Not sure where to start guys, so here goes......
Im putting a backlit number plate on my vehicle. I have bought a 12v interior strip light which has an 8 watt light in it. My original number plate lights have bulbs rated at 5 watt. What i need to do is put some sort of resistor in series on the positive wire to drop the voltage down from 12v to ???? in order to get a 5 watt output from the light. Does this make any sense? I have tried to obtain a 5 watt strip bulb but they are not available :(. Can any of you guys work out for me what resistor i will need to use or give me a formula in order for me to work it out myself?
Many thanks in advance!!
Formula #1
P = V * I
P = watts, V = volt, I = current
8 = 12 * I
I = .667A
Formual #2
R = V / I
R = resistance, V = vot, I = Current
R = 12/.667
R= 18 Ohms
The filament in your 8W bult should be very close to 18 ohms/
5W calculation:
P = V * I
5 = 12 * I
I = .42A
R=V/I
R=12/.42
R= ~28 ohms.
The higher the resistance, the lower the power output. To get the bulb to only use 5 watts of energy put a 10 ohm resistor in series with it. You should use at least a 10 watt resistor.
Why can't you just run the bult at full brightness? Using a resistor you will still have an 8 watt load, you'll just only have 5 watts worth of light. The other 3 watts will be dispersed as heat energy.
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Kevin Pierson
I don't believe you need to worry about ohms law formulas although KP presented it very nicely. The thing is that "wattage" in this case is only a rating that tells you how much POWER the bulb will use...it does not in itself tell us anything about the voltage rating of the strip light or that a resistor is actually needed. In other words if you replace a 40watt light bulb in your house with a 100watt bulb it does not require a different voltage or a resistor..it simply is brighter and uses more power (heat)..... all else being equal of course.
If the strip is too bright however then knock yourself out..but if brightness is not a concern and the bulb is a 12volt rated I wouldn't be concerned about any problems.
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The reason why i want it to be 5 watts is because the side lights are rated at 5 watts as well and i dont want the number plate light to overpower the side lights.