Practical case: High power circuit isolation

High power circuit isolation prototype (Maker Style)

Level: Basic – Control a high-power load using a low-voltage signal via galvanic isolation.

Objective and use case

You will build a driver circuit that uses a small 5 V signal to activate an electromechanical relay, which in turn switches a separate 12 V high-power circuit powering a bulb.

  • Why it is useful:

    • Automotive Systems: Allows a low-current ECU signal to switch high-current headlights.
    • Safety: Keeps high voltage/current (the load side) physically separated from the sensitive control logic (the user side).
    • Interface: Enables microcontrollers (like Arduino/ESP32) to control industrial equipment or AC appliances (simulated here with 12 V).

  • Expected outcome:

    • The 12 V bulb turns ON only when the 5 V control switch is closed.
    • An audible «click» is heard from the relay component when switching states.
    • Measurements: 0 V on the load when the control signal is 0 V; ~12 V on the load when the control signal is 5 V.

  • Target audience: Students dealing with electromechanical interfaces and circuit protection.

Materials

  • V1: 5 V DC voltage source, function: Control Logic Supply
  • V2: 12 V DC voltage source, function: High Power Load Supply
  • S1: SPST Toggle Switch, function: Control trigger
  • R1: 1 kΩ resistor, function: Base current limiter for Q1
  • Q1: 2N2222 NPN BJT Transistor, function: Relay coil driver
  • D1: 1N4007 Diode, function: Flyback protection (snubber)
  • K1: 5 V SPST Relay (coil resistance ~70 Ω), function: Galvanic isolation switch
  • L1: 12 V / 10 W Incandescent Bulb, function: High power load

Wiring guide

This guide uses specific node names to ensure correct connections in simulation and assembly. The circuit has two isolated sides: the Control Side (Nodes: V_CTRL, 0) and the Load Side (Nodes: V_HV, GND_LOAD).

Control Side (Low Power):
* V1 (+): Connects to Node V_CTRL.
* V1 (-): Connects to Node 0 (Common Ground).
* S1: Connects between V_CTRL and Node V_TRIG.
* R1: Connects between V_TRIG and Node V_BASE.
* Q1 (Base): Connects to Node V_BASE.
* Q1 (Emitter): Connects to Node 0.
* Q1 (Collector): Connects to Node COIL_LOW.
* K1 (Coil pin 1): Connects to Node V_CTRL.
* K1 (Coil pin 2): Connects to Node COIL_LOW.
* D1 (Anode): Connects to Node COIL_LOW.
* D1 (Cathode): Connects to Node V_CTRL (Reverse biased across coil).

Load Side (High Power):
* V2 (+): Connects to Node V_HV.
* V2 (-): Connects to Node GND_LOAD (Isolated from Node 0).
* K1 (Common Contact): Connects to Node V_HV.
* K1 (Normally Open Contact): Connects to Node BULB_IN.
* L1: Connects between Node BULB_IN and Node GND_LOAD.

Conceptual block diagram

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

Schematic

+-------------------------------------------------------------------------+
|               PRACTICAL CASE: HIGH POWER CIRCUIT ISOLATION              |
+-------------------------------------------------------------------------+

===========================================================================
  PART 1: CONTROL SIDE (5 V Logic)
  Nodes: V_CTRL, V_TRIG, V_BASE, COIL_LOW, 0 (GND)
===========================================================================

  (Trigger Signal Path)
  [ V1: 5 V (+) ] --> [ S1: Switch ] --> [ R1: 1k Ohm ] --> [ Q1: Base ]
                                                               |
                                                               | (Controls)
                                                               v
  (Coil Power Path)                                    [ Q1: Collector ]
  [ V1: 5 V (+) ] ---------> [ K1: Relay Coil ] --------------> |
                            [ || D1 Diode    ]                 |
                            [ (Rev Biased)   ]                 | (Conducts to)
                                                               |
                                                               v
                                                       [ Q1: Emitter ]
                                                               |
                                                               v
                                                       [ Node 0 (GND) ]


             ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~
             ~      MAGNETIC LINK (GALVANIC ISOLATION)   ~
             ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~ ~


===========================================================================
  PART 2: LOAD SIDE (12 V High Power)
  Nodes: V_HV, BULB_IN, GND_LOAD
===========================================================================

  (High Current Path)

  [ V2: 12 V (+) ] --> [ K1: Relay Switch ] --> [ L1: 12 V Bulb ] --> [ GND_LOAD ]
                      [   (COM -> NO)    ]
Electrical Schematic

Measurements and tests

Follow these steps to validate the isolation and switching capability:

  1. Coil Voltage Test:

    • Close switch S1.
    • Measure voltage between V_CTRL and COIL_LOW.
    • Result: It should read approximately 5 V (indicating the transistor is sinking current).

  2. Load Activation:

    • Keep S1 closed.
    • Observe L1 (Bulb).
    • Result: The bulb illuminates. Measure voltage across L1; it should be ~12 V.

