Solid State Relays

This document is part of a comprehensive tutorial on Industrial Digital I/O and Counter/Timer hardware. Learn about National Instruments product offerings for digital and timing I/O, the Industrial Feature Set including watchdog timers and isolation, complementary devices like relays, solenoids and encoders, concepts like sinking and sourcing, and see how these devices can be used in your industrial application.

For more information return to the Complete Industrial Digital I/O and Counter/Timer Tutorial

Introduction to Solid State Relays

A Solid State Relay (SSR) is an electronic switch that works without moving parts. They primarily consist of a low current control and a high current load, which are isolated optically or with transformers.

Recommended National Instruments Products

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NI SCXI-1163R

Technical Overview

SSRs are commonly designed to use a photo-sensitive Metal-Oxide-Semiconductor Field-Effect-Transistor (MOSFET) device with a Light-Emitting Diode (LED) to actuate the device. See figure 1.




Figure 1: Solid State Relay (SSR): Light from the encapsulated LED actuates the photo-sensitive MOSFET and allows current to flow through it.


SSRs are a faster alternative to electromechanical relays because their switching time is dependent on the time required to power the LED on and off - approximately 1 ms and 0.5 ms respectively. Because there are no mechanical parts, their life expectancy is higher than an electromechanical relay.

SSRs are useful for high-voltage applications because the LED actuation provides a galvanic isolation barrier between the control circuitry and the MOSFET. Because the MOSFET is doing the switching, however, there is no galvanic barrier between its contacts. When there is no gate drive on the MOSFET, the drain-source channel on the MOSFET has a very high resistance providing the disconnection between the contacts.

Because the connection is made via a transistor instead of physical metal, as in electromechanical relays, the contact resistance for an SSR is greater. Although technology improvements are continually improving the contact resistance of SSRs, it is still not uncommon to find them in production today with resistances of 100 Ohms or more.

SSRs are not as robust as electromechanical relays. Much like reed relays, they are highly susceptible to surge currents and damage when used at signal levels above their rating. Although there are no metal contacts to weld, damage to the MOSFET can render the relay unusable.

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