Supply circuit for electric razor motor - has bridge supplying three parallel branches ensuring for wide range of AC inputs

摘要

The a.c. input (1,2) is fed via a switch (3) and a first capacitor (4) and resistor (5) to a rectifier bridge (6). The bridge output (7,8) is fed to three parallel branches: one contains a 'crowbar' thyristor (9), one contains a Zener diode voltage detector (10,11) and the third contains a blocking diode (12) in series with the motor (13). As the bridge output rises, a capacitor (14) is charged via the blocking diode (12) and a PNP transistor. When the bridge voltage causes the Zener (10) to conduct, the thyristor fires, reverse biasing the blocking diode (12) and allowing the motor (13) to be fed by the capacitor (14) via the base-to-emitter diode (15). At the same time, the capacitor (4) on the a.c. side of the bridge charges up ready for the next cycle.