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Farmall & IHC Tractors Discussion Forum
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Diode

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Bugsy

11-04-2007 09:08:32




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I recently converted my C to a 12 volt system. When I completed the conversion everything seemed to be working fine. I shut the tractor off and with the switch off noticed a slight amperage draw at the amp meter. This a two wire alternator. I was told to splice a diode in the brown wire,and which direction to place it. Bingo, everything works great. I'm a electronic moron,so could someone explain to me why it had a amp draw with the switch off and why it doesn't with the diode. Thanks.

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vinmann

11-04-2007 22:22:33




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 Re: Diode in reply to Bugsy, 11-04-2007 09:08:32  
The battery terminal on the alternator has diodes connected to it and can not drain the battery so a diode on this terminal will not help. The sense terminal is typically connected to the battery terminal and is used by the regulator to set the output voltage and no current is drained through this terminal. The light terminal is used to energize the field until the alternator maintains the field on its own. This terminal is usually connected through a light bulb to a switched source usually the ignition switch. If a light bulb is not used the alternator will draw several amps while it is not spinning creating heat that you could feel and would back feed the coil if connected there. This would cause the engine the keep running after the ignition switch is turned off. A diode between this terminal and the coil would stop the run on.

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Bob

11-04-2007 22:09:50




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 Re: Diode in reply to Bugsy, 11-04-2007 09:08:32  
The simple answer is that if the alternator is wired correctly, and has NO problems with it's INTERNAL diodes, the EXTERNAL diode SHOULD NOT affect current draw with the engine STOPPED.

The diode in the lead to the #1 "switched excite" terminal simply prevents current from the power supply for the rotating field INSIDE the alternator from "backfeeding" into the ignition system, preventing the engine from being shut down.

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Once the engine IS STOPPED, there is no current being produced inside the alternator TO backfeed, and the diode should have NO voltage on either it's anode or cathode, and be serving NO purpose, at that time.

To recap, if the alternator is good, and wired correctly, the absense or presence of the diode has NO effect, ONCE THE ENGINE IS STOPPED.

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El Toro

11-04-2007 17:40:28




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 Re: Diode in reply to Bugsy, 11-04-2007 09:08:32  
Its used to prevent back feeding voltage to the ignition so the engine will shutoff too. Hal



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dlplost

11-04-2007 14:52:16




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 Re: Diode in reply to Bugsy, 11-04-2007 09:08:32  
A diode is like a one way valve to electricity, like a check valve in plumming..



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John T

11-04-2007 14:36:21




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 Re: Diode in reply to Bugsy, 11-04-2007 09:08:32  
Bugsby, I'm just not sure how you have that alternator wired (wonder if its correct) and just where you placed that diode cuz what youre saying makes me think shes wired wrong or else the alternator has a problem BECAUSE if the diode is in the lil side No 1 termianl Excitation circuit AS IT SHOULD BE it should be wired to the ignition switches IGN output terminal (same as input to coil) such that it get voltage ONLY when the ignition switch is on THEREFORE when the switch is off there shouldnt be any connection there and NO RESULTANT BATTERY DISCHARGE CURRENT..... ...at least from that circuit, possibly elsewhere from the alternator if it has a problem????? ?????

Nowwwww w if you had the alternators lil side No 1 excitation terminal circuit ALWAYS wired hot, it will draw current even when the switch is off tractor not running THATS BADDDDD DD ANDDDDD DDD IF you prevented that from drawing current by placing a diode in the circuit, fine??? buttttt ttttt ttttt t with that diode wired that direction she cant then draw excitation current as needed when you start the tractor for a fast alternator kick in.

Id sureeeee like to know first how shes wired and second if when you wired that diode in to stop the discharge she now charges as soon as you start the tractor????? ??? I have a suspicion you may have the alternators excitation cirvcuit always wired hot (not via ign switch or coil as should be) and second maybe you used a diode incorrectly????? ? Or else the alternator is bad and theres another reason for the discharge ????

YOU DONT WANT A DIODE IN THE MAIN BIG ALTERNATORS OUTPUT CIRCUIT I guess one could be used there BUTTTTT TTT it would have to be like a large high rated 50 amp diode versus the few amp diode required in the excitation circuit. Thats NOT the normal way of doing things ..... ...

SOOOOO OOOOO if you indeed had a discharge and also if indeed a diode stopped it, Id like to know where it is. The GM 10 SI family of 3 wire alternators has 2 lil side terminals (1 is Excitation, 2 is voltage sensor) and theres the big main output stud on the rear which wires to the load (NOT batetry/starter) side of the ammeter. The lil No. 1 wires to the IGN output on the ignition switch or else on the high input side of the coil. The lil No 2 usually jump/wires to the big main rear output stud. In the No 1 excitaion circuit I dont normally even use a diode but prefer the safety current limiting resistance of a 10 ohm resistor (better safe then sorry is my motto) although many use a diode NOT an problem whatsoever, its purpose, which a resisitor usually takes care of also, is to prevent the engine from running even after the switch is turned off by preventing backfeed.

Again this alllll ll depends on what alternator you have and how its wired..... ..... .

John T

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old

11-04-2007 11:14:44




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 Re: Diode in reply to Bugsy, 11-04-2007 09:08:32  
Would help a lot if we knew what alternator you used. I my self have never heard of a 2 wire but there is a 3 wire type.



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Bugsy

11-04-2007 13:24:24




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 Re: Diode in reply to old, 11-04-2007 11:14:44  
Let me think about this for awhile----- one-two-three. you're right Old it's a three wire. Bugsy.



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old

11-04-2007 16:44:04




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 Re: Diode in reply to Bugsy, 11-04-2007 13:24:24  
If thats so then you may have it wired wrong or it has a problem.



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Bigdog

11-04-2007 09:43:01




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 Re: Diode in reply to Bugsy, 11-04-2007 09:08:32  
Bugsy - there was a current draw because there is a path through the alternator windings from the battery to ground. A diode is a sort of electrical check valve. It will conduct current in only one direction. So when the alternator is charging current can flow from the alternator to the battery but when the alternator is not putting out the diode prevents current backfeeding to drain the battery.

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