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Farmall & IHC Tractors Discussion Forum
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Re: alternator diode question

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John T

01-24-2013 06:20:18
71.52.103.159



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Similar to the other gents, I use a 10 ohm safety current limiting voltage dropping resistor in the alternators external excitation circuit so the engine doesnt continue to run (via alternator backfeeding the coil) when the IGN switch is turned off. (talking about the GM 10SI 3 wire family of alternators that use external initial excitation voltage to start charging) A didoe serves the same purpose (has a 0.6 voltage drop) but doesnt provide the small degree of current limiting like a resistor does, which is why I use a resistor instead of a diode.

For another double duty method, you can instead use a small "incandescent" NOT LED indicator light which provides current limiting to prevent run on plus serves as an idiot light.

I think it was Bob M who noted you might even place a resistor in paralell with the idiot light so the alternator still excites even if the light were to burn out...

John T

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Owen Aaland

01-25-2013 02:39:05
216.47.34.216



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 Re: alternator diode question in reply to John T, 01-24-2013 06:20:18  
Quoting Removed, click Modern View to see

Actually this set up might be the best of both worlds.
The resistor will provide protection in case the battery to alternator wire becomes an open circuit which would cause the alternator output to all be on the #1 terminal circuit. The diode would provide a low resistance path to allow for a lower start up speed for the alternator.



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Owen Aaland

01-26-2013 02:01:00
216.47.32.228



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 Re: alternator diode question in reply to Owen Aaland, 01-25-2013 02:39:05  
[quote="Indiana Ken"](quoted from post at 10:11:52 01/25/13) Quoting Removed, click Modern View to see

It is basically a solution in search of a problem that you are not likely to ever see. It really only applies if the alternator wire on the #2 terminal is connected remotely such as at the starter solenoid instead of the alternator output terminal. Connected remotely gives better control over the battery voltage than having it connected at the alternator.

Connect them in parallel.
When starting the diode will provide low resistance to energize the alternator. It will also prevent any current from flowing back to the switch which ensures that the switch can control the power to the coil.
The problem with having only a diode in the circuit to energize the alternator is if the wire between the alternator output and the battery should become open all the current would be fed through the #1 terminal back to the switch. With no path back to the sensing wire the alternator voltage will rise until it destroys the alternator. The resistor allows some of the current to flow back through the circuit so the sensing wire can still control the voltage but limit the current to prevent over loading the circuit.

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Super-H-Mike

01-24-2013 06:48:00
76.164.147.173



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 Re: alternator diode question in reply to John T, 01-24-2013 06:20:18  
I agree, a small bulb like a 194 is simple and cheap plus it is visual. I tuck mine in the hood space where the steering goes through,you can see it from the seat, but doesnt show otherwise and protected from the weather.



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