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Solid State Relays (SSR) use an optocoupler system with a combination of an Emitter and a Photo Detector to drive an output MOSFET switch. SSRs are optically isolated coupling devices that provide separation between the input and output circuits, which operate from independent power sources. The voltage difference between input and output is dependent on the device, however many SSRs used in power applications can withstand isolation voltages of up to 1,500 V. CEL’s SSR, known as an “OC MOSFET” (Opto-Coupled MOSFET,) has the input and output isolated with a photocoupler and the MOSFET configured as an output switch. One key difference when comparing a standard transistor-output optocoupler is that the SSR acts more like a “switch” in the circuit, while the output transistor in a standard optocoupler retains transistor Collector – Emitter behavior. Basic operation theory is as follows: A control signal is applied to the OC MOSFET input terminals, which drives the Infrared Emitting Diode (IRED), which then sends its signal across the optical isolation gap. The Photo-Voltaic Diode (PVD) portion receives this infrared signal and triggers the output switch of the OC MOSFET, which, in turn, opens or closes the output terminals. As shown, the CEL SSR consists of an Emitter, Photo Detector, Control Circuit, and the output MOSFET. The optical isolation barrier is between the Emitter and the Photodetector.
PTM Published on: 2009-03-27