accord_94ex

I am hooking up my garage door opener through my high beams...I already soldered the switch and am ready to instal. My delimna is the original battery is only 3 volts while the high beam wire puts out ~12 volts...Do I need a resistor and if so, what amperage do I use in ohm's law? (P=IV)
Any help would be much appreciated...

xviper

Go to Radio Shack and get one of their DC voltage step down things. It plugs into a cig lighter (which you can cut off) and there is a selection of output voltages you can pick. No need to worry about formulas.

MacGyver

Unless you know the rough current pull on the opener, you won't be able to calculate a resistor value (although you could take a reasonable guess, measure the voltage drop, calculate the true current and try again). RS sells several different Zener values, so use 2 of the 5.1V. The car generally runs around 14.3V when running, so that should put you around 4V. There's no real reason to use Zeners other than the value is convenient, but RS's website doesn't show the actual value of their other diodes, so I can't suggest a part number...just look for some that when combined will give you a 10-11V drop.

CRX7

Just measure the voltage drop across a known resistor value and calculate.

V = IR

accord_94ex

Man, I am an idiot... P=IV isnt even an equation...I was getting the ohm's law (V=IR) and the universal gas constant (PV=nRT) mixed up. Oh well, whenever these exams are over my mind may clear up a littler bit. Thanks for the help guys, hopefully next time I go home I can mess with my car a little bit more.

MacGyver

Actually, P=IV is an equation...Power = Current * Voltage. It probably doesn't help you in this case, though.