Richard Notton wrote:
>Don't tap off one battery or use a plain resistor, the former requires the
>unloaded battery to do things it rather wouldn't and the latter is only
>good for an absolutely fixed load and battery voltage, neither of which you
>have.
>
>I am always fitting two way radios to 24V vehicles and use one of the
>commercial regulators, there are two types, non-isolated and isolated. The
>non-isolated types are invariably for negative earth vehicles whilst with
>the isolated there is no DC connection between input and output and vehicle
>ground polarity is immaterial.
Richard is correct. If you put a resistor in series with your 24/28v battery
supply it _will_ allow your appliance to work, but its not very kind to the
battery system. Also, each appliance would need its own resistor, if you used
just one common resistor then every time you switch on/off an appliance, the
voltage would change due to the varying load. This could be potentially
damaging to your equipment if you got your theory wrong.
Your best bet is either a proper regulator which provides a constant voltage
whatever load you use (within limits) or a voltage converter which uses a
more complicated circuit but is more efficient, as little heat (waste energy)
is dissipated, and so your batteries will last longer. In a catalogue in front
of me a 24 volt to 12 volt converter rated at 80 watts costs 68 pounds (around
110 US dollars).
Another poster wrote:
>It connects 24 volts to the fan for a very short period of time
>(microseconds) and then it shuts off the 24 volts for an equal amount of
>time. The converter does this repeatedly. Then the average voltage to the
>load is 12 volts.
IF it did this I suggest that the 24 volt spikes may well destroy your
appliance. A properly designed voltage converter uses the switched mode
technique and converts maybe 85% of your battery power into useful 12 volts.
A simple resistor and even a voltage regulator would only be 50% efficient.
Do it right - spend the money.
regards,
Richard Evans
England
Richard H. Evans R&D Engineer
Mail: BBC R&D Department, Kingswood Warren, Tadworth, Surrey, KT20 6NP
Tel: +44 1737 836678
Fax: +44 1737 836667
e-mail: richard.evans@rd.bbc.co.uk
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