Diode for alternator wiring - Which one is correct (picture)

AL Farmall Boy · Dec 8, 2010

Hey guys, I recently went to a local electrical supply warehouse and purchase some diodes for the 12 volt alternator wiring on my Farmall tractors. If I understand correctly, the diode should be 200 milliamps (or 20 amps). Below is a picture of a diode I purchased at Radio Shack several months ago that is 200 mA. Next to it is a diode I purchased at the supply warehouse that is rated at (I believe) 230 mA. They are much different in size and was wondering if they may have sold me the wrong thing, or if this is correct. The reason for the purchase at the supply warehouse is because I can get them for $0.32 each, compared to Radio Shack where they are around $3.00 each.

AL Farmall Boy · Dec 8, 2010

Just for clarifacation, the one from Radio Shack is on the Left; and the one from the supply warehouse is on the Right. Thanks.

Janicholson · Dec 8, 2010

One milliamp is 1/1000 amp. The diode needs to handle about 150ma. I like a one amp diode just to be robust. Either of those should work they just might be closer to a spark or stray current breakdown. The PIV peak inverse voltage rating in more important to guard against the failure. 250 volts would be about right as it is connected in the ignition circuit (usually) and these are noisy. Jim

AL Farmall Boy · Dec 8, 2010

Jim,

My question is: why is the diode with the higher rating ALOT smaller than the one from Radio Shack?? The smaller ones I got from the supply warehouse can be taken back. I bought 9 of them, so I would want to make sure I have the right/best one.

Janicholson · Dec 8, 2010

Neither is a 20 amp diode (just to be clear)
The rating may not reflect the size if the efficiency is better on one than the other (Full spec sheet needed) If the small one excites the alternator, give it a whirl. The filiment in a usable idiot light is way smaller then the leads on either diode. 150 ma is the filiment current in the bulb that will excite the alternator to charge. Jim

AL Farmall Boy · Dec 8, 2010

Thank you so much Jim. I am actually on my way to the electrical supply house, and will take them with me just to make sure of the rating of them. I am also going up there to purchase some new fuse holders (the original glass fuse style).

If you don't mind, please clarify these statements for me:

* The pilot light is used to excite the alternator and prevent current from "backfeeding".

* The diode is used to excite the alternator and prevent current from backfeeding.

* The 20 amp?? glass fuse holder (in the dash) is wired between the light switch and ammeter.

Brownie 450 · Dec 8, 2010

A 1N5408 is a 3 amp 1000 volt diode which is lots of over-kill. Price won't be much more than smaller units. A 1N4007 is a 1 amp unit with 1000 volts rating.

John T · Dec 8, 2010

Jim has ya well covered and I (as usual) agree with his excellent advise, but FWIW I dont really recommend diodes anymore, I say use EITHER at least a 10 (or up to a 100 will work) ohm 5 watt resistor ORRRRRRRR an incandescent (NOT LED too much resistance) dash type small idiot/indicator light. The light or resistor affords some degree of current limiting protection better then a diode would and the light gives you a toofer as it excites the alternator PLUS provides a good visual indication of whats going on. Either still prevents backfeeding the ignition as the diode does.

Of course you dont need a 5 watt resistor, a smaller wattage rating would suffice, but they usually have bigger more robust easier to wire up leads

Some "dudes" even wire a resistor in paralell with the idiot light so if the lamp burns out the alternator still gets excitation wooooooo hoooooooo

Wonder if Jimbo concurs?????

Ol John T and all

John M · Dec 8, 2010

I gave up on trying to wire in a diose or a light. I just put a Normally Open oil pressure switch somewhere where theres oil pressure and run the wire from the alt to the switch, then to the gauge.

Bus Driver · Dec 8, 2010

IH used a 25 ohm resistor in their earliest alternator equipped tractors. The typical indicator light has about 53 ohms resistance when the filament is hot. I use a 44 ohm 5 watt wirewound as it is a common size at the supply house.

AL Farmall Boy · Dec 8, 2010

(quoted from post at 01:05:44 12/09/10) IH used a 25 ohm resistor in their earliest alternator equipped tractors. The typical indicator light has about 53 ohms resistance when the filament is hot. I use a 44 ohm 5 watt wirewound as it is a common size at the supply house.

Thank you all for the help!!! I went to the supply house and made sure my diodes were correct, and it turns out they are 2.5 amp therefore waaayy overkill which is good.

Yes, I do too like the pilot lights because it adds a nice touch to the dash as well as serves a purpose as exciting the alternator. I generally like to use them when there is a hole already in the dash that needs to be filled. At $0.40 per diode, and around $8.00 per light; it is definitely more cost effective. That really doesn't matter though.

Question: Does the diode excite the alternator?? If not and there is no light, then what does??

John M · Dec 8, 2010

I just tell my parts guy to give me a normally open oil pressure switch. He know what Im talking about. Most of what I do is on Ms and Hs, I put a tee at the gauge, put the switch in the 3rd hole, and wire it up.

John T · Dec 8, 2010

Question: Does the diode excite the alternator?? If not and there is no light, then what does??

Its actually the current that passes through the diode orrrrrrrrrr resistor orrrrrrrrrrr an idiot light that excites the alternator. If theres no circuit to supply current REGARDLESS if it has to pass through a diode or idiot light or resistor shes not gonna initially excite.

John T

Bob · Dec 8, 2010

For long, trouble-free life, use a diode rated WAY beyond what MAY be needed. I use 1N5408 diodes, 3 Amps, @ 1000 PIV. They cost well under a buck apiece from the right sources, so durability needn't break the bank!

<img src = "http://i167.photobucket.com/albums/u125/27Grainfield/424IH/Alternator%20Stuff/AlternatorHarness31.jpg?t=1291655203">

AL Farmall Boy · Dec 8, 2010

(quoted from post at 04:26:48 12/09/10) For long, trouble-free life, use a diode rated WAY beyond what MAY be needed. I use 1N5408 diodes, 3 Amps, @ 1000 PIV. They cost well under a buck apiece from the right sources, so durability needn't break the bank!

<img src = "http://i167.photobucket.com/albums/u125/27Grainfield/424IH/Alternator%20Stuff/AlternatorHarness31.jpg?t=1291655203">

Ok, 1 more clarifacation that the wiring diagram didn't explain. Does the glass fuse holder go inline betwee the ammeter and the light switch connection?? (I am using Bob M's 12 volt alternator conv. diagram)

cannonball · Dec 10, 2010

many years ago I used a start button, I just pushed it one time after tractor was runnung

Janicholson · Dec 10, 2010

AL Farmall Boy · Dec 10, 2010

Thank you. I thought so. I bought several new fuse assemblies at the local electiral supply just like the origninal ones. I got 25 amp glass fuses for them, and think they will work just fine.

Diode for alternator wiring - Which one is correct (picture)

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