SSR
A solid-state relay (SSR) is an optoisolated semiconductor switch that performs the function of a coil relay without any mechanical motion. An LED inside the package illuminates a photo-detector, which gates a triac, SCR pair, or MOSFET output stage that switches the load.
The control side drives an LED through a current-limiting resistor; the LED's photons cross an isolation barrier (5–10 mm of clear silicone) into a photo-array or photo-triac on the load side. That detector gates the high-current output device — a back-to-back SCR pair for AC, or a back-to-back MOSFET pair for DC. Zero-cross AC SSRs wait for the next mains zero crossing before turning on, eliminating switching noise.
In plain terms
An optocoupler stapled to a beefy semiconductor switch: light crosses the isolation gap, the chip on the other side does the heavy work silently.
Why designers use it
- Silent, fast switching of resistive heaters and solenoids in industrial automation.
- Logic-level control of high-current loads with zero contact wear.
- Tight-cycle PWM control of heater elements for temperature loops.
Best for
- Heater control
- Silent switching
- Long-cycle PWM
Key specifications
- Control input: 3 – 32 VDC typ.
- Load current: 1 – 100 A
- Load voltage: 24 – 480 VAC / DC
- Isolation: 2.5 – 4 kVrms
- Switching: Random or zero-cross
When not to use it
- Fast switching — relay coils take milliseconds to settle.
- Battery-powered designs where coil current would dominate budget — use a MOSFET.
Common mistakes
- Forgetting the flyback diode across the coil.
- Driving the coil straight from a microcontroller pin without a transistor.
- Sizing contacts for steady current and forgetting inrush.
Where you will find it
- A 3D printer's heated bed is switched by a 25 A SSR driven by the printer's 5 V controller GPIO — the SSR's near-infinite mechanical life lets the firmware PWM the heater at 1 Hz for years without contact wear.
- Industrial reflow ovens use a 40 A SSR per heater zone; the PLC modulates duty cycle, and the SSR's silent zero-cross switching keeps mains harmonics low.
- A semiconductor wafer-test rig uses DC SSRs with back-to-back MOSFETs to switch low-voltage device-under-test currents without the contact bounce that a coil relay would inject.
A short history
The first commercially useful SSRs appeared in the early 1970s when optoisolators (developed by HP and General Electric in the 1960s) were combined with thyristor output stages. Crydom (founded 1969) and Opto 22 (founded 1974) commercialised the panel-mount SSR brick that became standard on factory floors. The zero-cross variant — which delays turn-on until mains crosses zero volts — was patented through the 1970s and became the default behaviour for resistive-load SSRs because it eliminates the conducted-noise spike that a random-fire SSR injects on every switch.
Good to know
- Industrial SSRs typically need a heatsink: a 25 A SSR dissipates ~30 W at full load (1.2 V across the triac).
- An SSR's 'turn-on' is technically when the next mains zero crossing arrives — up to 8 ms of latency at 60 Hz.
- The output of a triac SSR can never be fully off for AC if leakage exceeds the load; LED lighting often glimmers because of this.