Power supply cut=out circuit - uses thyristor control with thyristor firing being varied on load short circuit to reduce short circuit current

摘要

The power supply cutcut device is for short circuit protection and comprises a transformer primary (11) and switching transistor series connected across a d.c. power supply. A control circuit (90) applies positive firing pulses to the transistor to turn it on, these pulses being prolonged by feedback from a first transformer secondary to provide a linear increase in transistor current. A regulator (80) through a second secondary monitors the primary current and fires a thyristor (61) to short circuit the transistor input when the current attains a selected level. Output power is taken from a further secondary. To ensure dynamic compatibility between primary current and output power a polarisation circuit (50) adds a negative voltage to the thyristor gate on each discharge of the transformerprimary. In addition, to ensure full power on start up of the device a further polarisation circuit (50) applies a negative voltage to the gate on each charging of the primary. On short circuit of the output power the first polarisation circuit (50) ceases to operate thus keeping the short circuit current sufficiently low.