Working Principle and Structure of Solid-State Relays

A solid state relay (SSR) is a contactless electronic switching device that uses semiconductor components to control the on/off state of a circuit.
1. Core Components
A solid state relay mainly consists of the following parts:
(1) Input Circuit: Receives control signals, usually DC 3-32V or AC 90-280V.
(2) Isolation (Coupling) Circuit: Provides electrical isolation between the input and output circuits, typically using optocouplers or transformer coupling.
(3) Drive Circuit: Converts the input signal into a signal that drives the semiconductor switch.
(4) Output Circuit: The semiconductor device directly controls the load without mechanical contacts.
(5) Protection Circuit: Built-in RC snubber, TVS diode, etc., to suppress voltage spikes.
2. Operating Process
The operation of a solid state relay can be divided into two stages:
(1) On State:
· The input signal activates the optocoupler
· The drive circuit triggers the semiconductor device to conduct
· The load is powered
(2) Off State:
· The input signal disappears
· The semiconductor device turns off naturally (AC zero-crossing) or is forcibly turned off (DC)
3. Equivalent Circuit Model
The equivalent circuit model of a solid state relay mainly considers three parts: input, isolation, and output:
(1) Input Part: Usually the LED part of the optocoupler, equivalent to the characteristic of a diode.
(2) Isolation Part: The photoelectric conversion part of the optocoupler, equivalent to the signal transmission stage.
(3) Output Part: Semiconductor switching elements, such as bidirectional thyristors (AC SSR) or MOSFETs (DC SSR).