3-Wire Solenoid Wiring Diagram: Understanding the Three Terminal Connection
This is a free printable 3 wire solenoid wiring diagram: download the diagram as SVG or open it and print to paper or PDF.
A 3-wire solenoid wiring diagram shows how the common, normally-open, and normally-closed terminals of a solenoid valve or solenoid actuator connect to the control supply and the load circuit to achieve bi-state switching.
A solenoid is an electromechanical device that converts electrical current into linear mechanical force via an electromagnetic coil. In industrial and automotive applications, solenoids appear as directional control valves (hydraulic, pneumatic, and fuel), latching actuators, door locks, and starter motor engagement mechanisms.
The term '3-wire solenoid' typically refers to one of two distinct configurations. The first is a solenoid valve with a double-coil or latching design that has three connection points: a common return, an energise-forward winding, and an energise-reverse (or latch) winding. Applying voltage between the common and the first winding moves the valve in one direction; applying voltage between the common and the second winding moves it in the other direction. This configuration is common in automotive transmissions, hydraulic proportional valves, and irrigation solenoid manifolds.
The second common usage is a 3-wire connection to a single-coil solenoid where the three wires are: the supply positive, the supply negative (or common), and a feedback or diagnostic signal wire. This third wire may carry a position-sensor output, a coil-fault detection signal, or a pulse-width-modulation (PWM) control signal rather than being an additional coil winding.
In relay-equivalent solenoid valves, the three terminals may correspond to common (C), normally open (NO), and normally closed (NC) contacts — analogous to a relay. When the coil is de-energised, current flows through the NC path; when energised, current flows through the NO path. This arrangement allows the solenoid to switch fluid or electrical circuits in two states without needing a separate relay.
Correct coil voltage is critical. Applying AC voltage to a DC solenoid coil, or significantly exceeding the rated voltage, burns out the coil rapidly. Suppression diodes or varistors across the coil terminals are standard practice to prevent voltage spikes when the coil is de-energised.
How to wire 3 wire solenoid wiring diagram
- Identify the solenoid type and terminal function Determine whether the solenoid has a single coil with three wires (coil supply + and −, plus feedback) or a double-coil configuration (common + two control wires). Consult the data sheet or use a multimeter to identify terminals as described in the FAQ.
- Verify supply voltage requirements Confirm the coil-rated voltage and current from the data sheet. Match the power supply voltage exactly — use 24 V DC for a 24 V DC coil, 12 V DC for 12 V DC. Do not use AC on a DC coil or vice versa.
- Connect the power supply to the common terminal For a DC solenoid, connect the supply positive (+) to the appropriate power terminal. For a double-coil solenoid, the common terminal typically connects to supply return (negative). Confirm polarity on the data sheet — some solenoids are polarity-insensitive on the coil but not on the feedback or position-sensor terminals.
- Connect the control signal For a single-coil solenoid: connect the switch, relay output, or PLC output to the remaining coil terminal (the other end of the coil). When the control switches on, it completes the coil circuit. For a double-coil solenoid: connect two separate PLC or relay outputs — one to each control winding — ensuring they are never energised simultaneously.
- Install a suppression diode or varistor Connect a diode across the coil terminals with the cathode (banded end) toward the positive supply terminal. This ensures the diode is reverse-biased during normal operation and conducts only during the back-EMF spike when power is removed. For AC solenoids, use a varistor (MOV) across the coil instead of a diode.
- Connect the third wire (feedback/sensor/signal, if applicable) If the third wire is a position-feedback sensor output (NPN, PNP, or analogue), connect it to the corresponding input on the PLC or monitoring circuit. Ensure the signal wire voltage rating is compatible with the input card specification. Do not connect the sensor output wire to the power supply terminal.
- Test the solenoid operation Apply the control signal and verify the solenoid energises (audible click for a mechanical valve, position sensor state change if fitted). Measure current draw and compare with the data sheet. After de-energising, verify the solenoid returns to its default state and that there are no voltage spikes on the control circuit bus (verify with an oscilloscope if sensitive electronics are on the same supply rail).
Specifications
| Common industrial solenoid coil voltages | 12 V DC, 24 V DC, 48 V DC, 24 V AC, 110 V AC, 230 V AC |
|---|---|
| Typical solenoid holding current (24 V DC, small valve) | 0.2–0.5 A |
| Typical solenoid inrush current multiplier | 3–10× holding current for the first 20–200 ms |
| Suppression diode minimum reverse voltage rating | Minimum 10× supply voltage; 1N4007 (1000 V PIV) for ≤100 V DC supplies |
| Back-EMF spike amplitude (unsuppressed, 24 V DC coil) | Typically 100–600 V transient, depending on coil inductance and circuit impedance |
| Typical solenoid coil resistance (24 V DC holding type) | 20–100 Ω (measure for specific device) |
Safety warnings
- Solenoid installations in fixed electrical installations must comply with applicable electrical codes — IEC 60364, NEC/NFPA 70, BS 7671, or your regional equivalent. Installation should be performed by a qualified electrician or instrumentation technician. This diagram is illustrative and reference-only.
