Solid State Relay (SSR) Diagram

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A solid-state relay diagram shows how a semiconductor-based switch replaces mechanical contacts to control AC or DC loads from a low-power control signal, with complete galvanic isolation between control and load circuits.

A solid-state relay (SSR) is an electronic switching device that uses semiconductor components — typically optocouplers, triacs, SCRs (thyristors), or MOSFETs — to switch a load circuit in response to a low-level control signal, without any moving parts. The control side is optically or magnetically isolated from the load side, providing galvanic isolation equivalent to or better than a mechanical relay.

For AC load switching, two main variants exist. A zero-crossing SSR monitors the AC waveform and only turns the output on when the voltage passes through (or near) zero volts. This dramatically reduces electromagnetic interference (EMI) and extends the life of resistive loads such as heating elements. A random turn-on SSR (also called phase-control or random-fire SSR) switches at any point in the AC cycle, allowing true proportional (phase-angle) control — useful for lamp dimming and motor speed control, but it generates more EMI.

For DC load switching, SSRs use power MOSFETs as the output device. DC SSRs are available in normally-open and normally-closed configurations.

Because SSRs conduct current through semiconductor junctions rather than metal contacts, they have a conduction voltage drop (typically 1 V – 2 V across the output) that causes power dissipation proportional to load current. At higher currents this heat must be managed. Mounting on an appropriately sized heatsink is mandatory for any SSR operating above a few amperes. Thermal compound between the SSR base and heatsink improves heat transfer. Failure to heatsink adequately is the most common cause of SSR failure.

SSRs are widely used in temperature controllers, PLC output modules, medical equipment, and anywhere that silent operation, long service life, or resistance to vibration makes mechanical relays unsuitable.

How to wire solid state relay diagram

Select the correct SSR type for the load Determine whether the load is AC or DC, and the load voltage and current. For AC resistive heating, choose a zero-crossing AC SSR rated for at least 125 % of full load current at the supply voltage. For proportional control (dimming, motor speed), choose a random turn-on (phase-angle) SSR. For DC loads, choose a DC SSR with MOSFET output.
Calculate heatsink requirements Estimate power dissipation as: P = V_drop × I_load, where V_drop is typically 1 V – 1.5 V for triacs. For 10 A at 1.2 V drop, P = 12 W. Use the SSR's thermal resistance (junction-to-case, Rθjc) and the heatsink's thermal resistance (Rθsa) to verify the junction temperature stays below the maximum rated value at the highest expected ambient temperature.
Mount the SSR on the heatsink with thermal compound Apply a thin, even layer of thermal interface compound to the SSR mounting base. Secure the SSR to the heatsink with the appropriate fastener and torque specified in the datasheet. Ensure metal-to-metal contact over the full base area. Avoid air gaps, which act as insulators.
Wire the control (input) circuit Connect the DC control signal positive to the SSR input + terminal, and the control signal return to the SSR input – terminal. Most SSRs accept 3–32 V DC directly. No current-limiting resistor is required for most SSRs — the internal LED has a built-in series resistor. Verify with the datasheet.
Wire the load (output) circuit For AC SSRs: connect Line (live/phase) to one output terminal and the other output terminal to one end of the load. Return the other end of the load to Neutral. Polarity does not matter for AC. For DC SSRs: observe polarity — connect positive supply to the correct terminal as marked.
Add a snubber or MOV for inductive loads Inductive AC loads (motors, solenoids, transformers) generate voltage transients that can exceed the SSR's voltage rating and cause failure. Fit a metal oxide varistor (MOV) rated above the peak supply voltage across the load terminals, or use an RC snubber (typically 100 Ω + 47 nF in series) across the SSR output.
Test and verify operation Apply the control signal at its minimum specified voltage and confirm the load energises. Remove the control signal and confirm the load de-energises. Monitor SSR temperature under full load — if the heatsink becomes too hot to touch briefly (above approximately 60 °C surface), increase heatsink size or improve airflow.

Specifications

Typical control (input) voltage range	3–32 V DC (most general-purpose SSRs)
Typical input current draw	5–15 mA at nominal control voltage
Output voltage range (AC SSR)	24–480 V AC (varies by device)
Output voltage range (DC SSR)	5–200 V DC (varies by device)
Conduction voltage drop (output)	1.0–1.5 V typical (triac-based)
Off-state leakage current	1–10 mA typical
Control-to-load isolation voltage	≥ 2 500 V AC (optical isolation)
Zero-crossing switching window	± 5° of zero voltage crossing (typical)

Safety warnings

SSR output terminals operate at full mains voltage (120 V AC or 230 V AC). Fixed mains installations must comply with applicable electrical codes (NEC/NFPA 70, BS 7671, AS/NZS 3000, or IEC 60364) and be carried out by a licensed electrician. Always isolate the supply and verify it is dead before working on the load circuit.
Always fit an adequately sized heatsink. An SSR without sufficient thermal management will fail silently in a shorted (ON) state, leaving the load permanently energised — a potentially dangerous condition for heaters, motors, and other equipment.
SSRs fail predominantly in the shorted (closed) state rather than open. Do not rely on an SSR as a sole means of isolation. Always include an upstream fuse or circuit breaker and, for critical loads, a mechanical isolation device.
For inductive loads (motors, solenoids), always fit a transient voltage suppressor (MOV or snubber) across the SSR output terminals to prevent voltage spike damage to the SSR.
The leakage current through an off-state SSR can be sufficient to maintain a partial voltage on the load — do not assume the load is safe to touch simply because the control signal is removed. Verify with a meter.

