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SSR Switching Characteristics
SSR for AC Loads
1. Zero-crossing SSR
The zero-crossing SSR uses a phototriac coupler to isolate the input from the output (see the circuit configuration on the previous page). When the input signal is activated, the internal zero-crossing detector circuit triggers the triac to turn on as the AC load voltage crosses zero.
The load current is maintained by the triac’s latching effect after the input signal is deactivated, until the triac is turned off when the load voltage crosses zero. The following describes voltage and current wave forms for different types of loads:
●Resistive loads
Since resistive loads cause no phase shift between the voltage and current, the triac turns on when the AC load voltage crosses zero after the input signal is activated. The SSR turns off when the AC load voltage crosses zero and the load current is turned off after the input signal is subsequently deactivated.
●Inductive loads
The SSR turns on when the load voltage crosses zero after the input signal is activated. It turns off when the load current subsequently crosses zero after the input signal is deactivated. A phase difference between the voltage and current may supply a transient spike to the SSR when it is turned off. While the snubber circuit absorbs this spike, an excessively large spike may result in a dv/dt error in the SSR’s internal triac.
2. Random type SSR
Random type SSR uses a phototriac coupler to isolate the input from the output. When the input signal is activated, the output immediately turns on, since there is no zero-crossing detector circuit. The load current is maintained by the triac’s latching effect after the input signal is deactivated, until the AC load voltage crosses zero.
●Resistive loads
SSR for DC Loads
The SSR for DC loads uses a MOS-FET driver to isolate the input from the output.
The output immediately responds to the input, since the MOS-FET driver directly turns the output MOS-FET ON or OFF.
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