How to Select a Solid State Relay and Heat Sink
Most SSR failures come from three mistakes: the wrong switching type, too little current margin and too small a heat sink.
A solid state relay has no contacts to wear, but it is a semiconductor — it must be kept cool and protected. This guide walks through the decisions in order, using the Unison range we stock, so the relay you install lasts as long as the machine.
Zero-Cross or Random Switching?
| Choose | Zero-cross (Z) | Random / instant-on (R) |
|---|---|---|
| Turns on | At the next zero point of the AC wave | Immediately when the input is applied |
| Best for | Heaters, ovens, resistive loads | Transformers, some lamps, phase-angle control, fast response |
| Electrical noise | Very low | Higher |
| Unison codes | 901 ZDA, 801 ZDA, 803 ZDA, 802 ZDA | 901 RDA, 806 RDD (DC) |
How Much Current Margin?
Resistive heaters
Choose an SSR rated about 1.5–2× the heater current, then select a heat sink for the actual current.
Lamps and transformers
Inrush can be 10× running current. Use a much larger rating and a random-type relay for transformers.
Motors
Starting current is 6–8× running. Use a motor-rated or 3-phase SSR (e.g. Unison 905) with generous margin.
Example: a 4 kW, 230 V heater draws about 17 A. A 40 A relay on a suitable heat sink runs cool and lasts for years.
Size the Heat Sink
| Load current | Heat produced (≈1.2 W/A) | Typical Unison pairing |
|---|---|---|
| Up to 10 A | ≈ 12 W | 25 A 901/801 on small heat sink, or 25 A 803 slim |
| 10–18 A | ≈ 12–22 W | 25 A or 40 A 901 on G-68 |
| 18–30 A | ≈ 22–36 W | 40 A 803 on B-24, or 50 A 901 on B-48 |
| 30–60 A | ≈ 36–72 W | 50/90 A 803 on B-72, 90 A 901 on B-48 / A-100 |
Pairings follow Unison datasheet derating at 40–55 °C. Add a fan or step up a size in hot panels.
Protect the Relay
- Use a semiconductor fuse. Pick one whose I²t is below the SSR’s — the 801 25 A, for example, withstands 340 A²s. MCBs are too slow.
- Keep surge protection. Unison 45 mm and slim relays include a snubber and TVS; for very long cables or inductive loads, add an MOV across the output.
- Match the output type. AC loads need AC-output relays (…DA); DC loads need DC-output relays (…DD).
- Check the control signal. The input must reach at least 4 VDC with correct polarity; PLC outputs and controller SSR-drive outputs are ideal.
Common SSR Faults and Fixes
| Symptom | Likely cause | Fix |
|---|---|---|
| Load stays on permanently | Thyristor shorted by overheating or overcurrent | Replace SSR; upsize heat sink and fuse correctly |
| Load never turns on | Input below 4 V, reversed polarity or open load | Check input polarity, LED and load circuit |
| Small load glows when off | Off-state leakage through the snubber | Add a bleeder resistor or use a contactor for tiny loads |
| Relay runs very hot | Heat sink too small or no thermal compound | Fit the correct heat sink with compound; improve airflow |
| Terminals discoloured | Loose screws creating resistance | Re-terminate with ferrules and re-torque |
Our Quick Recommendations
Standard heater panels
Unison 901 ZDA on a proper heat sink. 901 & 801 guide →
Many zones, little space
Unison 803 slim on DIN rail. 803 slim guide →
OEM boards and DC loads
Unison 806 RDD or 802 ZDA. PCB relay guide →
Popular Unison SSRs
SSR Selection FAQs
Should I use a zero-cross or random SSR for heaters?
Zero-cross. It turns on at zero volts, which reduces electrical noise and stress on the heater.
How much bigger than the load should the SSR be?
About 1.5–2× for heaters, and much more for lamps, transformers and motors with high inrush.
Why does an SSR need a heat sink?
It drops about 1–1.5 V while conducting, turning roughly 1–1.5 W per amp into heat that must be removed.
Can I protect an SSR with an MCB?
Not reliably. Use a fast semiconductor fuse with an I²t rating below the relay’s.
Why does my lamp glow when the SSR is off?
A small leakage current flows through the snubber. Use a bleeder resistor or a mechanical relay for very small loads.
Which Unison SSR switches DC?
DC-output models ending in DD, such as the 806 RDD 100 05 01, or the MOSFET DC-to-DC versions of the 901 and 803.
Get Help Selecting an SSR
Tell us the load type, power, voltage, switching rate and panel temperature. We will recommend the Unison relay, heat sink and fuse.
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