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Old 11-11-2010, 11:38   #1
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AC Reverse Polarity 25,000 Ohm Resistor - What Kind ?

I have recently purchased a 1977 Hunter that does not have a reverse indicator light on the AC master. However a nice panel, with organized wiring I would like to install the indicator lights however I am unclear on the type of 25,000 Ohm resistor that is required. When I check on line for one there is multiple choices can anyone advise and suggest where also to purchase. Thank you.
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Old 11-11-2010, 12:44   #2
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The resistor should have a power rating higher than that of the indicator light. Typically, a 20 Watt resistor works well.

Using a 25,000 RPI circuit limits the current flow (when Reverse Polarity is indicated) to 4.8 milliAmps (120 V 25,000 Ω = 0.0048 A); which is just below the tripping threshold of U.S. Ground Fault Circuit Interrupters (GFCIs trip @ 5 mA),
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Old 11-11-2010, 14:13   #3
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I was able to purchase on line the appropriate resistor with the help of your information thank you.
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Old 11-11-2010, 17:58   #4
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A 25W resistor is massive overkill! With 125V applied, 25K resistor will dissipate 0.625W. A 2W resistor will give you plenty of margin. You may want to use a larger resistor for convenience in mounting.
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Old 11-11-2010, 20:42   #5
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With a neon lamp, a resistor would only need 1/4 watt rating.
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Old 14-11-2010, 15:58   #6
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Quote:
Originally Posted by senormechanico View Post
With a neon lamp, a resistor would only need 1/4 watt rating.

My circuit design days are long in the past. But there are several buts to consider. Yes, a neon will drop about 50v if memory serves me correctly which will lower the voltage across the resistor but there is more than power to worry about. For example, there is a voltage limitation on resistors. Consider a 1/4 watt, high ohmage resistor applied across a 220v source. There may be a voltage breakdown leading to failure. I do agree that power dissipation will be squat but maybe at least a 1/2 watt would be a better recomendation.

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Old 14-11-2010, 16:34   #7
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Neon bulbs fire at about 65 volts. Current is almost nothing.
I used to repair electronics and never saw any need for high voltage rated resistors until over about 1kv. Usually it was in CRT focus circuitry or similar.
I hope this isn't considered thread drift. I do more than my share of that!
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Old 15-11-2010, 02:08   #8
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standard 1/4 watt resistors are fine for Neon circuits the power dissipation is virtually zero and the resistors are rated for the voltage. Bigger is always better or safer.

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