what is SSR (solis state relay)?
What Is a Solid State Relay? How SSRs Work and How to Select One
A solid state relay, or SSR, switches a load in response to an electrical control signal. Its power stage uses semiconductors instead of moving contacts. This makes it useful for frequent switching, particularly in heater control, packaging equipment and other repetitive industrial processes.
Solid state relays are available with different control inputs, output technologies and load ratings. A model designed for a resistive AC heater is not automatically suitable for a DC solenoid, transformer or motor. Selection starts with the load and operating conditions, not the current rating printed on the housing.
Inside an SSR: input, isolation and power switching
The input circuit accepts a command from a PLC, a temperature controller or another compatible device. It does not necessarily contain a sensor measuring a physical process variable.
An isolation and trigger stage transfers that command to the power circuit. Optical coupling is common in industrial SSRs, but the isolation ratings and construction must be checked for the actual product.
The output stage carries load current. AC versions may use a triac or inverse-parallel thyristors. DC versions commonly use MOSFETs. These technologies have different switching and conduction characteristics.
Read the input and output ratings separately
A label reading Input: 3–32 VDC; Output: 24–380 VAC describes a DC-controlled relay for an AC load circuit within the stated ratings. It does not describe a relay for switching a DC load. These are illustrative values, not universal SSR specifications.
A conventional triac/SCR AC-output SSR must not be substituted for a DC-output model. Its normal turn-off depends on load current falling below the holding level around an AC current zero. A DC circuit does not provide that natural current zero, so the output can remain latched after the input is removed. Choose a suitable DC-output device and observe its polarity requirements. Special AC/DC output products must be explicitly rated for both.
Zero-cross versus random turn-on
A zero-cross SSR waits until AC load voltage is near zero before turning on. This can reduce switching noise with resistive heaters. It does not make the entire device immune to electrical interference.
A random turn-on SSR responds without waiting for a voltage zero crossing, subject to its specified delay. An appropriate external controller can use this behaviour for phase-angle control. With a triac/SCR output, however, removing the command still does not force arbitrary instantaneous turn-off.
Transformers, motors and loads with substantial inrush require application-specific selection. Zero-cross switching is not universally the best choice for every AC load.
SSR versus electromechanical relay
An electromechanical relay uses a magnetic actuator to move contacts. An SSR has no moving switching contacts. The comparison between mechanical and solid state relays therefore involves thermal behaviour, leakage and failure modes as well as switching frequency.
| Characteristic | Solid state relay | Electromechanical relay |
|---|---|---|
| Switching element | Semiconductor | Moving contact |
| Mechanical noise | No contact click | Audible operation is possible |
| Frequent operation | Well suited when correctly rated and cooled | Uses electrical and mechanical contact life |
| Losses | On-state voltage drop or resistance generates heat | Contact resistance and coil losses |
| OFF state | Leakage current may flow | Open contacts create a physical gap |
| Life limitations | Temperature, electrical stress and thermal cycling | Includes mechanical wear and contact erosion |
No moving contacts does not mean unlimited life. An overheated or electrically overstressed SSR can fail quickly. Equally, a mechanical contact gap does not automatically qualify a relay as a maintenance isolator or a safety switching device; its ratings and intended use still govern.
A heater-control example: what each component does
A Pt100 sensor or thermocouple measures process temperature. The controller's PID algorithm calculates the heating demand. The SSR switches electrical power to the heater. The SSR itself does not measure temperature or perform PID control.
With time-proportioning control, a two-second window might contain 0.8 seconds ON and 1.2 seconds OFF. That represents a 40% duty cycle. For a fixed-resistance heater drawing 2 kW when fully energized from a constant supply, the average electrical power over that window is approximately 0.8 kW. The SSR has not continuously reduced the supply voltage to 40%.
This is different from the high-frequency PWM switching used inside a motor drive. A conventional zero-cross AC SSR should not be driven with an arbitrary kilohertz PWM signal. Select the control cycle to suit both the load's thermal response and the switching devices.
Why heat sinking is part of the electrical design
An energized SSR is not an ideal short circuit. Triac/SCR outputs have an on-state voltage drop, while MOSFET outputs have an on-state resistance. Both produce heat.
Illustrative estimate: Assuming an approximately 1.5 V on-state drop at 20 A gives a rough loss estimate of 1.5 × 20 = 30 W. Actual AC-device dissipation must be obtained from the manufacturer's data and curves. For a MOSFET output, the basic conduction-loss relationship is I² × RDS(on), with resistance dependent on temperature.
A 40 A marking is not a promise of 40 A operation without a heat sink. Ambient temperature, heat-sink thermal resistance, mounting orientation, airflow and the interface material determine usable current. Thermal grease or a pad improves contact; it does not replace the heat sink. Several SSRs mounted together also raise the local cabinet temperature.
Selecting an SSR for the actual installation
Check the control interface. Verify input voltage, AC/DC type, required current and polarity. A controller terminal labelled “SSR output” usually supplies a low-power drive signal for an external SSR; it is not a heater power output. A relay contact, transistor output and 4–20 mA signal are not interchangeable interfaces.
Check the load behaviour. Evaluate output voltage, AC/DC compatibility, continuous current, inrush, minimum load current, off-state leakage and switching mode. Motor, transformer and capacitive loads need more than a steady-current comparison.
Start with a load calculation, then apply the thermal ratings. A resistive heater drawing 2 kW at 230 V takes approximately 8.70 A. That does not establish that a nominal 10 A SSR is suitable. Use the manufacturer's derating curves for the intended heat sink and cabinet temperature, together with the load's operating characteristics.
Coordinate protection. A fuse or protective device must be coordinated with the SSR's withstand capability. An ordinary circuit breaker may not clear a fault quickly enough to protect the semiconductor. Review the specified semiconductor fuse, let-through I²t and surge-current requirements. A varistor or other transient suppressor must also suit the voltage and load.
Why an OFF load may still show voltage
The output semiconductor and any RC snubber can pass a small off-state leakage current. This may leave an LED lamp glowing faintly, prevent a small load from releasing or produce a voltage reading on a high-impedance meter. Such a reading alone proves neither that the SSR has failed short nor that the circuit is safe to touch.
This leakage flows through the output path; it is not necessarily current leaking to earth. It should not be confused with the imbalance detected by a residual-current protective device. For sensitive loads, use a compatible low-leakage output or the manufacturer's calculated application circuit rather than adding an arbitrary resistor.
What happens if the SSR fails?
An SSR can fail open or fail shorted. A shorted output may continue powering a heater after the controller has removed its command. The input LED going out does not prove that load current has stopped.
Where uncontrolled heating creates a hazard, provide an appropriate independent temperature limit and a means of interrupting load power. A suitably controlled contactor or trippable breaker can perform that interruption as part of the designed protection system. The SSR handles routine process switching; the independent protective path addresses a failed-on condition.
A fuse does not necessarily clear a failed-on SSR. If the heater continues drawing its normal operating current, there may be no overcurrent to operate the fuse. Overcurrent protection and protection against uncontrolled heating solve different problems.
Frequently asked questions
Can an SSR directly replace a mechanical relay? Sometimes, but input compatibility, output type, leakage, minimum load and cooling requirements must all be checked.
Can an SSR control AC motor speed? A standard ON/OFF SSR is not a variable frequency drive. Even simple motor switching requires a model explicitly suitable for the motor load.
Does an illuminated input LED prove the SSR is working? No. It generally indicates an input command, not verified output conduction or load current.
Is an SSR maintenance-free? It needs no contact servicing, but terminals, cooling surfaces, fans, cabinet temperature and protective functions still require appropriate inspection.