Automatic Mains Voltage Selector

APPLICATION NOTES The contents of this column are based on information obtained from manufacturers in the electronics industry, or their representatives, and do not imply practical experience by Elektor Electronics or its consultants. AUTOMATIC MAINS VOLTAGE SELECTOR zel A newly developed opto-triac (SITAC) obviates the need for manual line voltage selection any time mains powered equipment is transferred between 110/117 and 220/240 VAC networks. The application circuit discussed here is a high voltage DC power supply as used in TV sets. With reference to the circuit diagram of Fig. 1, the SITAC Type BRT12 is turned off when the HV supply is powered from a line voltage between 180 and 270 VAC. This means that the 4 diodes in GI form a full-wave rectifier that provides about 300 VDC across the series connected smoothing capacitors at the output of the circuit. When the circuit is connected to a 110 VAC mains network, however, the SITAC is conductive, causing a low re- sistance conn...