: What happens to the inductor current when the switch opens?




Question 6: What parameter of the switch has a direct relationship with the magnitude of the output voltage?




Question 7: How can a Boost converter benefit battery-powered devices?




Question 8: What role does the Boost converter play in energy harvesting systems?




Question 9: What is the expected relationship between the output voltage and the input voltage source?




Question 10: Who is the target audience for this practical case?




Carlos Núñez Zorrilla
Carlos Núñez Zorrilla
Electronics & Computer Engineer

Telecommunications Electronics Engineer and Computer Engineer (official degrees in Spain).

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Practical case: Inductive peak protection

Inductive peak protection prototype (Maker Style)

Level: Medium | Objective: Analyze the transient voltage generated when disconnecting an inductor and mitigate it using a flyback diode.

Objective and use case

In this practical case, you will build a switched inductor circuit monitored by an oscilloscope to observe the destructive voltage spike (inductive kickback) that occurs when current is abruptly interrupted. You will then install a flyback diode in parallel with the inductive load to safely clamp this transient voltage.

Why it is useful:
* Prevents catastrophic overvoltage damage to sensitive switching components such as transistors, MOSFETs, and microcontroller pins.
* Significantly reduces electromagnetic interference (EMI) and radio frequency interference (RFI) caused by high-voltage arcing across mechanical switch contacts.
* Increases the reliability, safety, and lifespan of power supply systems, motor controllers, and relay-driven circuits.

Expected outcome:
* Without the diode, opening the switch will produce a massive negative voltage spike on the oscilloscope, often reaching hundreds of volts.
* With the flyback diode installed, the transient spike will be immediately clamped to a safe level of approximately -0.7 V.
* The stored magnetic energy will safely dissipate as a steadily decaying circulating current through the inductor-resistor-diode loop.

Target audience and level: Intermediate electronics students learning about reactive components, energy storage, and circuit protection techniques.

Materials

  • V1: 12 V DC supply, function: main power source
  • SW1: SPST toggle or push-button switch, function: circuit connection control
  • L1: 100 mH inductor, function: magnetic energy storage
  • R1: 100 Ω resistor, function: limits steady-state current to 120 mA
  • D1: 1N4007 rectifier diode, function: flyback protection

Wiring guide

  • V1: connects between node VCC (positive) and node 0 (ground).
  • SW1: connects between node VCC and node SW_OUT.
  • L1: connects between node SW_OUT and node L_MID.
  • R1: connects between node L_MID and node 0.
  • D1: connects between node 0 (Anode) and node SW_OUT (Cathode) for reverse bias during normal closed-switch operation.

Conceptual block diagram

Conceptual block diagram — Flyback Protection
Quick read: inputs → main block → output (actuator or measurement). This summarizes the ASCII schematic below.

Schematic

VCC (12 V) --> [ SW1: SPST Switch ] --(SW_OUT)--> [ L1: 100mH Inductor ] --(L_MID)--> [ R1: 100 Ω Resistor ] --> GND
                                         ^
                                         |
                              (Cathode)  |
                           [ D1: 1N4007 Flyback ]
                              (Anode)    ^
                                         |
                                        GND
Electrical Schematic

Electrical diagram

Electrical diagram for case: Inductive peak protection
Generated from the validated SPICE netlist for this case.

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Measurements and tests

  1. Connect the oscilloscope probe to node SW_OUT and attach the ground clip to node 0. Set the oscilloscope trigger to a falling edge, single-shot mode.
  2. Begin with the flyback diode (D1) completely disconnected from the circuit.
  3. Close the switch (SW1) to allow current to flow. Wait a moment for the magnetic field in the inductor to fully build up.
  4. Quickly open the switch (SW1). Observe the oscilloscope capture; you will see a massive negative voltage transient as the inductor acts as a current source, forcing current across the open switch gap.
  5. Connect the flyback diode (D1), verifying that the cathode (striped end) connects to node SW_OUT and the anode connects to node 0.
  6. Repeat the switching process. The oscilloscope trace will now show the negative transient safely clamped at roughly -0.7 V as the diode forward-biases to provide a safe discharge path.