Tools needed

Digital multimeter (AC voltage and resistance)
Infrared thermometer or thermal camera (heatsink temperature verification)
Insulated screwdrivers
Wire strippers and crimp tool
Torque screwdriver (for SSR mounting fastener)

Common mistakes

Omitting a heatsink entirely or using one that is undersized, causing thermal failure of the SSR, usually in a shorted state.
Using an AC SSR to switch a DC load — triac-output AC SSRs cannot interrupt DC current and will remain on permanently once triggered.
Connecting control-side wiring without verifying the input voltage range, resulting in no operation (voltage too low) or immediate damage (voltage too high).
Forgetting transient suppression on inductive loads, causing premature SSR failure from voltage spikes.
Relying on the SSR as the only isolation device, with no upstream fuse or circuit breaker to protect against SSR short-circuit failure.
Not applying thermal compound between the SSR body and heatsink, greatly increasing thermal resistance and accelerating thermal degradation.

Troubleshooting

Load stays on even with control signal removed: Cause: SSR has failed in the shorted (ON) state due to overcurrent, overtemperature, or voltage transient Fix: Isolate the supply immediately. Measure resistance across the SSR output terminals (with supply isolated) — near-zero resistance confirms a shorted output. Replace the SSR and fit a heatsink of greater capacity. Add an MOV or snubber if an inductive load was present.
Load does not energise with control signal applied: Cause: Insufficient control voltage, open control wiring, or failed SSR Fix: Measure DC voltage directly across the SSR input terminals. If within specified range and load does not turn on, the SSR may have failed open. Verify with a known-good substitute. If control voltage is too low, check the control source and wiring.
SSR overheats rapidly under load: Cause: Heatsink undersized or thermal interface compound missing Fix: Calculate required heatsink thermal resistance using SSR datasheet values. Ensure thermal compound is applied. Consider forced-air cooling (small fan) for higher current applications. If load current exceeds SSR rating, uprate to a higher-current SSR.
Interference (EMI) causing other equipment malfunction: Cause: Random turn-on SSR switching at non-zero-crossing, or inductive load without suppression Fix: Replace random turn-on SSR with a zero-crossing type if proportional control is not required. Fit an MOV across the load. Route control and load wiring separately, using shielded cable for the control signal if necessary.
Small lamp load does not extinguish fully when SSR is off: Cause: SSR off-state leakage current is sufficient to partially illuminate the lamp Fix: Fit a bleeder resistor across the load to provide a low-impedance path for leakage current. The resistor value and power rating must be chosen to keep voltage below the lamp's illumination threshold without dissipating excessive power.

Frequently asked questions

What is the difference between a zero-crossing and random turn-on SSR?

A zero-crossing SSR waits until the AC waveform crosses zero volts before turning on, minimising switching transients and EMI — ideal for resistive heaters. A random turn-on (phase-control) SSR switches at any point in the cycle, enabling proportional power control for dimming or speed control, but generates more interference and requires EMI filtering.

Why does an SSR need a heatsink?

The SSR's output semiconductor has a conduction voltage drop of approximately 1 V – 1.5 V. At 20 A, this dissipates 20–30 W inside the device. Without a heatsink, the junction temperature rises until the SSR enters thermal shutdown or fails permanently. Thermal compound and an adequately sized heatsink are always required above a few amperes of load current.

Can an SSR switch DC loads?

Yes. DC SSRs use MOSFET output stages and are available in ratings from a few amperes to over 100 A. Specify a DC SSR explicitly — an AC SSR (triac output) will not switch DC correctly because triacs require AC zero-crossing to commutate off. Check polarity of the load connection on DC SSRs.

What is leakage current in an SSR and why does it matter?

Even when an SSR is off, a small leakage current (typically 1 mA – 10 mA) flows through the output due to the nature of semiconductor devices. For most loads this is harmless, but it can prevent some small lamps from extinguishing fully, or cause unexpected behaviour in high-impedance circuits. A bleeder resistor across the load can mitigate this.

What input voltage do SSRs accept?

Most general-purpose SSRs accept a DC control input range of 3–32 V DC, making them directly compatible with 5 V TTL, 12 V systems, and 24 V PLCs without additional interface components. Some AC-input SSRs accept 90–280 V AC on the control side. Always verify the input range against your control signal voltage before connecting.

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