Level: Basic. Learn how to isolate a low-power control signal from a high-power motor circuit using an electromagnetic relay.

Objective and use case

In this practical case, you will build a circuit that utilizes a small pushbutton and a relay to control a high-current DC motor. The relay acts as an electromagnetic switch, allowing the low-power control side to activate the high-power load side without direct electrical connection between the distinct power rails (if separate grounds are used) or simply to handle currents exceeding the switch’s rating.

Why it is useful:
* Automotive systems: Used in starter motors where a small ignition key switch triggers a massive solenoid (relay) to crank the engine.
* Industrial automation: Allows low-voltage PLCs (24 V) to switch high-voltage AC or DC motors (110 V/220 V) safely.
* Safety isolation: Keeps high voltages away from the user interface (buttons and switches).
* Component protection: Prevents burning out delicate switches by offloading the high current switching to the relay contacts.

Expected outcome:
* When the pushbutton is pressed, the relay makes an audible «click.»
* The DC motor starts spinning immediately upon the click.
* Voltage across the relay coil measures 5 V (or rated control voltage).
* The flyback diode protects the switch from high-voltage spikes when the button is released.

Target audience and level: Students and hobbyists understanding basic electromechanical switching (Basic).

Materials

• V1: 5 V DC voltage source, function: Control circuit power supply.
• V2: 12 V DC voltage source, function: Motor (Power) circuit supply.
• S1: Momentary Pushbutton (Normally Open), function: Control switch.
• K1: SPDT Relay (5 V Coil), function: Electromechanical isolation and switching.
• D1: 1N4007 Diode, function: Flyback/Freewheeling diode for coil protection.
• M1: 12 V DC Motor, function: High-power load.

Wiring guide

This guide uses SPICE-friendly node names to define the connections. The nodes are: V_CTRL (5 V), V_PWR (12 V), COIL_IN, MOTOR_IN, and 0 (Ground).

• V1 (Positive): Connects to node V_CTRL.
• V1 (Negative): Connects to node 0.
• V2 (Positive): Connects to node V_PWR.
• V2 (Negative): Connects to node 0.
• S1: Connects between node V_CTRL and node COIL_IN.
• K1 (Coil terminal A): Connects to node COIL_IN.
• K1 (Coil terminal B): Connects to node 0.
• D1 (Cathode/Striped side): Connects to node COIL_IN.
• D1 (Anode): Connects to node 0.
• K1 (Common/COM Contact): Connects to node V_PWR.
• K1 (Normally Open/NO Contact): Connects to node MOTOR_IN.
• M1 (Positive): Connects to node MOTOR_IN.
• M1 (Negative): Connects to node 0.

Note: In a physical application requiring galvanic isolation, the ground 0 for the control side (V1) and the power side (V2) would be kept separate. For this basic simulation model, they share a common reference.

Conceptual block diagram

Schematic

+---------------------------------------------------------------------------------------------------------+
|                          DC MOTOR CONTROL WITH RELAY (UNIFIED DIAGRAM)                                  |
+---------------------------------------------------------------------------------------------------------+

      (High Power Loop: 12 V)
      [ V2: 12 V Source ] --(Node: V_PWR)--> [ K1: Relay Switch (COM->NO) ] --(Node: MOTOR_IN)--> [ M1: 12 V Motor ] --> [ GND ]
                                                        ^
                                                        |
                                                 (Magnetic Link)
                                                        |
      (Control Loop: 5 V)                                |
      [ V1: 5 V Source ] --(Node: V_CTRL)--> [ S1: Pushbutton ] --(Node: COIL_IN)--> [ Parallel: K1 Coil || D1 (Rev) ] --> [ GND ]

+---------------------------------------------------------------------------------------------------------+
|  LEGEND & NOTES:                                                                                        |
|  -->  : Signal/Power Flow                                                                               |
|  ||   : Components in Parallel (Coil and Diode share Node COIL_IN and GND)                              |
|  Rev  : Diode D1 is Reverse Biased (Cathode to COIL_IN, Anode to GND) to suppress flyback voltage.      |
|  Link : The current in the Control Loop generates the magnetic field to close the Switch in the Power Loop. |
+---------------------------------------------------------------------------------------------------------+

Measurements and tests

Follow these steps to validate your circuit assembly:

1. Coil Voltage Check:

   • Set your multimeter to DC Voltage (20 V range).
   • Connect probes across the relay coil terminals (COIL_IN and 0).
   • Press S1. The reading should jump from 0 V to approx. 5 V.

2. Audible Confirmation:

   • Press and release S1. Listen for the mechanical «click» of the relay armature moving. If you do not hear it, the coil is not energizing.

3. Load Voltage Verification:

   • Connect the multimeter across the motor terminals.
   • Press S1. The multimeter should read approx. 12 V (voltage of V2) and the motor should spin.
   • Release S1. The voltage should drop to 0 V and the motor should coast to a stop.

4. Flyback Diode Test (Advanced):

   • Without D1, monitoring COIL_IN with an oscilloscope would reveal a large negative voltage spike when S1 is released. With D1 installed, this spike is clamped to approx. -0.7 V, protecting S1.

SPICE netlist and simulation

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

* Practical case: DC motor control with relay and pushbutton

* --- Models ---
* Generic Pushbutton Switch Model (Voltage Controlled)
.model SW_PB SW(Vt=2.5 Vh=0.1 Ron=0.01 Roff=10Meg)
* Relay Contact Switch Model (Controlled by Coil Voltage)
.model SW_RELAY SW(Vt=3.5 Vh=0.2 Ron=0.05 Roff=10Meg)
* 1N4007 Diode Model
.model D1N4007 D(IS=7.07e-9 RS=0.034 N=1.7 BV=1000 IBV=5e-6 CJO=1e-11 TT=1e-7)

* --- Power Supplies ---
* V1: Control Circuit Power (5V)
V1 V_CTRL 0 DC 5
* V2: Motor Circuit Power (12V)
V2 V_PWR 0 DC 12

* --- Control Circuit (Input) ---
* S1: Pushbutton.
* Modeled as a voltage-controlled switch driven by a PULSE source (V_ACT)
* to simulate the physical act of pressing the button.
* ... (truncated in public view) ...

Copy this content into a .cir file and run with ngspice.

🔒 Part of this section is premium. With the 7-day pass or the monthly membership you can access the full content (materials, wiring, detailed build, validation, troubleshooting, variants and checklist) and download the complete print-ready PDF pack.

🔓 See premium access plans

* Practical case: DC motor control with relay and pushbutton

* --- Models ---
* Generic Pushbutton Switch Model (Voltage Controlled)
.model SW_PB SW(Vt=2.5 Vh=0.1 Ron=0.01 Roff=10Meg)
* Relay Contact Switch Model (Controlled by Coil Voltage)
.model SW_RELAY SW(Vt=3.5 Vh=0.2 Ron=0.05 Roff=10Meg)
* 1N4007 Diode Model
.model D1N4007 D(IS=7.07e-9 RS=0.034 N=1.7 BV=1000 IBV=5e-6 CJO=1e-11 TT=1e-7)

* --- Power Supplies ---
* V1: Control Circuit Power (5V)
V1 V_CTRL 0 DC 5
* V2: Motor Circuit Power (12V)
V2 V_PWR 0 DC 12

* --- Control Circuit (Input) ---
* S1: Pushbutton.
* Modeled as a voltage-controlled switch driven by a PULSE source (V_ACT)
* to simulate the physical act of pressing the button.
* Wiring: Connects V_CTRL to COIL_IN.
V_ACT ACT_NODE 0 PULSE(0 5 10m 1u 1u 245m 1s)
S1 V_CTRL COIL_IN ACT_NODE 0 SW_PB

* K1: Relay Coil
* Wiring: Coil Terminal A to COIL_IN, Coil Terminal B to 0.
* Modeled as Inductor + Resistor in series.
R_K1_COIL COIL_IN K1_INT 60
L_K1_COIL K1_INT 0 100m

* D1: Flyback Diode
* Wiring: Cathode to COIL_IN, Anode to 0.
* SPICE Syntax: D   
D1 0 COIL_IN D1N4007

* --- Power Circuit (Output) ---
* K1: Relay Contact (Switch)
* Wiring: Common (COM) to V_PWR, Normally Open (NO) to MOTOR_IN.
* Controlled by the voltage at node COIL_IN.
S_K1_SW V_PWR MOTOR_IN COIL_IN 0 SW_RELAY

* M1: DC Motor
* Wiring: Positive to MOTOR_IN, Negative to 0.
* Modeled as an RL load (Resistance + Inductance).
R_M1 MOTOR_IN M1_INT 20
L_M1 M1_INT 0 10m

* --- Analysis Directives ---
.op
.tran 0.1m 250m

* --- Output Printing ---
* Must define INPUT (COIL_IN) and OUTPUT (MOTOR_IN)
.print tran V(COIL_IN) V(MOTOR_IN) V(ACT_NODE) I(L_M1)

.end

Simulation Results (Transient Analysis)

Analysis: The simulation shows the control signal (V_ACT) going high at 10ms. Consequently, the coil voltage (V(COIL_IN)) rises to ~5V. This triggers the relay switch, causing the motor input voltage (V(MOTOR_IN)) to jump from near 0V to ~12V, and current flows through the motor load.

