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selecting resistor wattage

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Member - Posts: 17
Member spacespace
Joined: March 23, 2013
Location: New Jersey, United States
Posted: January 28, 2015 at 11:03 AM / IP Logged  

I'm building a street rod, all new wiring components and harness, and need help selecting the correct wattage for a resistor I need to use. I have a Painless Wiring universal harness and a Powermaster 140A alternator (1 wire). I am converting the alternator to 3 wire per input from Painless and Powermaster. The painless harness has a 12v switched "alternator exciter" wire that I'm running directly to terminal 1 of the alternator. I talked to the Powermaster tech yesterday and was told that I would need a resistor in series in the exciter wire to the alternator. Anywhere from 50 to 150 ohms would do. The tech didn't say, and I for got to ask, what the wattage of the resistor should be. As of now, I will not be including a warning light in the circuit, just the wire directly from the fuse block to the alternator.

If I did the math correctly using 12 volts and 150 ohms, I would need a 1 watt resistor? Am I on the right track, or not. Thanks in advance,


Gold - Posts: 4,913
Gold spacespace
Joined: November 03, 2008
Location: Australia
Posted: January 28, 2015 at 3:13 PM / IP Logged  
P=VV/R (or IIR).
But use 15V for the calc to cover typical vehicle voltages above 12V - ie, 14.4V.
Hence P = 15 x 15 / 150 = 1.5W & hence a 2W resistor.
Or conversely - to see what the resistor will handle - max V = 'root' (PR) = root of 1W x150Ohm = 12.3V for a 1W resistor - ie, 1W is too low.
However, that exciter current - aka tickle or trickle charge/current - is only that high with IGN on and the alternator NOT charging.
Usually bulbs are used - it is not a coincidence that charge lights are usually 2W - 3W bulbs.
i am an idiot 
Platinum - Posts: 13,680
Platinum spaceThis member consistently provides reliable informationspace
Joined: September 21, 2006
Location: Louisiana, United States
Posted: January 28, 2015 at 9:20 PM / IP Logged  
All above calculations are dropping the entire 12v across the resistor. I do not think that there will be more than a volt or 2 across the resistor. I think the 1 watt will be more than sufficient. Whatever you use, just monitor it for temperature increase for the first 10 minutes or so. Do not touch it, get a squirt bottle filled with water and spray it on the resistor. If it evaporates quickly, do not touch it.
Gold - Posts: 4,913
Gold spacespace
Joined: November 03, 2008
Location: Australia
Posted: January 28, 2015 at 11:03 PM / IP Logged  
Tho in practice that has its point, keep in mind that a 12V charge lamp does get 12V across it...
If te IGN is on for a long time without the alternator charng, then it's >1W.
But a larger resistance could be used eg 1W @ 15V means >225 Ohm and many of my alternators would initiate charging with >330 Ohm resistors.

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