Battery Polarity Protection

With battery-operated equipment there’s always the risk of batteries being inserted the wrong way around. Although a diode or a bridge rectifier can be used to solve the problem, the down side is considerable energy loss through dissipation of the forward biased device(s). An energy-wise alternative is now available in the form of an integrated circuit containing two analogue change-over switches.For sure, switching devices using CMOS technology have been around for a long time, but it was not until the requirements of mobile phones and portable test equipment forced chip designers to make substantial improvements to the venerable 4000 series of CMOS logic ICs. The ‘retrofit’ circuit described in this short article is based on the MAX4684 (from Maxim) and is capable of automatically swapping (or, if you like, correcting) the polarity of a battery set. The circuit can work from a voltage as low as 1.8 V, which may be supplied by two totally exhausted dry batteries, NiCd or NiMH cells in series.