# Resistor Selection 15ohm 50W wirewound?

Discussion in 'The Projects Forum' started by ddaytona1, Apr 19, 2010.

1. ### ddaytona1 Thread Starter Member

Nov 3, 2008
13
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I'm trying to determine what type of resistor to order ( and where to order from).

I have an automotive application where the voltage is 12V. Current is 3A. Power is ~ 36 Watts. This resistor needs to be wired in parallel to a light bulb in the instrument panel.

I need a 15 ohm resistor with greater than 36Watt capacity (I'll use 50W). My question is in what package? Ceramic? Wire wound? The photo shown of the resistor suggested by an article is rectangular.

Photo below shows two 30ohm resistors in parallel at 25W combined in parallel to get the 15ohms at 50 Watts. What is the package of these resistors?
Thanks.

Apr 5, 2008
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3. ### AlexR Well-Known Member

Jan 16, 2008
735
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The power dissipated by a resistor will be voltage drop across the resistor multiplied by the current through the resistor. There is no way that a 12volt battery can push 3Amp through a 15 Ohm resistor. Even assuming the worst case with a battery voltage of 15 Volt the maximum current you will get though a 15 Ohm resistor will be 1 Amp and the maximum power the resistor need to handle will be 15 Watt.
So all you need is a 15 Ohm 15 Watt resistor, or if you want a bit extra margin two 30 Ohm 10 watt resistors in parallel.

4. ### ddaytona1 Thread Starter Member

Nov 3, 2008
13
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OK, thanks for the inputs. All I need is to find a source to order the ceramic wirewound resistors.

5. ### SgtWookie Expert

Jul 17, 2007
22,182
1,728
Let me guess - you've replaced your brakelights with LEDs, and now your turn signals and emergency blinkers don't flash.

6. ### ddaytona1 Thread Starter Member

Nov 3, 2008
13
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Nope. I'm replacing the altenator gauge with an indicator light. I need to drop voltage across the light since I'm working with the altenator field.