Solid State Relay Basics

Solid State Relay Basics

Solid state relays can have a variety of different devices at the heart of their electronic circuits: thyristors, triacs, bipolar junction transistors, BJTs and MOSFETs provide ideal electronic switches within solid state relays.

To provide switching signals between the input and the switching elements, optical links are usually used. This provides almost complete electrical isolation between the input and output circuits.

Usually a switching device; thyristors, triacs, bipolar junction transistors, or MOSFETs are optical versions of the device that turn on in the presence of light.

Essentially, a solid state relay is a switch that inputs or controls voltage to light up a light-emitting diode. It acts as a transmitter for the optocoupler and then controls the switching devices: thyristors, triacs, bipolar transistors for MOSFETs.

The basic concept of solid state relay, SSR

A solid state relay consists of a transmitter Tx and a receiver Rx. They are physically located inside the solid state relay. The incoming control signal energizes the LED within the optocoupler, which illuminates the photosensitive output switching device, causing it to switch from its normal, non-energized state. Usually it turns on the output device, allowing current to flow through the SSR.

The transmitter and receiver are usually located within the same electronic component, which simplifies the structure of the solid state relay.

As can be seen from the figure, there is no electrical connection between the input and output electronic circuits. This separation, commonly referred to as galvanic separation, is the key to keeping the input and output circuits isolated from each other. Galvanic isolation between LEDs and optoelectronic devices is typically in the range of several kilovolts due to the separation between the optical transmitter and receiver or detector arrangement and the optically transparent insulating barrier placed between them.

When looking at SSR specifications, it's worth noting that isolation is specified in terms of voltage breakdown, the voltage that causes a breakdown between input and output. It is not the same as input to output resistance. Depending on the device, it can be anywhere from 1000 to 1 million MΩ - because it's so high, it's often considered an "infinite" resistance.

Although the basic conceptual diagram of a solid state relay only shows an LED that illuminates a light-sensitive semiconductor switch such as a thyristor or SCR, triac, transistor or MOSFET, other components are required in the SSR.

two main areas of solid state relays

SSR Inputs: Since the input LEDs need to operate at the desired input conditions, there are several aspects of the input circuit that need to be considered:

Input drive level: The input circuit needs to ensure that the light emitter, that is, the LED, can work at the specified drive level. This usually requires the inclusion of a current limiting resistor and any other electronic components to ensure that the LED is fully lit with the input signal. Solid State Relays can operate with input voltages above a few volts.

DC or AC input: If the SSR is designed to operate with a DC input, it can operate with minimal additional electronics - maybe just a current limiting resistor. If you plan to use AC operation, use a rectifier (usually a bridge rectifier) to rectify the input signal so that the LEDs are only driven with signals of the correct polarity. The LED lights will pulse in an AC waveform - twice the frequency if you use a bridge rectifier. This bridge rectifier can be incorporated as part of the solid state relay or added externally.

SSR output: The output of a solid state relay also needs to be understood, as there may be many additional electronic components in addition to the basic photosensitive switchgear.

A variety of different devices are available for the output of solid state relays: transistors, thyristors/thyristors, MOSFETs and triacs. The device type determines many of the characteristics of the SSR.

If the output is a single transistor, FET or thyristor/SCR, then this means that this SSR can only conduct in one direction and can only be used to control a DC load. AC operation usually requires a triac or two thyristors/SCRs in the output - sometimes pairs of MOSFETs are also used.

The specified maximum output range of a solid state relay can range from a few volts to hundreds of volts AC or DC, and can carry current levels as high as tens or even hundreds of amps, depending on the specifications of the particular device.

Synchronized and random switching solid state relays

When switching large currents and using semiconductor devices

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