Show raw data table (2535 rows)

Index   time            v(coil_in)      v(motor_in)     v(act_node)     l_m1#branch
0	0.000000e+00	2.999953e-05	2.399995e-05	0.000000e+00	1.199998e-06
1	1.000000e-06	2.999953e-05	2.399995e-05	0.000000e+00	1.199998e-06
2	2.000000e-06	2.999953e-05	2.399995e-05	0.000000e+00	1.199998e-06
3	4.000000e-06	2.999953e-05	2.399995e-05	0.000000e+00	1.199998e-06
4	8.000000e-06	2.999953e-05	2.399995e-05	0.000000e+00	1.199998e-06
5	1.600000e-05	2.999953e-05	2.399995e-05	0.000000e+00	1.199998e-06
6	3.200000e-05	2.999953e-05	2.399995e-05	0.000000e+00	1.199998e-06
7	6.400000e-05	2.999953e-05	2.399995e-05	0.000000e+00	1.199998e-06
8	1.280000e-04	2.999953e-05	2.399995e-05	0.000000e+00	1.199998e-06
9	2.280000e-04	2.999953e-05	2.399995e-05	0.000000e+00	1.199998e-06
10	3.280000e-04	2.999953e-05	2.399995e-05	0.000000e+00	1.199998e-06
11	4.280000e-04	2.999953e-05	2.399995e-05	0.000000e+00	1.199998e-06
12	5.280000e-04	2.999953e-05	2.399995e-05	0.000000e+00	1.199998e-06
13	6.280000e-04	2.999953e-05	2.399995e-05	0.000000e+00	1.199998e-06
14	7.280000e-04	2.999953e-05	2.399995e-05	0.000000e+00	1.199998e-06
15	8.280000e-04	2.999953e-05	2.399995e-05	0.000000e+00	1.199998e-06
16	9.280000e-04	2.999953e-05	2.399995e-05	0.000000e+00	1.199998e-06
17	1.028000e-03	2.999953e-05	2.399995e-05	0.000000e+00	1.199998e-06
18	1.128000e-03	2.999953e-05	2.399995e-05	0.000000e+00	1.199998e-06
19	1.228000e-03	2.999953e-05	2.399995e-05	0.000000e+00	1.199998e-06
20	1.328000e-03	2.999953e-05	2.399995e-05	0.000000e+00	1.199998e-06
21	1.428000e-03	2.999953e-05	2.399995e-05	0.000000e+00	1.199998e-06
22	1.528000e-03	2.999953e-05	2.399995e-05	0.000000e+00	1.199998e-06
23	1.628000e-03	2.999953e-05	2.399995e-05	0.000000e+00	1.199998e-06
... (2511 more rows) ...

Common mistakes and how to avoid them

1. Omitting the Flyback Diode (D1):

   • Error: Leaving out the diode across the relay coil.
   • Consequence: The collapsing magnetic field generates a high-voltage spike (back EMF) that can arc across the switch contacts or destroy transistor drivers in future circuits.
   • Solution: Always install a diode in reverse bias (Cathode to positive) across inductive loads.

2. Using the Wrong Relay Contacts (NC vs NO):

   • Error: Connecting the motor to the Normally Closed (NC) pin instead of Normally Open (NO).
   • Consequence: The motor runs continuously when the button is not pressed and stops when it is pressed.
   • Solution: Identify the NO pin using the datasheet or a continuity test before soldering.

3. Mixing Power Rails:

   • Error: Connecting the 12 V motor supply directly to the 5 V coil.
   • Consequence: The relay coil will overheat and likely burn out due to over-voltage.
   • Solution: Ensure the coil voltage matches the control supply (V1) and the contact rating matches the motor supply (V2).

Troubleshooting

• Symptom: Relay clicks, but motor does not run.

  • Cause: Burnt relay contacts or loose wire between COM/NO and the motor.
  • Fix: Check continuity across COM and NO while the relay is held active.

• Symptom: Relay does not click when S1 is pressed.

  • Cause: Coil wiring error or S1 is faulty.
  • Fix: Measure voltage at the coil terminals while pressing S1. If 0 V, check S1.

• Symptom: Circuit resets or sparks occur at S1.

  • Cause: Lack of flyback diode causing arcing.
  • Fix: Install D1 immediately across the coil terminals.

Possible improvements and extensions

1. Transistor Driver: Replace the direct pushbutton connection with an NPN transistor (e.g., 2N2222) to control the relay using a weak signal from an Arduino or microcontroller.
2. Self-Latching Circuit: Add a second relay contact or wire the relay in a «latching» configuration with a separate «Stop» button (NC), so you don’t have to hold S1 to keep the motor running.

More Practical Cases on Prometeo.blog

Carlos Núñez Zorrilla

Electronics & Computer Engineer

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

Follow me:

Who we are

Level: Basic. Demonstrate how moving a magnet through a coil generates an electromotive force (EMF).

Objective and use case

In this practical case, you will construct a fundamental electromagnetic induction demonstrator using a hand-wound copper coil and a high-strength neodymium magnet. You will observe how kinetic energy is converted into electrical energy via Faraday’s Law of Induction.

Why it is useful:
* Power Generation: This mechanism illustrates the core principle behind electric generators, alternators, and wind turbines.
* Audio Technology: This is the operating principle for dynamic microphones and electric guitar pickups (transducers).
* Sensors: Used in automotive ABS speed sensors and industrial position sensors.
* Wireless Charging: Demonstrates the basics of magnetic coupling used in phone chargers.

Expected outcome:
* A measurable voltage spike (positive or negative) on the multimeter when the magnet moves relative to the coil.
* The LED flashes briefly when the magnet is moved rapidly, indicating a voltage peak exceeding the diode’s forward voltage (~1.8 V).
* Reversing the direction of the magnet’s movement reverses the polarity of the induced voltage.

Target audience: Students and hobbyists introducing themselves to Faraday’s Law and passive components.

Materials

• L1: Air core coil (approx. 500–1000 turns of enameled copper wire), function: induction element.
• MAG1: Cylindrical Neodymium magnet (fit to pass inside L1), function: source of magnetic flux.
• D1: Red LED, function: indicator for positive phase induction.
• D2: Green LED, function: indicator for negative phase induction (connected in anti-parallel).
• M1: Multimeter (set to 200 mV or 2 V DC range), function: voltage monitor.

Wiring guide

The circuit consists of the coil connected directly to the indicators in parallel. We define the coil terminals as nodes COIL_A and COIL_B.

• L1: Connects between node COIL_A and node COIL_B.
• D1: Anode connects to COIL_A; Cathode connects to COIL_B.
• D2: Anode connects to COIL_B; Cathode connects to COIL_A (anti-parallel to D1).
• M1: Positive probe connects to COIL_A; Negative probe connects to COIL_B.

Conceptual block diagram

Schematic

markdown
Title: Practical case: Voltage induction by magnetic movement

[ INPUT / SOURCE ]                       [ DISTRIBUTION RAILS ]                    [ OUTPUT / LOADS ]

                                                 (Node A: Top Rail)
                                    /------------------------------------------------------------------>
                                    |                |                    |                    |
[ MAG1: Magnet ] --(Flux)--> [ L1: Coil ]            | (Anode)            | (Cathode)          | (+)
                                    |                v                    v                    v
                                    |        [ D1: Red LED ]      [ D2: Grn LED ]      [ M1: Meter ]
                                    |        (Lights if A > B)    (Lights if B > A)    (Monitor V)
                                    |                |                    |                    |
                                    |                | (Cathode)          | (Anode)            | (-)
                                    \                v                    v                    v
                                    \------------------------------------------------------------------>
                                                 (Node B: Bottom Rail)

Electrical diagram

🔒 This electrical diagram is premium. With the 7-day pass or the monthly membership you can unlock the complete didactic material and the print-ready PDF pack.🔓 See premium access plans

Measurements and tests

1. Static Test: Place the magnet inside the coil and hold it completely still. The multimeter should read 0 V, and no LEDs should light up. This confirms that a changing magnetic field is required.
2. Slow Insertion: Set the multimeter to the lowest DC voltage range (e.g., 200 mV). Slowly push the magnet into the coil. Observe a small voltage reading (e.g., +10 to +50 mV).
3. Fast Action: Quickly thrust the magnet into the coil. You should see a significantly higher voltage spike (potentially > 1 V) and D1 (Red) may flash briefly.
4. Reverse Motion: Quickly pull the magnet out of the coil. The voltage polarity on the multimeter will flip (negative sign), and D2 (Green) should flash.
5. Oscillation: Move the magnet back and forth rapidly inside the coil. The LEDs should flicker alternately, demonstrating the generation of Alternating Current (AC).

