I bought a nice AM radio for my 12 volt bug. Found out it is a 6 volt radio after purchase. What size and type of resistor should I use to be able to use this radio?
- posted
20 years ago
6 volt radio 12 volt system
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- posted
20 years ago
You really need to drop 7 volts, because this is really a 7V radio in a 14V system.
The voltage drop across a resistor is V x I, but the I (current) that the radio draws varies with the volume. This means that there is no simple resistance value that will do the job.
The 6V radio may draw up to 4 amps, and this means that your "resistor" (or whatever it turns out to be) will have to dissipate up to 7V x 4A = 28 Watts, which is a VERY big resistor. No matter what you do, you must provide for dissipating ~30 Watts of power, which will require a pretty good heat sink.
The best way to do this would be with a 6V Zener diode and a large transistor to drop the voltage. Someone who knew a bit of electronics might be able to help you draw up a circuit, but you will have to provide a heat sink.
NPN transistor, 25V, 50W, collector connected to 12V supply, emitter connect to the radio input. Mount the transistor to the heat sink with insulating mica & hardware.
6V Zener diode, 1W, cathode (banded end) connected to the transistor collector, other end connected to transistor base. ------------------------------------------------ Jim Adney snipped-for-privacy@vwtype3.org Madison, WI 53711 USA
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