Solid State Relay Definition
A solid state relay (SSR) is an electronic switching device that turns on or off when an external voltage (AC or DC) is applied to its control terminals. They have the same function as electromechanical relays, but because solid state electronics contain no moving parts and have a longer lifespan.
The SSR consists of a sensor that responds to the appropriate input (control signal), an electronic switching device that switches power to the load circuit, and a coupling mechanism that enables the control signal to activate the switch without mechanical parts. They can be designed to switch AC or DC loads.
Packaged SSRs use power semiconductor devices such as thyristors and transistors to switch currents up to about 100 amps.
Compared to electromechanical relays, SSRs have faster switching speeds and no physical contact wear.
SSRs cannot withstand large transient overloads like electromechanical relays and have higher "on" resistance.
Solid State Relay Characteristics
No mechanical components in solid state relays
Voltage rise is very low
Zero-Crossing Technology for Solid State Relays
How to Operate a Solid State Relay
SSRs based on a single MOSFET or multiple MOSFETs in a parallel array can work well for DC loads. MOSFETs have inherent substrate diodes that conduct in reverse, so a single MOSFET cannot block current flow in both directions. For AC (bidirectional) operation, two MOSFETs are arranged back-to-back with their source pins connected together. Their drain pins are connected to either side of the output. When the relay is closed, the substrate diodes are alternately reverse biased to block current flow. When the relay is open, the common source is always at the instantaneous signal level, and both gates are biased positive with respect to the source via the photodiodes.
Access to a common source is usually provided so that multiple MOSFETs can be paralleled when switching DC loads. When the control input is removed, a network is usually provided to speed up the turn-off of the MOSFET.
In AC circuits, an SCR or TRIAC relay inherently closes at the AC zero crossing when the load current is zero. The circuit never breaks in the middle of a sine wave peak, preventing large voltage transients that can occur due to a sudden collapse of the magnetic field around the inductor. By adding a zero detector (and without the unfavorable circuit inductance and resulting back EMF), a single SCR can be turned back on when a new wave begins. This feature is called zero-cross switching.
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