Practical case: DC motor control with relay and pushbutton

DC motor control with relay and pushbutton prototype (Maker Style)

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

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

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. |
+---------------------------------------------------------------------------------------------------------+
Schematic (ASCII)

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) ...

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* 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)

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

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Quick Quiz

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




Question 2: Why is a relay useful for component protection in this circuit?




Question 3: What happens immediately after the relay makes an audible 'click' in the expected outcome?




Question 4: Why is this circuit configuration commonly used in automotive starter systems?




Question 5: What is the role of the low-power control side in this setup?




Question 6: In industrial automation, what does this relay setup allow low-voltage PLCs to do?




Question 7: How does this relay setup provide safety isolation?




Question 8: What component is used as the user interface to trigger the circuit in this practical case?




Question 9: If separate grounds are used, what is the relationship between the control and load power rails?




Question 10: What physical phenomenon causes the relay to switch contacts?




Carlos Núñez Zorrilla
Carlos Núñez Zorrilla
Electronics & Computer Engineer

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

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