Supply protection

The use of external mains voltage adaptors for cassette recorders, port- able radios, home computers, pocket calculators, and so on, is common practice since the typical enclosure sizes of this type of electronic equip- ment either does not allow the incor- poration of a mains supply, or the device has been primarily intended for battery operation. Unfortunately, the degree of idio- syncrasy among manufacturers of adaptors is rather high; a standard for adaptor output voltage and output polarity is definitely hard to find. It may, therefore, be quite risky to power, say, the home computer from an adaptor which is not tailored to this (expensive) piece of electronic equipment. Here is a simple circuit to prevent a lot of trouble; its extremely low cost fully justifies incorporation in any equipment operated from an exter- nal DC supply. The supply protection consists of a mere four parts and a MT 1 MT2l---- U72,1 TAG 226 cuit has the wrong polarity, the zener diode conducts and cau...