mains interface

elektor july august 1985 0 00 mains interface This circuit is of use, for instance, when a computer is required to moni- tor a mains-operated equipment. Opto-isolator TIL111 ensures complete isolation between the mains and the computer. With the mains on, during every positive half-wave a current of about 1 mA flows through the LED in the opto-isolator. The associated transis- tor then conducts and its collector current of about 100 IAA is sufficient to drive T1. Remember, however, that this is a pulsating current: capacitor C1ensures that T1conducts continu- ously as long as the mains is on. If a 50 Hz square wave is required at the collector of T1, C1should, of course, be omitted. The two 100 k resistors in series with the LED should not be replaced by one 220 k resistor, because the maxi- mum permissible voltage drop across a standard "A W resistor should not exceed 150 Vrms•