Power sources

The necessary d.c. supplies for electronic circuits may be drawn from batteries or obtained by rectification of the a.c. mains. Batteries have the advantage of portability and complete absence of a.c. components in their output. There is, however, a danger of leakage if exhausted batteries are accidentally allowed to stay too long in equipment; this may endanger many hundreds of pounds worth of circuitry through corrosion damage.

The e.m.f. of a battery is not usually constant throughout its life, that of the common low cost zinc chloride and alkaline batteries, falling from 1.6 V to 1.3 V over the useful life of the cell. Mercury oxide cells have a much better e.m.f. characteristic, remaining at 1.3 V over virtually the whole of their life and then falling off rapidly so that there is no doubt when the end of their life is reached; they are, however, expensive. Silver oxide cells have a similarly constant e.m.f. of 1.55 V.

Rechargeable nickel–cadmium (NiCd) cells are available in an enormous range of sizes, ranging from tiny ‘button’ cells to the large batteries used for electric traction. The smaller sizes are usually hermetically sealed so that there is no risk of leakage and no need for topping up. NiCd batteries and the newer nickel-metal-hydride (NiMH) type make an ideal power source for portable electronics, since the need for battery replacement is avoided; the charger may be incorporated into the instrument giving facilities for mains or battery operation.