Solid-state relays

May 11, 2006
A solid-state relay conducts load current through one or more power transistors or thyristors.

Edited by Leland Teschler

A relatively low triggering energy on the SSR input switches the output on or off.

There are three main types of SSRs, classified by the type of input. Reed-relay coupled SSRs send the input signal through the coil of a reed relay. Closure of the reed switch triggers the thyristor into conduction. Transformer-coupled SSRs send an ac control signal through the primary of a small transformer. (In the case of dc control signals, the input first goes through a dc/ac converter before hitting the transformer primary.) Voltage from the transformer secondary triggers the thyristor. Finally, photocoupled SSRs isolate the input from the output through a photosensitive diode or transistor. A control current actuates a light source which. in turn, lets a photosensitive semiconductor conduct trigger-current to actuate the power device.

Optical coupling is the most widely used type because the components are small and inexpensive. Moreover, on/off switching times are faster, and the blocking voltages also typically far exceed those possible with transformer-coupled units.

The majority of SSRs can be switched by 3 to 32 Vdc and have input resistance of about 1 k. The lower switching voltage limit is set by the need for compatibility with TTL circuits. (TTL devices can sink 5 V to ground with enough current capacity to switch on the LED in an SSR.) Conversely, programmable logic controllers use 24 Vdc as a common signaling level for field devices and thus are also compatible with SSRs.

SSRs are usually designed to switch either ac or dc power, but not both. Ac relays often use either a triac or dual SCRs to switch the output. The most common type of ac relays handle either 120 or 240 V. But some ac-output SSRs can switch 500 Vac or even more.

To reliably switch highly inductive loads, many SCRs place a resistor/capacitor network across their output terminals to compensate for inductive loads. This snubber is the source of most ac leakage current, typically a few milliamps.

SSRs are also sometimes used because they can implement zerocross switching, turning on exactly as the ac-voltage waveform passes through zero.

The de facto standard package for SSRs is dubbed a hockey puck. It is a freestanding, panel-mounted, 17/8 21/8-in. block of plastic or epoxy with electrical terminals. Current capacities range from 1 to about 50 A. However, there are a variety of new DIN rail-type mounting schemes now emerging for these devices.

Opto 22 (opto22.com) provided information for this article.

About the Author

Leland Teschler

Lee Teschler served as Editor-in-Chief of Machine Design until 2014. He holds a B.S. Engineering from the University of Michigan; a B.S. Electrical Engineering from the University of Michigan; and an MBA from Cleveland State University. Prior to joining Penton, Lee worked as a Communications design engineer for the U.S. Government.

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