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A solid state relay (SSR) is an electronic switch that uses semiconductors instead of moving contacts to turn a load on and off in response to a control signal. TE Connectivity’s FAQ puts it this way: “An SSR is a relay with no moving contacts, and employs semiconductor switching elements like triacs, thyristors, and diodes.”
How a solid state relay works
An SSR has two sides: a control input and a separate output path that carries the load. When the input signal is applied, an internal circuit passes that signal to a semiconductor switching element. The output then changes from nonconducting to conducting. Manufacturers describe optical couplers and other coupling methods that keep the input and output electrically isolated. The exact implementation varies by design.
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The output element depends on what the relay switches:
Do these 3 things before closing this tab:
1Repair Windows errors before they cause bigger problems2Fix the driver behind crashes, sound loss and screen glitches3Clear out junk files and repair common Windows errors- AC SSRs commonly use triacs or thyristors.
- DC SSRs use transistor-based switching.
Panasonic’s examples show the difference. Its zero-crossing AC SSR uses a phototriac coupler and a detector that triggers a triac as the AC load voltage crosses zero. Its random-type AC SSR leaves out that detector and turns on in response to the input. Its DC SSR uses a MOSFET driver that directly switches the output MOSFET.
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Main types of SSR
Zero-crossing AC SSR
This type waits until the AC load voltage approaches a zero crossing before it turns on. Once on, the triac stays latched until the load current reaches zero after the input is deactivated (Panasonic).
Random-turn-on AC SSR
This type turns on when the control input activates, without waiting for a zero crossing. That matters where an application needs a particular turn-on point in the AC cycle. Neither label alone proves a relay suits every load.
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- Adopting AC solid-state relays with high quality
- Strong anti-interference ability and stable performance
- Low control current and high load power
- It has the function of reverse power connection, and the wrong module will not be damaged
- Each route has a status indicator light, and the status is clear and visible
DC-output SSR
This type has a DC-capable output stage, such as a MOSFET. Do not assume an AC-output SSR can switch a DC load, or the reverse. Check the exact datasheet and your load type.
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What an SSR is not
An SSR removes moving contacts, but it is not a perfect open circuit when off. TE notes that SSRs do not provide galvanic separation in the load circuit while off. Panasonic warns that leakage current can make some small loads malfunction.
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- Solid State Relay Module Board has been assembled, and electrical test passed
- Led indicator for each relay
- Size: 155 x 55 x 24 mm (approximately 6.1 x 2.17 x 0.95 inch)
- Has been assembled, and electrical test passed. Led indicator for each relay. Size: 155 x 55 x 24 mm (approximately 6.1 x 2.17 x 0.95 inch) Input control signal voltage: 0V - 0.5V Low stage (SSR is OFF), 0.5V - 2.5V (unknown state), 2.5V - 20V High state (SSR is ON); SSR Output (each channel);
- Load voltage range: 75 to 264VAC (50/60Hz); Load current: 0.1 to 2 AMP Standard interface that can be controlled directly by microcontroller (Compatible with Arduino , 8051, AVR, PIC, DSP, ARM, ARM, MSP430, TTL logic)
What to check when choosing one
| Check | Why it matters |
|---|---|
| AC or DC output, and actual load voltage and current | The output stage is built for one current type. |
| Zero-crossing or random turn-on | Sets the AC turn-on timing, which matters if switching timing matters to the load. |
| Load type and inrush | Lamps, motors, solenoids and transformers draw much more than steady-state current at start. |
| Thermal conditions, mounting and heat sink | Ratings depend on heat. Follow the datasheet’s derating curves. |
| Off-state leakage | The load must tolerate residual current. |
| Surge protection | Some designs call for a fuse, snubber, varistor or clamp. |
Heat and ratings
TE describes an SSR’s switching range and capacity as limited in part by the size and thermal resistance of its switching components. A heat sink adds size and weight. A printed current rating therefore does not replace the datasheet’s derating and heat-sink requirements. TE’s SSR FAQ says to apply heat-sink compound before mounting an SSR on a heat sink. Follow the specific device’s mounting instructions.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Inrush and surge examples
Panasonic Industry’s application guidance (the source page states no year) gives these examples:
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- LM YN 1-Channel Solid State Relay Module, optional positive or negative signal trigger, only need 0.1mA trigger current ,can drive the maximum control capacity of 3A solid state relay pull.
- The module uses high-quality components, red and blue signal indicator, industrial grade double-sided PCB board, stable performance.
- LM YN 1-Channel Solid State Relay Module with solid-state relay output resistance is very small features, can replace the mechanical relay contacts, but also has a small control current, fast action time, long working life and other characteristics, can be widely used in a variety of power Or signal control cases.
- DC 5-32V wide voltage power supply, with power supply anti-reverse ability;
- Output Rated load: AC / DC 0-50V / 2A; Maximum load: AC / DC 0-60V / 3A (within 100ms);
- Tungsten or halogen lamps draw roughly 7 to 8 times steady-state current with zero-crossing SSRs. For random-type SSRs, the cited worst case is roughly 9 to 12 times.
- An electric motor’s starting current is roughly 5 to 8 times steady-state load current, with a DC component superimposed.
These are examples from one manufacturer, not universal values for every lamp, motor, circuit or relay. Panasonic also advises considering varistor protection when high surge voltage is expected, and limiting inductive-load spikes with suitable protection. Whether you need a varistor, snubber or DC clamp diode depends on the load and circuit. Product ratings and installation details are model-specific, so check the current manufacturer datasheet.
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