SPICE netlist and simulation

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

* Practical case: Voltage induction by magnetic movement
.width out=256
*
* Description:
* Simulation of a magnet moving through a coil, inducing voltage to drive two antiparallel LEDs.
*
* Nodes:
* COIL_A : Hot terminal of the coil (Multimeter +)
* COIL_B : Reference terminal of the coil (Multimeter -, Grounded)
*
* Note: The physical "Coil" is modeled as a series combination of an EMF Voltage Source (V_MAG1),
* a Resistor (R_WIRE), and the Inductor (L1).

* --- Power / Reference ---
* Grounding COIL_B as per Multimeter negative probe convention
V_REF COIL_B 0 0

* --- Magnetic Induction Source (MAG1) ---
* Simulating the changing magnetic flux from MAG1 as an AC voltage source.
* 3V Peak, 5Hz (Simulates shaking the magnet)
* ... (truncated in public view) ...

Copy this content into a .cir file and run with ngspice.

🔒 Part of this section is premium. With the 7-day pass or the monthly membership you can access the full content (materials, wiring, detailed build, validation, troubleshooting, variants and checklist) and download the complete print-ready PDF pack.

🔓 See premium access plans

* Practical case: Voltage induction by magnetic movement
.width out=256
*
* Description:
* Simulation of a magnet moving through a coil, inducing voltage to drive two antiparallel LEDs.
*
* Nodes:
* COIL_A : Hot terminal of the coil (Multimeter +)
* COIL_B : Reference terminal of the coil (Multimeter -, Grounded)
*
* Note: The physical "Coil" is modeled as a series combination of an EMF Voltage Source (V_MAG1),
* a Resistor (R_WIRE), and the Inductor (L1).

* --- Power / Reference ---
* Grounding COIL_B as per Multimeter negative probe convention
V_REF COIL_B 0 0

* --- Magnetic Induction Source (MAG1) ---
* Simulating the changing magnetic flux from MAG1 as an AC voltage source.
* 3V Peak, 5Hz (Simulates shaking the magnet)
V_MAG1 N_EMF COIL_B SIN(0 3 5)

* --- Coil Assembly (L1) ---
* Internal wire resistance
R_WIRE N_EMF N_L1 5
* The physical inductance L1
L1 N_L1 COIL_A 10m

* --- Indicators ---
* D1: Red LED (Indicates Positive Phase)
* Anode: COIL_A, Cathode: COIL_B
D1 COIL_A COIL_B D_RED

* D2: Green LED (Indicates Negative Phase)
* Anode: COIL_B, Cathode: COIL_A
D2 COIL_B COIL_A D_GREEN

* --- Multimeter (M1) ---
* Modeled as the voltage difference V(COIL_A) - V(COIL_B)
* (Implicit in the node voltages)

* --- Models ---
* Generic LED Models
.model D_RED D(IS=1e-18 N=2 RS=10 BV=5)
.model D_GREEN D(IS=1e-18 N=2.5 RS=10 BV=5)

* --- Simulation Directives ---
.op
* Transient analysis: 1ms step, 500ms duration (2.5 cycles at 5Hz)
.tran 1m 500m

* --- Output ---
* Monitoring the induced voltage at COIL_A
.print tran V(COIL_A) I(L1)

Simulation Results (Transient Analysis)

Analysis: The transient analysis shows an AC voltage at COIL_A oscillating between approx +2.6V and -2.8V at 5Hz. Current flows through L1, peaking around 66mA. The voltage levels are sufficient to forward bias the LEDs (D_RED and D_GREEN) alternately, consistent with the intended indication of positive and negative phases.

Show raw data table (522 rows)

Index   time            v(coil_a)       l1#branch
0	0.000000e+00	4.375392e-35	-8.75078e-36
1	1.000000e-05	9.424778e-04	1.884985e-15
2	2.000000e-05	1.884955e-03	3.769970e-15
3	4.000000e-05	3.769910e-03	7.539938e-15
4	8.000000e-05	7.539814e-03	1.507987e-14
5	1.600000e-04	1.507958e-02	3.015936e-14
6	3.200000e-04	3.015878e-02	6.031856e-14
7	6.400000e-04	6.031451e-02	1.206316e-13
8	1.280000e-03	1.206046e-01	2.412214e-13
9	2.280000e-03	2.147012e-01	4.294658e-13
10	3.280000e-03	3.085859e-01	6.175653e-13
11	4.280000e-03	4.021661e-01	8.067202e-13
12	5.280000e-03	4.953494e-01	1.005111e-12
13	6.280000e-03	5.880438e-01	1.262566e-12
14	7.280000e-03	6.801579e-01	1.873422e-12
15	8.280000e-03	7.716008e-01	4.548512e-12
16	9.280000e-03	8.622822e-01	1.907006e-11
17	1.028000e-02	9.521126e-01	1.003825e-10
18	1.128000e-02	1.041003e+00	5.511221e-10
19	1.228000e-02	1.128867e+00	3.003086e-09
20	1.328000e-02	1.215616e+00	1.605415e-08
21	1.428000e-02	1.301164e+00	8.389370e-08
22	1.528000e-02	1.385424e+00	4.276266e-07
23	1.628000e-02	1.468291e+00	2.121308e-06
... (498 more rows) ...

Common mistakes and how to avoid them

1. Using weak magnets: Standard black ferrite magnets are often too weak to generate visible voltage on an LED. Solution: Use rare-earth Neodymium magnets.
2. Moving too slowly: Faraday’s Law (V = – N · d\Phi / dt) depends on the rate of change. Solution: Move the magnet as quickly as possible to maximize the voltage spike.
3. Insulation issues: Enameled wire has a clear coating that blocks electricity. Solution: Ensure the ends of the coil wire are sanded or scraped down to bare copper before connecting to the LEDs or multimeter.

Troubleshooting

• Symptom: Multimeter shows voltage, but LEDs never light up.
  • Cause: The induced voltage is lower than the LED forward voltage threshold (~1.8 V).
  • Fix: Add more turns to the coil (increase $N$) or move the magnet faster.
• Symptom: No reading on the multimeter even with fast movement.
  • Cause: Open circuit or poor connection at the coil tips.
  • Fix: Check continuity (resistance mode) across the coil terminals; it should read a few Ohms, not infinite.
• Symptom: Voltage reading is erratic or hard to see.
  • Cause: Digital multimeters have a slow sample rate.
  • Fix: Use the «Max/Min» hold function if available, or use an analog (needle) multimeter which responds better to transient pulses.

Possible improvements and extensions

1. Shake Flashlight: Add a bridge rectifier (4 diodes) and a large capacitor (e.g., 1000 µF) to store the energy generated by shaking the magnet, allowing the LED to stay lit for a few seconds after motion stops.
2. Core Comparison: Insert an iron bolt inside the coil (making it an iron-core inductor) and move a magnet near the head of the bolt to observe how the ferromagnetic core concentrates the magnetic flux and affects induction.

More Practical Cases on Prometeo.blog

Carlos Núñez Zorrilla

Electronics & Computer Engineer

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

Follow me:

Who we are

Level: Basic – Observe how an inductor filters high frequencies in an RL series circuit.

Objective and use case

In this practical exercise, you will build a passive RL low-pass filter using a series inductor and a shunt resistor. This circuit demonstrates the inductive reactance property, where impedance increases with frequency, effectively blocking high-frequency signals while allowing low-frequency signals to pass through to the output.

Why it is useful:
* Audio Electronics: Used in crossover networks to direct low frequencies (bass) to woofers while blocking treble.
* Power Supplies: Essential for smoothing output currents and reducing ripple in DC/DC converters.
* Noise Suppression: Filters out high-frequency interference (EMI) on signal lines.
* Signal Conditioning: Removes high-frequency noise from sensor data before processing.

Expected outcome:
* Low Frequency Input (< Cutoff): The output amplitude (VOUT) is approximately equal to the input amplitude (VIN).
* Cutoff Frequency (fc): The output amplitude drops to roughly 70.7% of the input amplitude (-3dB point).
* High Frequency Input (> Cutoff): The output amplitude is significantly attenuated (reduced).
* Target audience: Basic electronics students and hobbyists exploring AC circuit theory.

Materials

• V1: Function Generator (Sine wave source), function: AC signal injection
• L1: 10 mH inductor, function: series reactive element (impedance increases with frequency)
• R1: 100 Ω resistor, function: load/shunt resistor (output taken here)
• Scope: Dual-channel Oscilloscope, function: visual comparison of Input vs. Output

Wiring guide

Construct the circuit using the following node connections. The output voltage is measured across the resistor.

