Solid State Relays

Publish Date: Sep 19, 2014 | 135 Ratings | 3.27 out of 5 | Print


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

Table of Contents

  1. Introduction to Solid State Relays
  2. Recommended National Instruments Products
  3. Technical Overview

1. 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.

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2. Recommended National Instruments Products

NI PXI-2533 256-Crosspoint SSR 4x64 Matrix
NI PXI-2534 256-Crosspoint SSR 8x32 Matrix
NI 6509 High-Density Industrial Digital I/O
NI 6514 Low-Cost Industrial Digital I/O
NI 6515 Low-Cost Industrial Digital I/O
NI 6528 Industrial Digital I/O
NI SCXI-1128 High-Voltage FET Multiplexer/Matrix Switch
NI SCXI-1163R 32-Channel Optically Isolated Solid-State Relays

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3. Technical Overview

SSRs are commonly designed with a photo-sensitive Metal-Oxide-Semiconductor Field-Effect-Transistor (MOSFET) device with a Light-Emitting Diode (LED) which is used 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.

Benefits of Solid State Relays

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.  Additionally, SSRs are completely silent in their operation given their purely electronic nature.

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.

Weaknesses of Solid State Relays

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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