Solid State Relay(SSR) is a kind of non-contact switch, there is no moving parts inside SSR, turn on and turn off load is completed by components. Control loop and load loop isolates and transmits through Photoelectronic Coupler or transformer.
Input circuit is mainly DC, AC input is also included. DC input circuit can be divided into resistance input and constant input. The control current of resistance input circuit is changed linearly with input voltage; For constant input circuit, the current will not increase obviously when the voltage up to a certain point, so the solid state relay is available for widely input voltage range.
Output circuit is composed by output devices and absorptive loop, even feedback circuit. The output parts mainly contain triode transistor, thyristor, triac, MOSFET, IGBT and so on.