How to Choose a Solid State Relay: Input Type, Current Sizing, Switching Mode and Cooling

A solid state relay switches an industrial heating or motor load with semiconductors instead of moving contacts. This page is a selection reference: input and output combinations, current and voltage sizing, switching mode, and the heat sink calculation that decides whether the relay survives. Full model data is on the solid state relay category page.

1. Input / output combinations

Type	Control input	Load	Typical use
DC to AC (DA)	3–32 VDC from a PLC, controller SSR-drive output or microcontroller	24–480 VAC	Electric heaters, ovens, dryers, sealing bars — the dominant industrial case
AC to AC (AA)	70–280 VAC	24–480 VAC	Retrofits where only an AC command signal is available
DC to DC (DD)	3–32 VDC	DC load	Low-voltage DC heaters, valves and LED loads
Analog (LA)	4–20 mA or 0–10 V	AC load	Continuous power regulation instead of on/off cycling

For a controller with an SSR-drive output, the DC-to-AC type is the correct pairing: see the SSR-DA single-phase series for single-phase loads and the three-phase BSSR-3 series for three-phase heater banks.

2. Sizing current and voltage

• Current: take the steady-state load current and multiply by 2. A 10 kW single-phase 220 V heater draws about 45 A, so specify a 90 – 100 A relay. Derating is not optional — the printed rating assumes an ideal heat sink.
• Voltage: the load rating must exceed the mains peak plus switching transients. Use a 480 VAC-rated relay on a 380 V three-phase supply.
• Cold inrush: quartz, infrared and MoSi2 elements can draw 5–15 times their rated current when cold. Size on inrush, not on rated power.
• Inductive loads: motors and transformers need extra headroom and an RC snubber; do not select on nameplate current alone.

3. Zero-crossing or random turn-on

Switching mode	Choose it for
Zero-crossing	Resistive heating loads. The relay conducts at the AC zero point, which minimises electrical noise and stress. This is the default on the DA series.
Random turn-on	Inductive loads and phase-angle control, where conduction must start at an arbitrary point in the cycle.
Phase-angle SCR module	Continuous power modulation from a 4–20 mA or 0–10 V signal — use an SCR voltage regulator rather than an on/off SSR.

4. Heat sink calculation

An SSR dissipates roughly 1–1.5 W per ampere of load current across the semiconductor junction. That heat has to leave the cabinet or the relay fails short-circuit, leaving the heater permanently energised.

• Under 10 A continuous: a metal panel plate may be sufficient.
• 10–25 A: fit a matched aluminium SSR heat sink.
• Above 25 A, or any enclosed cabinet: heat sink plus forced-air cooling.
• Always use thermal paste on the mounting face, mount fins vertically, and leave clear air space above and below.

5. Selection checklist

• Control signal range matches the relay input (3–32 VDC for a controller SSR-drive output).
• Load current rating is at least twice the steady-state current, checked against cold inrush.
• Load voltage rating exceeds supply voltage with margin.
• Switching mode matches the load: zero-crossing for resistive, random for inductive.
• Heat sink sized for the actual current, with paste applied and vertical fin orientation.
• Single-phase or three-phase body chosen to match the heater configuration.
• Fuse protection: a semiconductor-rated fast fuse ahead of the relay.

6. FAQ

Why did my SSR fail closed and leave the heater on?

Almost always thermal. An undersized or unpasted heat sink lets the junction overheat, and a failed SSR usually fails short-circuit. Add a hardwired over-temperature cut-out so a shorted relay cannot run the heater away.

Can one SSR switch a three-phase heater?

No. Use a three-phase body such as the BSSR-3, or one single-phase relay per switched phase, driven from the same control signal.

What is the leakage current in the off state?

A few milliamps typically flows through the snubber network even when the relay is off. Never treat an SSR as an isolator for maintenance — isolate with a mechanical disconnect.

SSR or mechanical contactor?

SSR for short cycle times and tight control, because contactor life is measured in operations. Contactor for slow thermal masses and for the safety isolation function, where no panel heat and full isolation matter more than cycle speed.

Technical support

Beijing Strong Electric manufactures solid state relays and SCR power modules, PID temperature controllers and thermocouple and RTD sensors. For sizing help email sales@strong-ele.com.