  3. Switch Latency (Oscilloscope required):

    • Connect Channel 1 to V_TRIG and Channel 2 to BULB_IN.
    • Toggle S1 from OFF to ON.
    • Result: You will observe a delay (typically 5–15 ms) between the signal rising on Ch1 and power appearing on Ch2. This is the mechanical switching time of the relay armature.

SPICE netlist and simulation

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

* High power circuit isolation
*
* This netlist simulates a relay driver circuit with a high-power load.
* It includes a low-voltage control side (5V) and an isolated high-voltage load side (12V).
*

* --- Analysis Setup ---
.tran 10u 10m
.print tran V(V_TRIG) V(BULB_IN) V(COIL_LOW) I(L_K1_COIL)

* --- Control Side (Low Power) ---

* Supply V1: 5V DC
V1 V_CTRL 0 DC 5

* Switch S1: Modeled as a Pulse Voltage Source to simulate user actuation
* Connects to V_TRIG to drive the base resistor.
* Timing: Off for 1ms, On for 4ms, then Off.
V_S1 V_TRIG 0 PULSE(0 5 1m 10u 10u 4m 10m)

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

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

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* High power circuit isolation
*
* This netlist simulates a relay driver circuit with a high-power load.
* It includes a low-voltage control side (5V) and an isolated high-voltage load side (12V).
*

* --- Analysis Setup ---
.tran 10u 10m
.print tran V(V_TRIG) V(BULB_IN) V(COIL_LOW) I(L_K1_COIL)

* --- Control Side (Low Power) ---

* Supply V1: 5V DC
V1 V_CTRL 0 DC 5

* Switch S1: Modeled as a Pulse Voltage Source to simulate user actuation
* Connects to V_TRIG to drive the base resistor.
* Timing: Off for 1ms, On for 4ms, then Off.
V_S1 V_TRIG 0 PULSE(0 5 1m 10u 10u 4m 10m)

* Resistor R1: 1k Base Current Limiter
R1 V_TRIG V_BASE 1k

* Transistor Q1: 2N2222 NPN Relay Driver
* Connections: Collector=COIL_LOW, Base=V_BASE, Emitter=0
Q1 COIL_LOW V_BASE 0 2N2222MOD

* Relay Coil K1 (Coil Side)
* Modeled as Inductance + Resistance in series between V_CTRL and COIL_LOW
R_K1_COIL V_CTRL INT_COIL 70
L_K1_COIL INT_COIL COIL_LOW 50m

* Diode D1: Flyback protection (Snubber)
* Anode=COIL_LOW, Cathode=V_CTRL
D1 COIL_LOW V_CTRL 1N4007MOD

* --- Load Side (High Power) ---

* Ground Isolation: High resistance path to global ground 0 to prevent singular matrix
R_ISO GND_LOAD 0 100Meg

* Supply V2: 12V DC
V2 V_HV GND_LOAD DC 12

* Relay Contact K1 (Switch Side)
* Modeled as a Voltage Controlled Switch
* Controlled by the voltage across the coil: V(V_CTRL) - V(COIL_LOW)
* Connects V_HV to BULB_IN when coil is energized
S_K1 V_HV BULB_IN V_CTRL COIL_LOW RELAY_SW_MOD

* Load L1: 12V / 10W Bulb
* Resistance ~ 14.4 Ohms (R = V^2 / P = 144 / 10)
R_L1 BULB_IN GND_LOAD 14.4

* --- Component Models ---

* NPN Transistor Model
.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=46.91E-9 TF=411.1E-12 ITF=0.6 VTF=1.7 XTF=3 RB=10 RC=0.3 RE=0.2)

* Diode Model
.model 1N4007MOD D(IS=7.02767n RS=0.0341512 N=1.80803 EG=1.11 XTI=3 BV=1000 IBV=5u CJO=10p VJ=0.7 M=0.5 FC=0.5 TT=100n)

* Relay Switch Model
* Threshold Vt=2.5V (Coil is 5V), Hysteresis Vh=0.5V
.model RELAY_SW_MOD SW(Vt=2.5 Vh=0.5 Ron=0.1 Roff=100Meg)

.op
.end

Simulation Results (Transient Analysis)

Simulation Results (Transient Analysis)

