What kind of accessory?


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Posted by David Sherman on Sunday, May 29, 2005 at 1:15PM :

In Reply to: Electrical experts help needed... posted by Eric B. on Sunday, May 29, 2005 at 10:52AM :

There are different solutions that are best depending on what you're powering. For small to medium, steady loads like lights or a fan motor, a simple resistor works fine. Just measure how many amps the load draws at 6 volts, divide the amps into 6 volts, and it will give you the resistance you need (if it draws 2 amps, you need a 6/2 = 3 ohm resistor. Then multiply the amps by 6 volts to get the wattage (power-handling capacity) you need, such as 6 * 2 = 12 watts. Best multiply that by 2 and round up to the next standard value to have some safety factor so you don't overheat anything.

For small, fluctuating, loads such as a fuel gage or possibly a radio (one without a big amplifier), the small 3-terminal devices like JC Whitney might work. Some contain a simple 2-resistor voltage divider, and some contain a 3-terminal regulator like an LM7806 (available at radio shack). The regulators are electronic and provide the steadiest 6 volts, independent of load, but they are more succeptible to being fried by reverse polarity or voltage spikes. They are usually good for up to about 1 amp if mounted on some kind of heat sink (the tab is grounded internally and can be mounted directly on the back of the instrument panel if you have a negative-ground system. They have internal over-current and over-temperature protection and will shut down if overloaded.

For higher currents (more than a few amps) a resistor or linear regulator will need to be large and dissipate a lot of heat (the same amount of power as the load is drawing) and is probably not practical. At that point, you'd need either a transistorized switching converter (I know they make them for going from 12 to 24 and from 24 to 12 volts, but don't know about 6V) or a motor-generator set (might be some old army surplus unit out there somewhere, but I don't know what it would be).



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