Solid-state Relays

What Is a Solid State Relay?

A Solid State Relay (SSR) is a relay that does not have a moving contact. In terms of operation, SSRs are not very different from mechanical relays that have moving contacts. SSRs, however, employ semiconductor switching elements, such as thyristors, triacs, diodes, and transistors.

Structure and Operating Principle

SSRs use electronic circuits to transfer a signal.

Features

SSRs are relays that use semiconductor switching elements. They use optical semiconductors called photocouplers to isolate input and output signals.
The photocouplers change electric signals into optical signals and relay the signals through space, thus fully isolating the input and output sections while relaying the signals at high speed.
Also, SSRs consist of electronic components with no mechanical contacts. Therefore, SSRs have a variety of features that mechanical relays do not incorporate.
The greatest feature of SSRs is that SSRs do not use switching contacts that will physically wear out.

Mechanical Relays (General-purpose Relays)

Example of an Electromagnetic Relay (EMR)
An EMR generates electromagnetic force when the input voltage is applied to the coil. The electromagnetic force moves the armature. The armature switches the contacts in synchronization.

Solid State Relays (SSRs)

Representative Example of Switching for AC Loads

Types of SSRs

OMRON classifies the SSRs according to type, as shown in the following table.

Type	Load current	Points	Typical Relays
SSRs integrated with heat sinks	150 A or lower	The integrated heat sink enables a slim design. These relays are mainly installed in control panels.	G3PJ, G3PA, G3PE, G3PH etc.
SSRs with separate heat sinks	90 A or lower	Separate installation of heat sinks allows the customers to select heat sinks to match the housings of the devices they use. These relays are mainly built into the devices.	G3NA, G3NE, etc.
Relays with the same shapes	5 A (10 A) or lower	These relays have the same shape as plug-in relays and the same sockets can be used. They are usually built into control panels and used for I/O applications for programmable controllers and other devices.	G3F(D), G3H(D), G3R-I/O, G3RZ, G3TA etc.
PCB-mounted SSRs	5 A or lower	SSRs with terminal structure for mounting to PCBs. The product lineup also includes MOS FET relays, which are mainly used for signal switching and connections.	G3MC, G3M, G3S, G3DZ, etc.

*MOS FET relays have control circuits that are different from those of traditional SSRs.
Refer to MOS FET Relays of Further Information for the MOS FET relay structure, glossary, and other information.

Control Methods

ON/OFF Control

ON/OFF control is a form of control in which a heater is turned ON and OFF by turning an SSR ON and OFF in response to voltage output signals from a temperature controller. The same kind of control is also possible with an electromagnetic relay, but an SSR must be used to control the heater if it is turned ON and OFF at intervals of a few seconds over a period of several years.

Low-cost, noiseless operation without maintenance is possible.

Phase Control (Single Phase)

With phase control, the output is changed every half-cycle in response to the current output signals in the range 4 to 20 mA from a temperature controller. Using this form of control, high-precision temperature control is possible, and is used widely with semiconductor equipment.

Precise temperature control is possible.
The heater’s service life is increased.

Optimum Cycle Control

The basic principle used for optimum cycle control is zero cross control, which determines the ON/OFF status each half cycle. A waveform that accurately matches the average output time is output.
The accuracy of the zero cross function is the same as for conventionally zero cross control. With conventional zero cross control, however, the output remains ON continuously for a specific period of time, whereas with optimum cycle control, the ON/OFF status is determined each cycle to improve output accuracy.

Many heaters can be control using communications.
Noise-less operation with high-speed response is possible.

Cycle Control

With cycle control (with the G32A-EA), output voltage is turned ON/OFF at a fixed interval of 0.2s. Control is performed in response to current output from a temperature controller in the range 4 to 20 mA.

Noiseless operation with high-speed response is possible.

Precautions for Cycle Control

With cycle control, an inrush current flows five times every second (because the control cycle is 0.2 s).
With a transformer load, the following problems may occur due to the large inrush current (approximately 10 times the rated current), and controlling the power at the transformer primary side may not be possible.

(1)The SSR may be destroyed if there is not sufficient leeway in the SSR rating.

(2)The breaker on the load circuit may be tripped.