Excitation circuit for permanent magnet - has storage capacitor for de-excitation following mains interruption by triggering of diac connected to thyristor gate

摘要

The circuit finds application in magnetically operated tools were both high initial attraction forces and high residual magnetisation are required. A storage capacitor (8) charges during the excitation phase by cyclically switching a series thyristor (1). When fully charged the excitation phase ends and the thyristor remains off. The capacitor (8) then charges a reference capacitor (9) which is cyclically drained if the a.c. supply (20, 21) remains on. With the supply interrupted the reference capacitor potential reaches the break-over voltage of a diac (7) connected to the gate of the de-excitation thyristor (3). It triggers and a reverse current pulse from the storage capacitor is applied to the winding (18) resulting in demagnetisation.