What Is a Solid State Relay (SSR)?
A solid state relay (SSR) is a type of relay that switches with semiconductor elements instead of a moving mechanical contact. It has no wearing parts inside.
How Does It Work?
The input side is usually isolated from the load with an optocoupler. The control signal passes to the other side via light, and the output triac (for AC) or MOSFET (for DC) switches the load.
| Property | SSR | Mechanical Relay |
|---|---|---|
| Contact | Semiconductor | Mechanical |
| Lifetime | Very long | Limited |
| Speed/Sound | Fast, silent | Slow, clicks |
Advantages
Since there are no moving parts, it's silent, fast and long-lived. There's no arcing; it's ideal where frequent switching is needed (such as temperature control).
Disadvantages
It drops some voltage and heats up while conducting, so most SSRs need a heat sink. Also, it passes a small leakage current even when off and is more expensive than a mechanical relay.
Where Is It Used?
Temperature/heater control, industrial ovens, frequently-switched AC loads and devices that must run silently.
Zero-Cross and Random Type
AC SSRs come in two switching types. The zero-cross type only turns on the load as the mains voltage passes through zero; this reduces the inrush current and noise and is ideal for resistive loads. The random (phase-angle) type switches instantly and is used in applications needing phase control such as light dimming.
Common Mistakes
- No heat sink: an SSR overheats at high current.
- Zero-cross selection: choosing the wrong SSR type for a phase-controlled load.
- Leakage current: expecting exactly zero current in the "off" state.
Frequently Asked Questions
Why does an SSR heat up?+
Does it switch DC?+
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