Solid State Relay Drive
A solid state relay is used to switch the mains supply to the primary of the variac. The secondary cannot be
switched because the secondary voltage falls below the 28 V required to turn on the back to back SCRs in
the relay.
The solid state relay is turned on by a technique which reduces the in-rush current in the variac and loading
transformer combination. A phased turn-off prevents residual magnetization of the loading transformer and
ensures proper re-energisation. These features significantly reduce nuisance effects caused by protective
device operation when a test commences.
Current Cessation Detector
This circuit uses a comparator to detect cessation of the main output current, and has a built-in time
constant to prevent impulsive operation. The output signal is used by the microprocessor system to
acknowledge current cessation and stop the test accordingly.
Circuit Description
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Summary of Contents for PCITS2000/2
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