Analysis: The simulation shows the trigger signal (V_TRIG) going high (5V) between 1ms and 5ms. During this window, the coil current (I(L_K1_COIL)) rises, causing the relay switch to close and V(BULB_IN) to switch to ~12V. After 5ms, the trigger drops, coil current decays (snubber active), and the load voltage returns to near zero.
Show raw data table (4100 rows) 
Index   time            v(v_trig)       v(bulb_in)      v(coil_low)     l_k1_coil#branc
0	0.000000e+00	0.000000e+00	1.722670e-06	5.000000e+00	1.002664e-11
1	1.000000e-07	0.000000e+00	1.722670e-06	5.000000e+00	1.002626e-11
2	2.000000e-07	0.000000e+00	1.722670e-06	5.000000e+00	1.002547e-11
3	4.000000e-07	0.000000e+00	1.722670e-06	5.000000e+00	1.002342e-11
4	8.000000e-07	0.000000e+00	1.722670e-06	5.000000e+00	1.001814e-11
5	1.600000e-06	0.000000e+00	1.722670e-06	5.000000e+00	1.000316e-11
6	3.200000e-06	0.000000e+00	1.722670e-06	5.000000e+00	9.969744e-12
7	6.400000e-06	0.000000e+00	1.722670e-06	5.000000e+00	1.000801e-11
8	1.280000e-05	0.000000e+00	1.722670e-06	5.000000e+00	1.002921e-11
9	2.280000e-05	0.000000e+00	1.722670e-06	5.000000e+00	9.970357e-12
10	3.280000e-05	0.000000e+00	1.722670e-06	5.000000e+00	1.004993e-11
11	4.280000e-05	0.000000e+00	1.722670e-06	5.000000e+00	9.955463e-12
12	5.280000e-05	0.000000e+00	1.722670e-06	5.000000e+00	1.004077e-11
13	6.280000e-05	0.000000e+00	1.722670e-06	5.000000e+00	9.984500e-12
14	7.280000e-05	0.000000e+00	1.722670e-06	5.000000e+00	1.001134e-11
15	8.280000e-05	0.000000e+00	1.722670e-06	5.000000e+00	1.001578e-11
16	9.280000e-05	0.000000e+00	1.722670e-06	5.000000e+00	1.000519e-11
17	1.028000e-04	0.000000e+00	1.722670e-06	5.000000e+00	1.003686e-11
18	1.128000e-04	0.000000e+00	1.722670e-06	5.000000e+00	9.961732e-12
19	1.228000e-04	0.000000e+00	1.722670e-06	5.000000e+00	1.005266e-11
20	1.328000e-04	0.000000e+00	1.722670e-06	5.000000e+00	9.963169e-12
21	1.428000e-04	0.000000e+00	1.722670e-06	5.000000e+00	1.003205e-11
22	1.528000e-04	0.000000e+00	1.722670e-06	5.000000e+00	9.984436e-12
23	1.628000e-04	0.000000e+00	1.722670e-06	5.000000e+00	1.001919e-11
... (4076 more rows) ...

Common mistakes and how to avoid them

  1. Omitting the flyback diode (D1):

    • Error: The transistor Q1 fails permanently after a few switches.
    • Solution: Always place a diode in reverse bias parallel to the relay coil to absorb the high-voltage spike generated when the magnetic field collapses.

  2. Sharing Grounds unintentionally:

    • Error: Connecting GND_LOAD to Node 0 on the breadboard.
    • Solution: While the circuit will work, you lose galvanic isolation. Keep the high-power return path physically separate from the logic ground.

  3. Insufficient Base Current:

    • Error: Using a resistor R1 that is too high (e.g., 100 kΩ). The relay does not click or clicks weakly.
    • Solution: Ensure the transistor is in saturation. For a 2N2222 driving a standard relay, 1 kΩ is usually sufficient.

Troubleshooting

  • Symptom: Relay clicks, but the bulb does not light up.

    • Cause: Issue on the Load Side (Secondary circuit).
    • Fix: Check V2 supply, verify the bulb L1 is not burnt, and ensure connections to the Relay COM/NO pins are tight.

  • Symptom: No sound from relay, Bulb OFF.

    • Cause: The coil is not energizing.
    • Fix: Check voltage at Node V_BASE. If 0 V, check S1. If ~0.7 V, check if Q1 is installed correctly (E-B-C pinout).

  • Symptom: Transistor gets extremely hot.

    • Cause: Coil current is too high for the selected transistor.
    • Fix: Verify the relay coil resistance. If it draws >600 mA, the 2N2222 might be underpowered; use a power transistor (e.g., TIP31) or a MOSFET.

Possible improvements and extensions

  1. Status Indicator: Add a small LED and a 330 Ω resistor in parallel with the Relay Coil to visually indicate when the control signal is active.
  2. Solid State Upgrade: Replace the mechanical relay (K1) and transistor driver with an Optocoupler and a MOSFET (or Triac for AC) to eliminate mechanical wear and reduce switching latency.

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Quick Quiz

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




Question 2: Which component is typically responsible for driving the relay coil in this type of driver circuit?




Question 3: What is the purpose of the flyback diode (D1) placed across the relay coil?




Question 4: What voltage is specified for the Control Logic Supply (V1)?




Question 5: What physical indication is expected from the relay component when it switches states?




Question 6: Why is this relay circuit useful in automotive systems?




Question 7: What is the function of the base resistor (R1) connected to the transistor?




Question 8: What voltage measurement is expected on the load when the control signal is 0 V?




Question 9: Which component represents the high-power load in this specific circuit?




Question 10: What is the main safety benefit of using a relay for galvanic isolation?




Carlos Núñez Zorrilla
Carlos Núñez Zorrilla
Electronics & Computer Engineer

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

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