battery-operated (text) Contentspre-amplifier - part 2
power supply and construction
Following the detailed description of the preamplifierin last month’s instalment, this secondand final part of the article deals with the powersupply and the construction of the completeunit. More so than usual, the power supplyforms an important part of the overall design. Itconsists of 12 NiMH batteries, size AA(RG/HP7) with a nominal capacity of 1.2 Ah,and a moderately fast charger that takes aboutthree hours to fully charge a set of flat batteries(note that it is not a good idea to dischargethese batteries completely – the nominal voltagelevel is maintained at 1.2 V during 80% ofthe discharge cycle).
Since the current drain of the preamplifieris not more than 21 mA, a set offully charged 12 NiMH batteries willenable the preamplifier to operateabout 50 hours continuously. As thistype of operation is seldom, if ever,required, there is normally plenty oftime for the batteries to be recharged.Note, by the way, that NiMH batteriesare free of the undesirable memoryeffect. Nevertheless, NiCd batteriesmay be used if for one reason oranother NiMH batteries cannot beobtained. Unlike NiMH batteries, NiCdcells will show some degradation ofcapacity in the long term.The charger is based on the wellknownType TEA1101 IC, which is eminentlysuitable for this purpose since ituses the ΔU method for controlling thecharging current.