- Always isolate and verify dead all control and power supplies before working on solenoid wiring. Some solenoid valves control hazardous fluids (gas, fuel, steam, high-pressure hydraulic) — ensure the associated process is also isolated and depressurised before touching any valve or actuator.
- Never simultaneously energise both coil windings of a double-coil (bistable) solenoid. This creates opposing magnetic forces that may stall the actuator and will overheat both coil windings.
- Verify coil voltage before applying power. Over-voltage burns out the coil winding; AC voltage on a DC coil causes rapid overheating. Always check the data sheet nameplate on the coil body.
- Solenoids controlling gas, fuel, or hazardous media in explosive atmospheres must be Ex-rated and certified for the specific zone and gas group. Standard solenoids must not be used in hazardous areas.
Tools needed
- Digital multimeter (resistance, DC voltage, and DC current ranges)
- Wire strippers and crimp tool for control cable
- Oscilloscope (for verifying spike suppression on sensitive control circuits)
- Screwdrivers appropriate for the solenoid connector terminal type
- Multimeter probe extensions or test leads for in-circuit measurement
Common mistakes
- Applying AC supply voltage to a DC-rated solenoid coil, causing immediate overheating and coil burnout.
- Installing the suppression diode in forward-bias orientation — the diode then short-circuits the supply when the coil is energised, blowing the fuse or damaging the control output.
- Omitting the suppression diode entirely, causing PLC digital outputs or relay contacts to arc and fail prematurely.
- Energising both windings of a bistable solenoid simultaneously, damaging both coils.
- Connecting the feedback/sensor third wire to the power supply rail instead of the signal input of the controller.
- Sizing the relay contacts for holding current only, ignoring the inrush current that can weld or erode relay contacts on each energisation.
Troubleshooting
- Solenoid does not energise when control signal is applied
- Cause: No supply voltage at coil terminals, blown fuse, open coil winding, or faulty relay/PLC output Fix: Measure supply voltage at the solenoid coil terminals while applying the control signal. If no voltage, trace back through relay contacts and fuse. If voltage is present but the solenoid does not operate, measure coil resistance — an open coil reads infinite resistance and requires coil replacement.
- Solenoid coil overheats rapidly
- Cause: Wrong supply voltage (over-rated or AC on DC coil), coil shorted internally (low resistance), or duty cycle exceeded Fix: Measure supply voltage and compare with the data-sheet rated voltage. Measure coil resistance and compare with the data-sheet value — a significantly lower resistance indicates a partial short. Verify the solenoid's duty cycle rating — some solenoids are intermittent duty only and cannot be held energised continuously.
- PLC output module or relay contacts fail repeatedly
- Cause: Absent or incorrectly installed suppression diode, causing back-EMF spikes to damage the output device Fix: Verify the suppression diode is installed across the coil terminals with cathode toward the positive supply. Replace the damaged diode if it has failed short-circuit. Check the diode's voltage and current rating is adequate for the coil.
Frequently asked questions
What is a normally open vs. normally closed solenoid?
A normally open (NO) solenoid valve allows flow when the coil is de-energised and closes when energised. A normally closed (NC) solenoid valve blocks flow when de-energised and opens when energised. The choice depends on the fail-safe requirement — NC valves are typically used for fuel or gas lines so a power failure cuts the flow.
Why is a suppression diode placed across a solenoid coil?
When the current through an inductive coil is interrupted, the collapsing magnetic field generates a voltage spike (back-EMF) that can be many times the supply voltage. On a 24 V DC system, spikes of several hundred volts are possible. A diode connected in reverse-bias across the coil clamps this spike, protecting transistors, relays, and PLCs in the control circuit from damage.
Can I run a DC solenoid from AC supply voltage?
No. A DC solenoid coil on an AC supply will overheat and fail rapidly because the alternating current does not produce a steady holding force — the solenoid 'buzzes' and the coil's RMS current is higher than designed. Use AC solenoids on AC supplies and DC solenoids on DC supplies, and always verify the coil voltage rating against the supply.
How do I identify the three terminals on an unmarked solenoid?
Use a multimeter on resistance mode. For a double-winding solenoid, you will find two coil resistances and a common terminal. Measure resistance between all three pin combinations — the terminal that shares a resistance reading with both of the other terminals is the common. For a C/NO/NC solenoid, test with the coil de-energised: continuity exists between C and NC, and open circuit exists between C and NO.
What current does a typical 24 V DC solenoid draw?
This varies considerably with the solenoid size and design, but a common small industrial solenoid valve draws 0.2–1.0 A at 24 V DC in the energised (holding) state, with an inrush current 3–10 times higher during the first milliseconds of energisation. Always check the manufacturer's data sheet for inrush and holding current to size the control circuit and fusing correctly.
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