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Posted by David Sherman [24.32.202.83] on Thursday, January 28, 2010 at 22:42:02 :
In Reply to: Sorry David! posted by Ron in San Mateo [75.18.201.86] on Thursday, January 28, 2010 at 21:27:18 :
If you're a bit handy with electronics, the minimum you'd need is a LM7806 on a heat sink. That's a 3-terminal regulator that is pre-set for 6 volts out. When I build "voltage reducers" for vehicles, I add half a dozen other parts to make them more immune to electrical brutality -- a couple of 1N4001s to protect against voltage reversals, a 1 watt 30 volt zener to protect against spikes (the regulator chips can usually survive up to 30 volts on their inputs), and a 10uF tantalum capacitor on input and output to further smooth out spikes and glitches. For a wiper control, I'd use an LM317 or similar adjustable regulator with a pot to adjust the voltage. After all that other stuff, making it adjustable doesn't add much complexity.
I suspect that the "voltage reducers" sold for running 6 volt gauges on 12 volt systems are just the bare LM7806 stuck in a larger package. They'll probably work fine as long as you're nice to them. The trouble is automotive environments aren't nice, electrically. It seems strange to add a half dozen parts to protect one part (sometimes you have to do the same in audio amplifiers, though), but it's worth it in the end. With the extra parts I listed, the regulator would survive load dump transients, battery disconnects with the generator/alternator running, reversed polarity, shorts to ground, jumping from the wrong voltage, and pretty much anything else short of a lightning strike.
Also, to really hold up under bad conditions, it should go in a sealed box, be potted, or at least be dipped in conformal coating. Damp circuitry with DC voltages constantly applied to it causes metal to be electroplated off of some parts and onto others. If somebody wanted to actually get into production with them, if they thought there was a market for good-quality regulators that weren't cheap, I would be happy to draw them a schematic of a good solid design that would withstand all kinds of abuse.