What resistor should I use for this?
I'm buying this 18-LED strip for my car's dash and gauge cluster, as the stock lights are burnt out.
The whole set of 18 LEDs draws 102mA at 12 VDC, and produces 1200mcd per bulb. I'm buying two of these strips, and each strip will be wired in parallel.
I've heard talk that a total of 90ohms of resistance is fair enough to bring down the brightness, but I want to know what wattage the resistors should be, or if I can wire 1/4 watt resistors in series BEFORE the circuit splits off for each strip.
The problem is, that these LEDs are going to be very bright. I've been looking around, and have found quite a bit of resistors. I've been told that using about 90ohms of resistance should be sufficient, although I'm not quite sure what kind of resistors I should get.
Depends. Are the 18 LEDs sets internally wired in parallel with individual resistors? More likely there are 2 or 3 in series in each string, then a series resistor (or a constant current driver) for each string.
But if the string draws 100 mA at 12 volts, and you want to drip it by 2 volts (to 10 volts), for example, the the resistor would be R = E/I = 2/0.1 = 20 ohms, and the dissipation is I²R = (0.1)(0.1)20 = 0.2 watts, no problem, but I'd use a 1/2 watt unit. If you used a 90 ohm under the same conditions, which seems high, the power would be (0.1)(0.1)90 = 0.9 watts, so I'd use a 2 watt unit.
Always use a unit with a power rating at least 2x the actual power.
Note that auto batteries actually are at 15 volts when being charged, which is most of the time.
Your best bet may be to get a bunch of 20 ohm 1/2 watt resistors and put them in series to get the desired brightness. Be sure to check with the engine running and it not.
Yahoo has lost the ability to edit answers, so you can email me if I can help further.
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