• V1 (Signal Source): Connects between node VIN (Positive) and node 0 (GND).
• L1: Connects between node VIN and node VOUT.
• R1: Connects between node VOUT and node 0 (GND).
• Oscilloscope Channel 1: Connect probe tip to VIN and ground clip to 0.
• Oscilloscope Channel 2: Connect probe tip to VOUT and ground clip to 0.

Conceptual block diagram

Schematic

[ V1: Func Gen ] --(Node VIN)--> [ L1: 10mH ] --(Node VOUT)--> [ R1: 100 Ω ] --> GND (0)
       |                        (Series Inductor)      |          (Load)
       |                                               |
       +--------(Probe)-------> [ Scope CH1 ]          +--------(Probe)-------> [ Scope CH2 ]

Electrical diagram

🔒 This electrical diagram is premium. With the 7-day pass or the monthly membership you can unlock the complete didactic material and the print-ready PDF pack.🔓 See premium access plans

Measurements and tests

Follow these steps to validate the frequency response of the filter.

1. Setup: Configure the Function Generator (V1) to output a Sine Wave with 5 Vpp amplitude.
2. Low Frequency Test (Pass Band):
   • Set V1 frequency to 100 Hz.
   • Observe Channel 1 (Input) and Channel 2 (Output) on the oscilloscope.
   • Result: The output wave (VOUT) should be almost identical in amplitude to the input (VIN).
3. Cutoff Frequency Test (fc):
   • Calculate the theoretical cutoff: fc = (R / (2\pi L)) ≈ (100 / (2\pi × 0.01)) ≈ 1.59 kHz.
   • Set V1 frequency to 1.6 kHz.
   • Result: VOUT should be approximately 3.5 Vpp (roughly 0.707 × 5 Vpp). You will also notice a phase lag of -45°.
4. High Frequency Test (Stop Band):
   • Set V1 frequency to 50 kHz.
   • Result: The output wave (VOUT) should be very small (highly attenuated) compared to the input.

SPICE netlist and simulation

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

* Practical case: Simple RL Low-Pass Filter
.width out=256

* --- Component Definitions ---

* V1: Function Generator (Sine wave source)
* Wiring: Connects between node VIN (Positive) and node 0 (GND)
* Configuration: Sine wave, 0V offset, 5V amplitude, 2kHz frequency
* (Note: Cutoff frequency fc = R/(2*pi*L) approx 1.6kHz. 2kHz chosen to show attenuation)
V1 VIN 0 SIN(0 5 2k)

* L1: 10 mH inductor
* Wiring: Connects between node VIN and node VOUT
L1 VIN VOUT 10m

* R1: 100 Ohm resistor
* Wiring: Connects between node VOUT and node 0 (GND)
R1 VOUT 0 100

* --- Analysis Commands ---
* ... (truncated in public view) ...

Copy this content into a .cir file and run with ngspice.

🔒 Part of this section is premium. With the 7-day pass or the monthly membership you can access the full content (materials, wiring, detailed build, validation, troubleshooting, variants and checklist) and download the complete print-ready PDF pack.

🔓 See premium access plans

* Practical case: Simple RL Low-Pass Filter
.width out=256

* --- Component Definitions ---

* V1: Function Generator (Sine wave source)
* Wiring: Connects between node VIN (Positive) and node 0 (GND)
* Configuration: Sine wave, 0V offset, 5V amplitude, 2kHz frequency
* (Note: Cutoff frequency fc = R/(2*pi*L) approx 1.6kHz. 2kHz chosen to show attenuation)
V1 VIN 0 SIN(0 5 2k)

* L1: 10 mH inductor
* Wiring: Connects between node VIN and node VOUT
L1 VIN VOUT 10m

* R1: 100 Ohm resistor
* Wiring: Connects between node VOUT and node 0 (GND)
R1 VOUT 0 100

* --- Analysis Commands ---

* Transient Analysis
* Step size: 1us
* Stop time: 2ms (sufficient to capture several cycles at 2kHz)
.tran 1u 2m

* Operating Point Analysis (DC check)
.op

* --- Output Directives ---

* Print Input (VIN) and Output (VOUT) voltages for simulation logging
* Scope Channel 1: VIN
* Scope Channel 2: VOUT
.print tran V(VIN) V(VOUT) I(L1)

.end

Simulation Results (Transient Analysis)

Analysis: The simulation shows a sinusoidal input (VIN) and a sinusoidal output (VOUT). At 2kHz, the output amplitude (approx 3V peak) is attenuated relative to the input (5V peak) and phase-shifted, consistent with RL low-pass filter behavior near its cutoff frequency.

Show raw data table (2012 rows)

Index   time            v(vin)          v(vout)         l1#branch
0	0.000000e+00	0.000000e+00	0.000000e+00	0.000000e+00
1	1.000000e-08	6.283185e-04	6.282557e-08	6.282557e-10
2	1.084006e-08	6.811008e-04	6.854662e-08	6.854662e-10
3	1.252017e-08	7.866654e-04	8.087543e-08	8.087543e-10
4	1.588039e-08	9.977945e-04	1.108531e-07	1.108531e-09
5	2.260084e-08	1.420053e-03	1.920880e-07	1.920880e-09
6	3.604174e-08	2.264569e-03	4.396687e-07	4.396687e-09
7	6.292353e-08	3.953601e-03	1.275216e-06	1.275216e-08
8	1.166871e-07	7.331665e-03	4.307397e-06	4.307397e-08
9	2.242143e-07	1.408778e-02	1.581244e-05	1.581244e-07
10	4.392686e-07	2.759992e-02	6.055593e-05	6.055593e-07
11	8.693773e-07	5.462350e-02	2.367416e-04	2.367416e-06
12	1.729595e-06	1.086651e-01	9.340244e-04	9.340244e-06
13	2.729595e-06	1.714719e-01	2.318447e-03	2.318447e-05
14	3.729595e-06	2.342516e-01	4.313902e-03	4.313902e-05
15	4.729595e-06	2.969943e-01	6.913992e-03	6.913992e-05
16	5.729595e-06	3.596901e-01	1.011228e-02	1.011228e-04
17	6.729595e-06	4.223291e-01	1.390231e-02	1.390231e-04
18	7.729595e-06	4.849014e-01	1.827756e-02	1.827756e-04
19	8.729595e-06	5.473972e-01	2.323151e-02	2.323151e-04
20	9.729595e-06	6.098065e-01	2.875758e-02	2.875758e-04
21	1.072959e-05	6.721195e-01	3.484918e-02	3.484918e-04
22	1.172959e-05	7.343264e-01	4.149966e-02	4.149966e-04
23	1.272959e-05	7.964173e-01	4.870237e-02	4.870237e-04
... (1988 more rows) ...

Common mistakes and how to avoid them

1. Measuring across the Inductor: If you measure voltage across L1 instead of R1, you create a High-Pass filter (passing high frequencies). Solution: Ensure the oscilloscope probe monitors the node between L1 and R1 relative to Ground.
2. Using DC Input: An inductor acts as a short circuit in DC (after the transient). Solution: Ensure the function generator is set to AC (Sine Wave) to observe reactance effects.
3. Inductor Saturation: Using a very small inductor core with high current can saturate the magnetic field, distorting the waveform. Solution: Use an appropriate inductor or keep signal current within the component’s rating.

Troubleshooting

• Symptom: VOUT is zero at all frequencies.
  • Cause: Open circuit in the wiring or broken inductor wire.
  • Fix: Check continuity of L1 and connections at VIN and VOUT.
• Symptom: VOUT equals VIN at all frequencies.
  • Cause: The inductor L1 is shorted or R1 is disconnected (open).
  • Fix: Measure the resistance of L1 (should be non-zero but low) and ensure R1 is properly grounded.
• Symptom: No attenuation observed at 50 kHz.
  • Cause: Inductor value is too small or Resistor value is too large (cutoff frequency is too high).
  • Fix: Verify component values. Try increasing L1 or decreasing R1 to lower the cutoff frequency.

Possible improvements and extensions

1. Bode Plotting: Manually record the amplitude of VOUT at 10 different frequencies from 100 Hz to 100 kHz and plot the results on semi-log graph paper to visualize the -20dB/decade roll-off.
2. Second Order Filter: Add a capacitor in parallel with R1 to create an RLC low-pass filter, creating a steeper roll-off (-40dB/decade) and potentially introducing resonance.

More Practical Cases on Prometeo.blog

Carlos Núñez Zorrilla

Electronics & Computer Engineer

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

Follow me:

Who we are

Level: Basic. Observe the delay in lamp activation due to self-induction.

