Solid state relays SSR

Solid state relays (SSR) switch loads electronically, without moving contacts, which gives silent and wear-free operation even at high switching frequencies. SCN supplies SSRs from celduc: 1-phase, 2-phase and 3-phase relays, DC solid-state motor reversers, and accessories such as heatsinks and thermal seals. The range includes models from 30 A to 125 A, for example in the SGT and SMB series. Typical applications are heating control, plastics machinery, packaging machines, motor switching and lighting. Our engineers help you choose the right load current, number of phases, control voltage and heatsink for your application.

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Solid state relays SSR – contactless switching of heaters, motors and other AC loads

A solid state relay switches a load electronically, with no moving contacts, which gives silent, wear-free operation even at high switching frequencies. SSRs are used for temperature control of heating elements in plastics machines, ovens and packaging lines, and for frequent switching of motors, lamps and valves. SCN supplies solid state relays from celduc and DC motor reversers from Nominal Controls.

Under 2 and 3 phase you find celduc SOB and SOBR relays that switch two phases from 2×15 A to 2×75 A at up to 600 V AC, and three-phase SGT, SMB and SGB relays from 3×24 A to 3×125 A at up to 660 V AC, with 4–30 V DC or 90–280 V AC/DC control and versions with push-in terminals. Accessories include WF heatsinks from 3 down to 0.3 K/W, thermal pads, DIN rail adaptors, mounting kits and the ESUC module that monitors current for up to five heaters. The DC Solid-State Motor Reverser DRV handles up to 1.5 kV and 200 A. More components are under Components.

How to choose: pick the number of phases and a current rating with margin, use zero-cross switching for resistive heaters, match the control voltage, then size the heatsink in K/W for the load current and cabinet temperature.

Frequently asked questions

What is a solid state relay?

A solid state relay, SSR, is an electronic switch without moving contacts that turns an AC or DC load on when a small control signal is applied. Since nothing wears mechanically, an SSR handles millions of cycles, switches silently and suits frequent switching such as heater control. The trade-off is heat: an SSR typically dissipates about 1 to 1.5 W per ampere and needs a heatsink.

How does a solid state relay work?

An SSR switches the load with semiconductors, thyristors or triacs for AC and MOSFETs for DC, driven through an optocoupler that isolates the control circuit from the load. A zero-cross SSR turns on when the mains voltage passes zero, which limits interference and suits resistive loads such as heaters. A random turn-on SSR switches immediately and suits inductive loads and phase angle control.

How do you wire a solid state relay?

Connect the control signal, typically 4–30 V DC from a PLC output or temperature controller, to the input terminals with the correct polarity, and put the SSR output in series with the load in the phase line. A three-phase SSR such as the celduc SGT switches all three phases, while the SGB switches two and leaves the third connected. Mount it on a heatsink with a thermal pad and protect it with a fast semiconductor fuse.

How do you test a solid state relay?

Test an SSR with a load connected, because a multimeter alone on the output reads misleading values caused by leakage current. With no control signal the load should stay off; with the control voltage applied the load should run and the voltage across the SSR output should drop to typically 1 to 1.6 V. A relay that conducts without a control signal has a shorted output.

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