SPICE netlist and simulation

Reference SPICE Netlist (ngspice) — excerptFull SPICE netlist (ngspice)

* Inductive peak protection
.width out=256

V1 VCC 0 DC 12

* SW1 modeled as a voltage-controlled switch connecting VCC to SW_OUT
S1 VCC SW_OUT SW_CTRL 0 SW_MODEL
V_SW_CTRL SW_CTRL 0 PULSE(0 5 100u 1u 1u 500u 1000u)
.model SW_MODEL SW(VT=2.5 VH=0.1 RON=0.01 ROFF=100Meg)

L1 SW_OUT L_MID 100m
R1 L_MID 0 100

* ... (truncated in public view) ...

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* Inductive peak protection
.width out=256

V1 VCC 0 DC 12

* SW1 modeled as a voltage-controlled switch connecting VCC to SW_OUT
S1 VCC SW_OUT SW_CTRL 0 SW_MODEL
V_SW_CTRL SW_CTRL 0 PULSE(0 5 100u 1u 1u 500u 1000u)
.model SW_MODEL SW(VT=2.5 VH=0.1 RON=0.01 ROFF=100Meg)

L1 SW_OUT L_MID 100m
R1 L_MID 0 100

* Flyback protection diode
D1 0 SW_OUT 1N4007
.model 1N4007 D(IS=1e-9 N=1.9 RS=0.03 BV=1000 IBV=5e-08 CJO=10p VJ=0.7 M=0.5 TT=1e-07)

.op
.tran 1u 2000u
.print tran V(SW_CTRL) V(SW_OUT) V(L_MID) V(VCC) I(L1)

.end

Simulation Results (Transient Analysis)

Simulation Results (Transient Analysis)
Analysis: The transient analysis spans 0 s to 2 ms and captures the switching interval. The switching node and inductor current remain bounded, consistent with the flyback path protecting the switch. Main ranges: l1#branch 120 nA -> 62.7 mA; v(sw_out) -884 mV -> 12 V; v(l_mid) 12 uV -> 6.27 V.
Show raw data table (2088 rows) 
Index   time            v(sw_ctrl)      v(sw_out)       v(l_mid)        v(vcc)          l1#branch
0	0.000000e+00	0.000000e+00	1.199996e-05	1.199996e-05	1.200000e+01	1.199996e-07
1	1.000000e-08	0.000000e+00	1.199996e-05	1.199996e-05	1.200000e+01	1.199996e-07
2	2.000000e-08	0.000000e+00	1.199996e-05	1.199996e-05	1.200000e+01	1.199996e-07
3	4.000000e-08	0.000000e+00	1.199996e-05	1.199996e-05	1.200000e+01	1.199996e-07
4	8.000000e-08	0.000000e+00	1.199996e-05	1.199996e-05	1.200000e+01	1.199996e-07
5	1.600000e-07	0.000000e+00	1.199996e-05	1.199996e-05	1.200000e+01	1.199996e-07
6	3.200000e-07	0.000000e+00	1.199996e-05	1.199996e-05	1.200000e+01	1.199996e-07
7	6.400000e-07	0.000000e+00	1.199996e-05	1.199996e-05	1.200000e+01	1.199996e-07
8	1.280000e-06	0.000000e+00	1.199996e-05	1.199996e-05	1.200000e+01	1.199996e-07
9	2.280000e-06	0.000000e+00	1.199996e-05	1.199996e-05	1.200000e+01	1.199996e-07
10	3.280000e-06	0.000000e+00	1.199996e-05	1.199996e-05	1.200000e+01	1.199996e-07
11	4.280000e-06	0.000000e+00	1.199996e-05	1.199996e-05	1.200000e+01	1.199996e-07
12	5.280000e-06	0.000000e+00	1.199996e-05	1.199996e-05	1.200000e+01	1.199996e-07
13	6.280000e-06	0.000000e+00	1.199996e-05	1.199996e-05	1.200000e+01	1.199996e-07
14	7.280000e-06	0.000000e+00	1.199996e-05	1.199996e-05	1.200000e+01	1.199996e-07
15	8.280000e-06	0.000000e+00	1.199996e-05	1.199996e-05	1.200000e+01	1.199996e-07
16	9.280000e-06	0.000000e+00	1.199996e-05	1.199996e-05	1.200000e+01	1.199996e-07
17	1.028000e-05	0.000000e+00	1.199996e-05	1.199996e-05	1.200000e+01	1.199996e-07
18	1.128000e-05	0.000000e+00	1.199996e-05	1.199996e-05	1.200000e+01	1.199996e-07
19	1.228000e-05	0.000000e+00	1.199996e-05	1.199996e-05	1.200000e+01	1.199996e-07
20	1.328000e-05	0.000000e+00	1.199996e-05	1.199996e-05	1.200000e+01	1.199996e-07
21	1.428000e-05	0.000000e+00	1.199996e-05	1.199996e-05	1.200000e+01	1.199996e-07
22	1.528000e-05	0.000000e+00	1.199996e-05	1.199996e-05	1.200000e+01	1.199996e-07
23	1.628000e-05	0.000000e+00	1.199996e-05	1.199996e-05	1.200000e+01	1.199996e-07
... (2064 more rows) ...