Objective and use case

In this session, you will build a circuit that demonstrates how an inductor opposes rapid changes in current flow. By placing a large inductor in series with a lamp (with a parallel bypass resistor), you will create a visual «soft-start» effect where the light starts dim and gradually brightens.

Why it is useful:
* Inrush Current Limiting: Used in power supplies and large motors to prevent blown fuses when devices are first turned on.
* Soft-Start Circuits: Protects delicate filaments and components from thermal shock.
* Filtering: Smoothes out noise and ripples in DC power lines.

Expected outcome:
* When the switch is closed, the lamp will turn on immediately but dimly.
* Over a short period (0.5 to 2 seconds, depending on the inductance), the lamp will become fully bright.
* This visualizes the inductor initially acting as an «open circuit» (blocking current) and transitioning to a «short circuit» (allowing full flow).
* Target audience: Basic electronics students and hobbyists.

Materials

• V1: 12 V DC power supply or battery.
• S1: SPST mechanical switch (toggle or push-button).
• L1: 1 H to 2 H iron-core inductor, function: creates opposition to current change (e.g., a transformer primary winding used as a choke).
• R1: 220 Ω resistor (1 Watt or higher), function: bypass path for visual contrast.
• X1: 12 V / 100 mA incandescent lamp (small bulb), function: visual output load.

Wiring guide

Construct the circuit using the following connections. The node names (e.g., VCC, SW_OUT) help identify the electrical points.

• V1 (DC Source): Connect the positive terminal to VCC and the negative terminal to 0 (GND).
• S1 (Switch): Connect between VCC and node SW_OUT.
• L1 (Inductor): Connect between node SW_OUT and node LAMP_IN.
• R1 (Resistor): Connect between node SW_OUT and node LAMP_IN (this places R1 in parallel with L1).
• X1 (Lamp): Connect between node LAMP_IN and 0 (GND).

Conceptual block diagram

Schematic

(Node: SW_OUT)          (Node: LAMP_IN)
                                              /--> [ L1: Inductor ] --\
[ V1: 12 V Source ] --(VCC)--> [ S1: Switch ] --                        --> [ X1: Lamp ] --> GND
                                              \--> [ R1: Resistor ] --/

Electrical diagram

🔒 This electrical diagram is premium. With the 7-day pass or the monthly membership you can unlock the complete didactic material and the print-ready PDF pack.🔓 See premium access plans

Measurements and tests

Follow these steps to validate the phenomenon:

1. Initial State: Ensure the switch S1 is open. The lamp X1 should be off.
2. Observation: Keep your eyes on the lamp X1.
3. Action: Close switch S1.
4. Visual Validation:
   • Phase 1 (Instant): The lamp lights up at roughly 30–50% brightness. (Current is flowing through R1, as L1 opposes the sudden change).
   • Phase 2 (Delay): The lamp brightness ramps up smoothly to 100%. (As the magnetic field in L1 stabilizes, it allows full current to pass, bypassing R1).
5. Voltage Measurement (Optional): If you have a multimeter, place probes across the Inductor (SW_OUT to LAMP_IN).
   • At the moment of contact, voltage is high (approx 6–8 V).
   • After 1–2 seconds, voltage drops to near 0 V.

SPICE netlist and simulation

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

* Title: Practical case: Opposition to DC current change
.width out=256
* Description: Demonstrates inductive opposition to current change (dim-to-bright lamp effect)

* --- Power Supply ---
* 12V DC Supply
V1 VCC 0 DC 12

* --- User Interface (Switch Control) ---
* Generates a control pulse to simulate pressing the button.
* Button Press: Starts at 10ms, Duration 300ms.
V_BTN_CTRL CTRL 0 PULSE(0 5 10m 1u 1u 300m 600m)

* --- Components ---

* S1: SPST Mechanical Switch
* Connected between VCC and SW_OUT.
* Modeled as a voltage-controlled switch driven by the control pulse.
S1 VCC SW_OUT CTRL 0 SW_IDEAL

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

Copy this content into a .cir file and run with ngspice.

🔒 Part of this section is premium. With the 7-day pass or the monthly membership you can access the full content (materials, wiring, detailed build, validation, troubleshooting, variants and checklist) and download the complete print-ready PDF pack.

🔓 See premium access plans

* Title: Practical case: Opposition to DC current change
.width out=256
* Description: Demonstrates inductive opposition to current change (dim-to-bright lamp effect)

* --- Power Supply ---
* 12V DC Supply
V1 VCC 0 DC 12

* --- User Interface (Switch Control) ---
* Generates a control pulse to simulate pressing the button.
* Button Press: Starts at 10ms, Duration 300ms.
V_BTN_CTRL CTRL 0 PULSE(0 5 10m 1u 1u 300m 600m)

* --- Components ---

* S1: SPST Mechanical Switch
* Connected between VCC and SW_OUT.
* Modeled as a voltage-controlled switch driven by the control pulse.
S1 VCC SW_OUT CTRL 0 SW_IDEAL

* L1: 1.5H Iron-core Inductor
* Creates opposition to current change.
* Connected between SW_OUT and LAMP_IN.
L1 SW_OUT LAMP_IN 1.5

* R1: 220 Ohm Resistor
* Bypass path for visual contrast (parallel to L1).
* Connected between SW_OUT and LAMP_IN.
R1 SW_OUT LAMP_IN 220

* X1: 12V / 100mA Incandescent Lamp
* Modeled as a resistor: R = V / I = 12 / 0.1 = 120 Ohms.
* Connected between LAMP_IN and 0 (GND).
R_X1 LAMP_IN 0 120

* --- Models ---
* Ideal switch model: Low resistance when ON, High when OFF.
.model SW_IDEAL sw(vt=2.5 ron=0.01 roff=100Meg)

* --- Simulation Setup ---
* Transient analysis to capture the inductive time constant (approx 20ms).
* Simulation time: 500ms to allow full settling.
.op
.tran 1m 500m

* --- Output Directives ---
* V(SW_OUT): Input voltage to the LR network (Switch Output).
* V(LAMP_IN): Voltage across the Lamp (Visual Output).
.print tran V(SW_OUT) V(LAMP_IN) I(L1)

.end

Simulation Results (Transient Analysis)

Analysis: The simulation shows the switch closing at 10ms (Index 26), causing V(SW_OUT) to jump to ~12V. V(LAMP_IN) rises to ~4.2V initially due to the inductive kick/impedance, then settles. The current I(L1) is initially very low and rises, demonstrating the inductive opposition to current change.

Show raw data table (564 rows)

Index   time            v(sw_out)       v(lamp_in)      l1#branch
0	0.000000e+00	1.439998e-05	1.439998e-05	1.199999e-07
1	1.000000e-05	1.439998e-05	1.439998e-05	1.199999e-07
2	2.000000e-05	1.439998e-05	1.439998e-05	1.199999e-07
3	4.000000e-05	1.439998e-05	1.439998e-05	1.199999e-07
4	8.000000e-05	1.439998e-05	1.439998e-05	1.199999e-07
5	1.600000e-04	1.439998e-05	1.439998e-05	1.199999e-07
6	3.200000e-04	1.439998e-05	1.439998e-05	1.199999e-07
7	6.400000e-04	1.439998e-05	1.439998e-05	1.199999e-07
8	1.280000e-03	1.439998e-05	1.439998e-05	1.199999e-07
9	2.280000e-03	1.439998e-05	1.439998e-05	1.199999e-07
10	3.280000e-03	1.439998e-05	1.439998e-05	1.199999e-07
11	4.280000e-03	1.439998e-05	1.439998e-05	1.199999e-07
12	5.280000e-03	1.439998e-05	1.439998e-05	1.199999e-07
13	6.280000e-03	1.439998e-05	1.439998e-05	1.199999e-07
14	7.280000e-03	1.439998e-05	1.439998e-05	1.199999e-07
15	8.280000e-03	1.439998e-05	1.439998e-05	1.199999e-07
16	9.280000e-03	1.439998e-05	1.439998e-05	1.199999e-07
17	1.000000e-02	1.439998e-05	1.439998e-05	1.199999e-07
18	1.000010e-02	1.439998e-05	1.439998e-05	1.199999e-07
19	1.000026e-02	1.439998e-05	1.439998e-05	1.199999e-07
20	1.000031e-02	1.439998e-05	1.439998e-05	1.199999e-07
21	1.000039e-02	1.439998e-05	1.439998e-05	1.199999e-07
22	1.000041e-02	1.439998e-05	1.439998e-05	1.199999e-07
23	1.000045e-02	1.439998e-05	1.439998e-05	1.199999e-07
... (540 more rows) ...

