Is a solid state relay right for you?

A solid state relay (SSR) is an electrical switch that can be used to turn things on and off using another electrical input. Unlike Electromechanical Relays (EMRs), the SSR uses semiconductor switches and optoelectronics instead of magnetism and moving contacts.

It is important to realise there are two common types of solid state relay, those designed for switching AC loads and those designed for switching DC loads. A DC SSR will at best only turn off for half the time with an AC supply – this could cause serious issues related to a half-wave rectified current. If an AC relay is used with a DC supply and the current does start to flow it will probably not stop, even with the control signal removed.

With no wearing parts, solid state relays are less sensitive to environmental factors such as mechanical shock, vibration and external magnetic fields. This provides an increased lifetime and greater dependability of the relay. Interestingly, an EMR has an average lifespan of one million cycles, whereas an SSR has a lifespan of roughly 100 times that. Finder’s solid state relays are electronically clean with low emissions, no contact bounce, and no electrical arcing. The modular design also allows for quick and tidy easy wiring.

At full rated current an SSR generates about 10X as much heat as an electromechanical of similar current rating, so SSRs need proper thermal management whereas EMRs don’t. Spacing, ventilation and PCB layout guidelines in Finder’s data sheets need to be followed to allow heat removal, to avoid overheating and any damages. For the high current 77 ‘Hockey Puck’ Series, Finder has its 077.XX DIN rail mount heat sinks available.

Finder’s thermostat (7T Series) or fan (7F Series) would complement the SSR perfectly and would help regulate optimal temperature and keep components performing at a consistently high level.

Finder’s solid state relays are used in many industrial applications. They are particularly suitable for resistive loads and for industrial applications where high switching frequencies and high inrush currents can be problematic and are ideal for controlling motors, valves and electromagnets in energy, automation, machine and rail applications.

A solid state relay is ideal when fast, frequent switching is required. For example, precision temperature control of an industrial oven might require switching off the mains supply to the heating elements every two or three seconds. Finder’s 77 Series would be the best component to operate this with input AC or DC, 230 V AC or 400 V AC output and rated voltage range (19…305) V AC or (48…480) V AC and is most suited to heater controls.

Interfacing

Finder’s solid state relays are excellent in interfacing applications. Small SSR’s such as Finder’s 34, 38 and 39 Series draw only a few mA, so they sip at the limited output currents available from PLCs.

With a high 6A rating the Finder Type 34.81.7.xxx.9024 would allow a PLC to control a small DC motor or DC coil contactor perfectly. The 2A rating of the Finder Type 34.81.7.xxx.8240 would allow a PLC to control a small AC motor, AC Coil contact, AC solenoid or mains voltage heater. In both cases with a control current of only around 7mA.