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Solid-state relays incorporate an LED to control the gate of a photo-sensitive metal-oxide-semiconductor field-effect transistor (MOSFET). This relay structure provides an isolation barrier between the control circuitry and the load allowing the relay to switch high voltage. It takes about 1 ms (depending on the relay) for the LED to power on, and 0.5 ms for the LED to power off. These times restrict the switching speed of this relay, yet SSRs are faster than electromechanical relays.
The following figure shows a diagram of a typical solid-state relay. The digital control signal powers an LED that turns the MOSFET circuitry on and off. The LED is required to isolate the drive circuitry from the DUT signal.