Common mistakes and how to avoid them

1. Using an LED instead of an incandescent lamp: LEDs respond too quickly and have non-linear resistance, making the «ramp up» effect very hard to see. Solution: Always use an incandescent bulb or a coil-based relay for this demo.
2. Inductor value too small: If you use a small air-core inductor (e.g., 100 µH), the delay will be microseconds, invisible to the eye. Solution: Use a large iron-core inductor, such as the primary coil of a mains transformer (ensure it is rated for the DC current).
3. Omitting the parallel resistor: Without R1, the lamp might simply stay off for a split second and then pop on, which can look like a switch bounce rather than a smooth transition. Solution: R1 provides an immediate «dim» reference state, making the transition to «bright» much more obvious.

Troubleshooting

• Lamp lights fully bright instantly: The inductor value is too low, or the inductor is shorted. Check if you are using an air-core coil; switch to an iron-core one.
• Lamp never gets fully bright: The inductor might have a very high internal DC resistance (thin wire). Measure the resistance of the inductor coil; if it is comparable to the resistor R1, the current will never fully bypass the resistor.
• Sparks at the switch when turning off: Inductors generate back-EMF voltage when the circuit breaks. R1 acts as a snubber here, but if sparks persist, ensure your switch is rated for inductive loads.

Possible improvements and extensions

1. Oscilloscope Visualization: Connect channel 1 of an oscilloscope across the Lamp. You will see an exponential curve rising, allowing you to calculate the Time Constant (\tau = L / R).
2. Variable Delay: Replace R1 with a potentiometer and experiment with how changing the parallel resistance affects the initial «dim» brightness and the perceived transition speed.

More Practical Cases on Prometeo.blog

Carlos Núñez Zorrilla

Electronics & Computer Engineer

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

Follow me:

Who we are

Level: Basic – Demonstrate the relationship between current and magnetic field using an iron core.

Objective and use case

In this experiment, you will build a functional electromagnet by winding insulated copper wire around a ferromagnetic core (iron nail or bolt) and powering it with a DC source.

• Why it is useful:
  • Electromechanical Relays: Used to switch high-voltage circuits using low-voltage signals.
  • Electric Motors: Fundamental principle for converting electrical energy into mechanical motion.
  • Solenoids: Used in electronic door locks, valves, and automotive starters.
  • Industrial Lifting: Large electromagnets used to lift scrap metal in junkyards.
• Expected outcome:
  • When the switch is open, the core exhibits no magnetic properties; iron filings or paperclips remain on the table.
  • When the switch is closed, current flows through the coil, generating a magnetic field.
  • The iron core concentrates the magnetic flux, allowing the device to lift small metallic objects (paperclips, washers).
  • Releasing the switch stops the current, causing the objects to drop immediately.
• Target audience: Students and hobbyists learning basic electromagnetism.

Materials

• V1: 4.5 V DC Battery pack (3x AA batteries), function: energy source.
• S1: Momentary Push-button Switch (NO), function: current control.
• L1: Solenoid Coil (approx. 50-100 turns of enameled copper wire), function: generates magnetic field.
• CORE: Large Iron Nail or Bolt (Soft Iron), function: magnetic core for L1.
• R1: 1 Ω Power Resistor (5W) or similar, function: current limiting (optional but recommended to protect battery).
• X1: Iron filings or small steel paperclips, function: test load to visualize attraction.

Wiring guide

• V1 (Positive): Connects to node VCC.
• V1 (Negative): Connects to node 0 (GND).
• S1: Connects between node VCC and node SW_OUT.
• R1: Connects between node SW_OUT and node COIL_IN.
• L1: Connects between node COIL_IN and node 0 (GND).
  • Note: The wire for L1 must be physically wrapped tightly around the CORE.

Conceptual block diagram

Schematic

[ V1: 4.5 V Battery ] --(VCC)--> [ S1: Push Button ] --(SW_OUT)--> [ R1: 1 Ω Resistor ] --(COIL_IN)--> [ L1: Coil + Iron Core ] --> GND
                                                                                                                |
                                                                                                         (Magnetic Field)
                                                                                                                |
                                                                                                                V
                                                                                                       [ X1: Paperclips ]

Electrical diagram

🔒 This electrical diagram is premium. With the 7-day pass or the monthly membership you can unlock the complete didactic material and the print-ready PDF pack.🔓 See premium access plans

Measurements and tests

1. Baseline Check: Before connecting the battery, place the CORE (with the wire wrapped around it) near the iron filings (X1). Confirm there is no attraction.
2. Activation: Press and hold S1 to close the circuit.
3. Observation: While holding S1, move the tip of the CORE near the iron filings or paperclips.
4. Verification: Observe that the metal objects stick to the CORE.
5. Deactivation: Release S1. The current stops flowing, the magnetic field collapses, and the objects should fall off.
6. Current Check (Optional): Connect a multimeter in series between S1 and R1 to measure the current flow (Amps) during activation.

SPICE netlist and simulation

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

* Practical case: The coil as a simple electromagnet
.width out=256

* --- Power Source ---
* V1: 4.5 V DC Battery pack (3x AA batteries)
V1 VCC 0 DC 4.5

* --- Control Signal for Switch S1 ---
* Simulates the user pressing the button (S1).
* Logic: 0V (Released) -> 5V (Pressed).
* Timing: Press at 1ms, hold for 50ms, release.
V_S1_CTRL S1_GATE 0 PULSE(0 5 1m 1u 1u 50m 100m)

* --- Circuit Components ---

* S1: Momentary Push-button Switch (NO)
* Function: Connects VCC to SW_OUT when S1_GATE is High.
S1 VCC SW_OUT S1_GATE 0 SW_MODEL

* R1: 1 Ohm Power Resistor
* ... (truncated in public view) ...

Copy this content into a .cir file and run with ngspice.

🔒 Part of this section is premium. With the 7-day pass or the monthly membership you can access the full content (materials, wiring, detailed build, validation, troubleshooting, variants and checklist) and download the complete print-ready PDF pack.

🔓 See premium access plans

* Practical case: The coil as a simple electromagnet
.width out=256

* --- Power Source ---
* V1: 4.5 V DC Battery pack (3x AA batteries)
V1 VCC 0 DC 4.5

* --- Control Signal for Switch S1 ---
* Simulates the user pressing the button (S1).
* Logic: 0V (Released) -> 5V (Pressed).
* Timing: Press at 1ms, hold for 50ms, release.
V_S1_CTRL S1_GATE 0 PULSE(0 5 1m 1u 1u 50m 100m)

* --- Circuit Components ---

* S1: Momentary Push-button Switch (NO)
* Function: Connects VCC to SW_OUT when S1_GATE is High.
S1 VCC SW_OUT S1_GATE 0 SW_MODEL

* R1: 1 Ohm Power Resistor
* Function: Current limiting between Switch and Coil.
R1 SW_OUT COIL_IN 1

* L1: Solenoid Coil (approx 50-100 turns on Soft Iron Core)
* Function: Generates magnetic field.
* Value: 5mH (Estimated for described coil).
L1 COIL_IN 0 5m

* D1: Flyback Diode (Added per review)
* Function: Protects S1 by clamping inductive kickback when switch opens.
* Connection: Anode to GND (0), Cathode to COIL_IN.
D1 0 COIL_IN D_1N4007

* --- Models ---
* Switch Model: Low resistance ON, High resistance OFF.
.model SW_MODEL sw (vt=2.5 vh=0.2 ron=0.05 roff=100Meg)

* Diode Model: Standard Silicon Rectifier (1N4007).
.model D_1N4007 D (IS=2.5n RS=0.04 N=1.7 BV=1000 IBV=5u)

* --- Analysis ---
* Transient analysis for 100ms to capture energizing and de-energizing.
.tran 10u 100m
.op

* --- Output Directives ---
* V(S1_GATE): Input Control
* V(COIL_IN): Output Voltage at Coil
* V(SW_OUT): Voltage after Switch
* I(L1): Current through Coil (Magnetic Field Strength)
.print tran V(S1_GATE) V(COIL_IN) V(SW_OUT) I(L1)

.end

Simulation Results (Transient Analysis)

Analysis: The provided log data only covers the initial OFF state (0s) and the final OFF state (100ms). The signals are effectively zero (nano-amps range), confirming the circuit returns to rest, although there is some negligible numerical ringing (+/- 80mV) at the coil input in the final steps.

Show raw data table (10053 rows)

Index   time            v(s1_gate)      v(coil_in)      v(sw_out)       l1#branch
0	0.000000e+00	0.000000e+00	0.000000e+00	4.500000e-08	4.500000e-08
1	1.000000e-07	0.000000e+00	-1.58289e-19	4.500000e-08	4.500000e-08
2	2.000000e-07	0.000000e+00	-1.58289e-19	4.500000e-08	4.500000e-08
3	4.000000e-07	0.000000e+00	-1.58289e-19	4.500000e-08	4.500000e-08
4	8.000000e-07	0.000000e+00	-2.44581e-19	4.500000e-08	4.500000e-08
5	1.600000e-06	0.000000e+00	3.684064e-19	4.500000e-08	4.500000e-08
6	3.200000e-06	0.000000e+00	-3.03688e-19	4.500000e-08	4.500000e-08
7	6.400000e-06	0.000000e+00	2.882625e-19	4.500000e-08	4.500000e-08
8	1.280000e-05	0.000000e+00	-3.16655e-19	4.500000e-08	4.500000e-08
9	2.280000e-05	0.000000e+00	2.975540e-19	4.500000e-08	4.500000e-08
10	3.280000e-05	0.000000e+00	-3.05533e-19	4.500000e-08	4.500000e-08
11	4.280000e-05	0.000000e+00	2.975540e-19	4.500000e-08	4.500000e-08
12	5.280000e-05	0.000000e+00	-3.05533e-19	4.500000e-08	4.500000e-08
13	6.280000e-05	0.000000e+00	2.975540e-19	4.500000e-08	4.500000e-08
14	7.280000e-05	0.000000e+00	-3.05533e-19	4.500000e-08	4.500000e-08
15	8.280000e-05	0.000000e+00	2.975540e-19	4.500000e-08	4.500000e-08
16	9.280000e-05	0.000000e+00	-3.05533e-19	4.500000e-08	4.500000e-08
17	1.028000e-04	0.000000e+00	2.975540e-19	4.500000e-08	4.500000e-08
18	1.128000e-04	0.000000e+00	-3.05533e-19	4.500000e-08	4.500000e-08
19	1.228000e-04	0.000000e+00	2.975540e-19	4.500000e-08	4.500000e-08
20	1.328000e-04	0.000000e+00	-3.05533e-19	4.500000e-08	4.500000e-08
21	1.428000e-04	0.000000e+00	2.975540e-19	4.500000e-08	4.500000e-08
22	1.528000e-04	0.000000e+00	-3.05533e-19	4.500000e-08	4.500000e-08
23	1.628000e-04	0.000000e+00	2.975540e-19	4.500000e-08	4.500000e-08
... (10029 more rows) ...

Common mistakes and how to avoid them

1. Overheating the battery/wire: Creating a coil with very low resistance (short wire) draws excessive current. Solution: Use a longer wire (more turns) or include the limiting resistor R1.
2. Using a non-magnetic core: Wrapping wire around aluminum, plastic, or wood. Solution: Ensure the core is ferromagnetic (iron or steel) to concentrate the magnetic field lines.
3. Leaving the switch closed too long: This drains the battery rapidly and heats the coil. Solution: Use a momentary push-button and only pulse the power for short tests.

Troubleshooting

• Symptom: No magnetic attraction when switch is pressed.
  • Cause: Dead battery or broken circuit connection (enamel insulation not stripped at connection points).
  • Fix: Check battery voltage; ensure the ends of the magnet wire are sanded down to bare copper before connecting to the circuit.
• Symptom: Very weak magnetic pull.
  • Cause: Too few turns on the coil or low current.
  • Fix: Add more turns of wire around the nail; ensure windings are tight and neat.
• Symptom: Wire gets extremely hot immediately.
  • Cause: Short circuit condition (resistance too low).
  • Fix: Add the series resistor R1 or increase the length of the wire used for L1.

Possible improvements and extensions

1. Variable Strength: Add a potentiometer (rheostat) in series to vary the current and observe how the lifting capacity changes (number of paperclips lifted).
2. Core Comparison: Replace the iron nail with an air core (remove the nail) or a brass rod to demonstrate the importance of permeability in electromagnets.

More Practical Cases on Prometeo.blog

Carlos Núñez Zorrilla

Electronics & Computer Engineer

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

Follow me:

Who we are

Level: Basic. Compare switching efficiency and drive requirements of BJT and MOSFET transistors.

Objective and use case

You will build two parallel switching circuits using a BJT (Bipolar Junction Transistor) and a MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor) to drive identical LED loads. By measuring input currents and output voltage drops, you will observe the fundamental differences in how these devices control power.

Why it is useful:
* Efficiency: Understanding which transistor dissipates less power (heat) in a specific application.
* Microcontroller interfacing: Learning which device connects directly to logic pins without loading the processor.
* Drive requirements: Distinguishing between current-controlled devices (BJT) and voltage-controlled devices (MOSFET).
* Component selection: Making informed decisions for motor drivers, relay controls, and high-power switching.

Expected outcome:
* Input Current: The BJT will draw measurable current into its Base, while the MOSFET Gate current will be near zero.
* Voltage Drop: You will measure different voltage drops (VCE vs VDS) across the transistors when ON.
* LED Action: Both LEDs will light up, visually confirming the switching action.

Target audience and level:
Students and hobbyists learning component characteristics.

Materials

• V1: 5 V DC supply, function: Main power source.
• S1: SPST toggle switch, function: Input control signal.
• Q1: 2N2222 NPN Transistor, function: Current-controlled switch.
• M1: 2N7000 N-Channel MOSFET, function: Voltage-controlled switch.
• R1: 1 kΩ resistor, function: Current limiting for BJT Base.
• R2: 10 kΩ resistor, function: Pull-down for switch signal.
• R3: 330 Ω resistor, function: Current limiting for BJT load (LED).
• R4: 330 Ω resistor, function: Current limiting for MOSFET load (LED).
• D1: Red LED, function: Load indicator for BJT.
• D2: Green LED, function: Load indicator for MOSFET.

Wiring guide

Construct the circuit following these connections using the node names provided.

Control Signal Section:
* S1 connects between node VCC and node CTRL.
* R2 connects between node CTRL and node 0 (GND).

BJT Circuit (Current Controlled):
* R1 connects between node CTRL and node B_BASE.
* Q1 Base connects to node B_BASE.
* Q1 Emitter connects to node 0.
* Q1 Collector connects to node B_COLL.
* D1 Anode connects to node VCC.
* D1 Cathode connects to node D1_K.
* R3 connects between node D1_K and node B_COLL.

MOSFET Circuit (Voltage Controlled):
* M1 Gate connects directly to node CTRL.
* M1 Source connects to node 0.
* M1 Drain connects to node M_DRAIN.
* D2 Anode connects to node VCC.
* D2 Cathode connects to node D2_K.
* R4 connects between node D2_K and node M_DRAIN.

Conceptual block diagram

Schematic

+-------------------------------------------------------------------------+
|               PRACTICAL CASE: COMPARING BJT AND MOSFET SWITCHES         |
+-------------------------------------------------------------------------+

1. CONTROL SIGNAL GENERATION
   (Creates the "CTRL" signal used by both circuits below)

   VCC (5 V) --> [ S1: Switch ] --+--(Node: CTRL)
                                 |
                                 +--> [ R2: 10k Pull-Down ] --> GND


2. BJT CIRCUIT (Current Controlled)
   (Requires Base Resistor R1 for current limiting)

   [ Node: CTRL ] --(Signal)--> [ R1: 1k ] --(I_Base)--> [ Q1: Base ]
                                                             |
                                                         (Controls)
                                                             |
                                                             v
   VCC --> [ D1: Red LED ] --> [ R3: 330 ] --> [ Q1: Collector ]
                                                             |
                                                         (Switch)
                                                             |
                                                             +--> [ Q1: Emitter ] --> GND


3. MOSFET CIRCUIT (Voltage Controlled)
   (Gate connects directly; controlled by Voltage Field)

   [ Node: CTRL ] --(Voltage)--------------------------> [ M1: Gate ]
                                                             |
                                                         (Controls)
                                                             |
                                                             v
   VCC --> [ D2: Grn LED ] --> [ R4: 330 ] --> [ M1: Drain ]
                                                             |
                                                         (Switch)
                                                             |
                                                             +--> [ M1: Source ] --> GND

Measurements and tests

Perform the following steps to validate the differences between the transistors.

1. Switch ON: Close switch S1 to apply 5 V to the control node. Ensure both D1 (Red) and D2 (Green) turn on.
2. Test 1: Input Current (Current Gain vs. Field Effect):
   • Measure the voltage across R1 (1 kΩ). Use Ohm’s Law ($I = V/R$) to calculate the Base current (IB) flowing into Q1.
   • Result: You should calculate approximately 4.3 mA.
   • Try to measure current flowing into the Gate of M1.
   • Result: It should be effectively 0 mA (typically nano-amps), proving the MOSFET is voltage-controlled.
3. Test 2: Switching Efficiency (Voltage Drop):
   • Measure the voltage from Q1 Collector to Emitter (VCE).
   • Result: Expect a drop of roughly 0.1 V to 0.2 V (Saturation voltage).
   • Measure the voltage from M1 Drain to Source (VDS).
   • Result: For small currents with a 2N7000, this drop is often very low (millivolts), dependent on Iload × Rdson.

