Need a bit of advice - I know v=ir, but not sure how to put it into practice. I need to dim a car bulb, it's 21w (12v), and I want it dimmed to about 10w (approx - doesn't have to be exact). For the formula, what figures do I use for voltage and current, what it was initially, what it would be after resistance, or the amount it would drop by? Also, if there are two positive feeds, one with a resistor, one without, and a common earth, what effect will that have on the resistance? Will it be reduced to zero, or will it be halved? Again, only roughly speaking, don't need exact figures!
With a simple fixed resistance the bulb resistance is V2/W = 144/21 or 6.9 ohms; you'd be wanting to give the bulb about 6V so a series resistance of 6.9 ohms would do the trick. However it gets very complicated with filament bulbs as the resistance is very dependent on the filament temperature (brightness). So a bit of experimentation needed, the actual value will be around 4.7 ohms or less and it will need to be around a 10W rating and able to lose heat easily.
