Voltage converter 1: The unit described is capable of converting a 70-260 V r.m.s. alternating voltage into a 180-350 V direct voltage. For that, a full-wave rectifier, contained in an MC34161, is used as a voltage doubler at low input voltages and as a standard rectifier at high input voltages. In this way, an input voltage variation of x4 is reflected in an output voltage change of not greater than x2. The MC34161 has an integral voltage reference source that provides a voltage of 2.54 V at pin 1. The level of a signal applied to pin 2 is compared internally with a potential of 1.27 V.
Voltage divider R2-R3 ensures that the internal comparator changes state when the input voltage rises above 135 V: pin 5 then becomes logic high. The potential. at pin 2 is then lower than 1.27 V. The triac is switched off, and this removes the center link between the output capacitors, C2 and C3 so that voltage doubling cannot take place. Diodes D1-D4 then operate as a normal bridge rectifier.
When the input voltage is lower than 135 V, the potential at pin 2 remains higher than 1.27 V: the internal transistor at pin 6 is then off. Because of R4, pin 3 then carries the reference voltage of 2.54 V. That in turn results in the internal transistor at pin 5 being switched on, so that triac Trii switch-es from the blocking state to the conducting state. Diodes D2 and D3, and capacitors C2 and C3, then function as a voltage doubler. Zener diode D5, in conjunction with R1 and C4, ensures that the IC is supplied from a stable 12 V source. The time taken by the circuit to switch from standard rectifier to voltage doubler is determined by Ri-Ci. The working voltage of capacitors C2 and C3 must be >250 V.
Warring: the circuit carries high voltages and must, therefore, be treated with the greatest care.