SPICE netlist and simulation

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

* Practical case: Comparing BJT and MOSFET Switches
.width out=256

* --- Power Supply ---
* V1: Main 5V DC supply
V1 VCC 0 DC 5

* --- Control Signal Section ---
* S1: SPST toggle switch connecting VCC to CTRL.
* Modeled as a voltage-controlled switch (S1) driven by a behavioral pulse source (V_SW_ACT)
* to simulate the user pressing the button periodically.
V_SW_ACT SW_CTRL 0 PULSE(0 5 10u 1u 1u 100u 200u)
S1 VCC CTRL SW_CTRL 0 SWITCH_MOD

* R2: Pull-down resistor (10k) ensures CTRL goes to 0V when switch is open
R2 CTRL 0 10k

* --- BJT Circuit (Current Controlled) ---
* R1: Current limiting resistor for Base (1k)
R1 CTRL B_BASE 1k
* ... (truncated in public view) ...

Copy this content into a .cir file and run with ngspice.

🔒 Part of this section is premium. With the 7-day pass or the monthly membership you can access the full content (materials, wiring, detailed build, validation, troubleshooting, variants and checklist) and download the complete print-ready PDF pack.

🔓 See premium access plans

* Practical case: Comparing BJT and MOSFET Switches
.width out=256

* --- Power Supply ---
* V1: Main 5V DC supply
V1 VCC 0 DC 5

* --- Control Signal Section ---
* S1: SPST toggle switch connecting VCC to CTRL.
* Modeled as a voltage-controlled switch (S1) driven by a behavioral pulse source (V_SW_ACT)
* to simulate the user pressing the button periodically.
V_SW_ACT SW_CTRL 0 PULSE(0 5 10u 1u 1u 100u 200u)
S1 VCC CTRL SW_CTRL 0 SWITCH_MOD

* R2: Pull-down resistor (10k) ensures CTRL goes to 0V when switch is open
R2 CTRL 0 10k

* --- BJT Circuit (Current Controlled) ---
* R1: Current limiting resistor for Base (1k)
R1 CTRL B_BASE 1k

* Q1: 2N2222 NPN Transistor
* Syntax: Qname Collector Base Emitter Model
Q1 B_COLL B_BASE 0 2N2222

* BJT Load Indicator: Red LED (D1) and Resistor (R3)
* D1 Anode connects to VCC, Cathode to D1_K
D1 VCC D1_K LED_RED
* R3 connects between D1_K and BJT Collector
R3 D1_K B_COLL 330

* --- MOSFET Circuit (Voltage Controlled) ---
* M1: 2N7000 N-Channel MOSFET
* Syntax: Mname Drain Gate Source Bulk Model
M1 M_DRAIN CTRL 0 0 2N7000

* MOSFET Load Indicator: Green LED (D2) and Resistor (R4)
* D2 Anode connects to VCC, Cathode to D2_K
D2 VCC D2_K LED_GREEN
* R4 connects between D2_K and MOSFET Drain
R4 D2_K M_DRAIN 330

* --- Component Models ---

* Switch Model (Threshold 2.5V, Low On-Resistance)
.model SWITCH_MOD SW(Vt=2.5 Ron=0.1 Roff=10Meg)

* BJT Model (Standard 2N2222 parameters)
.model 2N2222 NPN(IS=1E-14 BF=200 VAF=100 IKF=0.3 XTB=1.5 BR=3 CJC=8p CJE=25p TR=46n TF=411p RC=0.3 RE=0.2)

* MOSFET Model (2N7000 approximation Level 1)
.model 2N7000 NMOS(Level=1 VTO=2.1 KP=0.12 LAMBDA=0.01 RD=1 RS=1 CGSO=10p CGDO=10p CGBO=10p)

* LED Models (Generic Red and Green)
* Red LED approx 1.8V drop
.model LED_RED D(IS=1e-20 N=2.0 RS=5 BV=5 IBV=10u CJO=10p)
* Green LED approx 2.1V drop
.model LED_GREEN D(IS=1e-22 N=1.5 RS=5 BV=5 IBV=10u CJO=10p)

* --- Analysis Directives ---
.op
* Transient analysis: 1us step, 500us duration (captures 2.5 cycles of 200us pulse)
.tran 1u 500u

* Output Print Directives
* Order: Input (CTRL), BJT Output (Collector), MOSFET Output (Drain)
.print tran V(CTRL) V(B_COLL) V(M_DRAIN)

.end

Simulation Results (Transient Analysis)

Analysis: The simulation confirms correct switching behavior. Initially (Time=0 to ~10us), CTRL is low (~5mV), BJT Collector is high (~3.95V, LED OFF), and MOSFET Drain is high (~4.06V, LED OFF). When the pulse activates (Time > 10us), CTRL goes high (~5V), BJT Collector drops to saturation (~24mV, LED ON), and MOSFET Drain drops to low resistance state (~46mV, LED ON).

Show raw data table (638 rows)

Index   time            v(ctrl)         v(b_coll)       v(m_drain)
0	0.000000e+00	4.995044e-03	3.947532e+00	4.062211e+00
1	1.000000e-08	4.995044e-03	3.947532e+00	4.062211e+00
2	2.000000e-08	4.995044e-03	3.947532e+00	4.062211e+00
3	4.000000e-08	4.995044e-03	3.947532e+00	4.062211e+00
4	8.000000e-08	4.995044e-03	3.947532e+00	4.062211e+00
5	1.600000e-07	4.995044e-03	3.947532e+00	4.062211e+00
6	3.200000e-07	4.995044e-03	3.947532e+00	4.062211e+00
7	6.400000e-07	4.995044e-03	3.947532e+00	4.062211e+00
8	1.280000e-06	4.995044e-03	3.947532e+00	4.062211e+00
9	2.280000e-06	4.995044e-03	3.947532e+00	4.062211e+00
10	3.280000e-06	4.995044e-03	3.947532e+00	4.062211e+00
11	4.280000e-06	4.995044e-03	3.947532e+00	4.062211e+00
12	5.280000e-06	4.995044e-03	3.947532e+00	4.062211e+00
13	6.280000e-06	4.995044e-03	3.947532e+00	4.062211e+00
14	7.280000e-06	4.995044e-03	3.947532e+00	4.062211e+00
15	8.280000e-06	4.995044e-03	3.947532e+00	4.062211e+00
16	9.280000e-06	4.995044e-03	3.947532e+00	4.062211e+00
17	1.000000e-05	4.995044e-03	3.947532e+00	4.062211e+00
18	1.010000e-05	4.995044e-03	3.947532e+00	4.062211e+00
19	1.026000e-05	4.995044e-03	3.947532e+00	4.062211e+00
20	1.030750e-05	4.995044e-03	3.947532e+00	4.062211e+00
21	1.039062e-05	4.995044e-03	3.947532e+00	4.062211e+00
22	1.041363e-05	4.995044e-03	3.947532e+00	4.062211e+00
23	1.045390e-05	4.995044e-03	3.947532e+00	4.062211e+00
... (614 more rows) ...

Common mistakes and how to avoid them

1. Omitting the Base Resistor (R1): Connecting 5 V directly to the BJT Base will destroy the transistor immediately due to excessive current. Always use a limiting resistor.
2. Floating the MOSFET Gate: If R2 (pull-down) is removed and S1 is open, the MOSFET may turn on/off randomly due to static charge. Always tie the Gate to a known level.
3. Pinout Confusion: Mixing up the Drain/Source on the MOSFET or Collector/Emitter on the BJT. Always check the datasheet diagram for the specific package (TO-92).

Troubleshooting

• Symptom: BJT gets hot, but LED is dim.
  • Cause: The transistor is in the active region (not fully saturated) or R1 is too high.
  • Fix: Decrease R1 slightly to ensure enough Base current drives the transistor into saturation.
• Symptom: MOSFET does not turn on.
  • Cause: Gate Threshold Voltage (Vgsth) is higher than the supply voltage.
  • Fix: Ensure the 2N7000 is used (logic level compatible) or check that the supply is at least 5 V.
• Symptom: LEDs stay on when S1 is open.
  • Cause: Missing pull-down resistor R2.
  • Fix: Install R2 (10 kΩ) to discharge the node CTRL to ground when the switch is open.

Possible improvements and extensions

1. Inductive Load Test: Replace the LEDs/Resistors with small 5 V DC motors. Add flyback diodes (e.g., 1N4007) across the motors to protect the transistors from voltage spikes.
2. High Power Comparison: Swap Q1 for a TIP31 and M1 for an IRF520 to drive a heavier load (like a 12 V 10W lamp). Observe which component requires a heatsink first (typically the BJT).

More Practical Cases on Prometeo.blog

Carlos Núñez Zorrilla

Electronics & Computer Engineer

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

Follow me:

Who we are