Common mistakes and how to avoid them

  • Reversing the diode polarity: Placing the diode with the anode pointing to the positive voltage node creates a direct short circuit to ground when the switch is closed. This will destroy the diode or trigger the power supply’s overcurrent protection. Always ensure the cathode faces the higher potential.
  • Using a diode with inadequate current rating: The flyback diode must safely handle a peak forward current equal to the steady-state current of the inductor just before switching. Always use properly rated rectifier, Schottky, or fast-recovery diodes.
  • Omitting the series resistor: Connecting a pure inductor directly across a high-current DC source acts as a near short-circuit once the magnetic field is fully established. Always include a current-limiting series resistor, or ensure the inductor (such as a relay coil) has sufficient internal DC resistance.

Troubleshooting

  • Symptom: The power supply shuts down or its current limit LED turns on immediately upon closing the switch.
    • Cause: The flyback diode is installed backwards, creating a short circuit from the power source to ground.
    • Fix: Disconnect power immediately and flip the diode so its striped end (cathode) faces the switch node.
  • Symptom: A massive voltage spike still appears on the oscilloscope even with the diode supposedly installed.
    • Cause: The diode may have blown open due to a previous overcurrent event, or the breadboard connection is loose.
    • Fix: Verify diode continuity using a multimeter’s diode mode, and check the physical seating of the pins at the switch and ground nodes.
  • Symptom: The oscilloscope trace shows high-frequency ringing instead of a clean clamp.
    • Cause: Parasitic capacitance in the switch, wiring, or oscilloscope probes interacting with the inductor.
    • Fix: Ensure the oscilloscope probe is properly compensated (x10 mode recommended for high voltage spikes) and keep ground leads as short as physically possible.

Possible improvements and extensions

  • Automated switching with a MOSFET: Replace the mechanical switch with an N-channel MOSFET driven by a square wave generator (configured as a low-side switch) to observe repetitive clamping on the oscilloscope in real-time.
  • Fast discharge using a Zener diode: Add an appropriately rated Zener diode in series with the standard flyback diode (anode connected to anode). This allows the inductor to discharge its energy much faster by clamping the voltage at a higher, but strictly controlled, level.

More Practical Cases on Prometeo.blog

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Go to Amazon

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Quick Quiz

Question 1: What is the main purpose of the flyback diode in this inductive load circuit?




Question 2: Which component stores energy in its magnetic field while current is flowing?




Question 3: What happens to the inductor current immediately after the switch opens?




Question 4: Why can an unprotected inductive load damage a switching device?




Question 5: In the SPICE model, what does the pulsed switch-control source represent?




Question 6: During normal energized operation, what should the flyback diode ideally do?




Question 7: Which measurement is most useful to observe the switching transient in the simulation?




Question 8: What role does R1 play in this practical model?




Question 9: Why is this circuit relevant for relays, solenoids and small motors?




Question 10: What should a correct validation show for this case before publication?




Carlos Núñez Zorrilla
Carlos Núñez Zorrilla
Electronics & Computer Engineer

Telecommunications Electronics Engineer and Computer Engineer (official degrees in Spain).

Follow me: