diode for delco alternator need information

wilson ind

Well-known Member
George Marsh John T and others Fixing to put delco alternator on a couple of slow running engines. Using 3 wire alternator what diode should I buy to install from ignition switch ? Yes could use one wire if I opened alternator case and changed the regulator internally. I find on slow engines the 3 wire is most dependable. Thanks guys.
 
Three wire setup gives more accurate charge rate since it senses battery voltage in a more accurate way.

3 amp diodes work fine. 1N5402, 1N5404, etc. Less then $2 each at Radio Shack.

When I install I use two in parallel for redundancy.
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I also prefer a 3 wire over a one wire alternator.

A standard diode rated 1.5 Amp @ 50 volts or more will work. The diode is a one way device that passes current in one direction and blocks it in the reverse direction.

The amp rating is the max amount of current it will withstand in the forward direction and the voltage rating is voltage it will withstand in reverse direction.

Diode ratings above will work in alternator circuit, one with higher rating gives extra reserve. You have a lot of choices available

I like Radio Shack 276-1661 as they have rugged leads.
 
Mornin neighbor, good question: When wiring a GM 3 wire alternator to prevent feedback which might allow the gas engine to run even with the IGN turned off (alternator back feeds ignition coil), I have used:


1) Resistors in the 10 ohms or so range and even though such power isn't required, I might use a 5 or 10 watt just because they have bigger stronger leads which may be easier to wire and attach.

2) Incandescent small tail/marker light bulbs because their resistance can prevent run on PLUS they serve as a charge indicating "Idiot Light" My friend Bob M has, I believe, placed a resistor in parallel with the bulb so if the bulb burns out it still works.

3) Diodes anywhere in the 3 to 10 amp range with a voltage withstand rating of maybe 50 or more volts. Again, a bigger unit (though not needed) will have bigger stronger leads. Rat Shack has them. DONT FORGET THEY ARE POLARITY SENSITIVE so wire them with the - towards the alternator.

Ive even had some tractors that didn't run on EVEN WITH NO DIODES and I've seen them with much higher (then 5 or 10 ohms) ohms resistors that still worked.

Resistor or an incandescent idiot light or a diode will work, your money your tractor your choice.

Keep warm over there Wilson

John T
 
Delco 12SI draws 2-3 amps on the excite-field circuit. I've never blown a pair of 3 amp diodes in parallel yet and I've got 8 tractors wired that way (with Delcos or Hitachis).

Biggest problem I've had when using resistors instead of diodes is run-on with diesels. Many diesels with 12 volt electric shut-offs will keep going even with only 3 volts. Diode at less then $2 is kind of a no-brainer. Works with gas and diesels and always allows shut-off.

Another way to wire a Delco SI series with no resistor or diode is with a generic automotive type ignition switch. The kind that has separate ACC and IGN terminals. Wire the ignition to the IGN term, and the alternator excite wire to the ACC term, and you're all set. The two are isolated when you turn the key off.
 
I use the Radio Shake 276-1661. Comes in a pack of 4 for around $3 and is way over kill so they last forever.
 
Has anyone ever just taken a diode trio from a Delco alt and cut two off and used that?? I thought I would try that if I ever needed one, I have a pail full of used Delco alt parts. I have a few using the old Delco with the external alt and they don't need a diode. I have also built some with an online kit to convert to a one wire and they charge right at a dead idle, no need to rev to get them to kick in, and no diode needed.
 
Hi
Years ago I purchased a kit for that alternator that
allowed it to work at low speed.
Found one as item 371120835385 on Ebay.
I'm sure they have others.
 
Wilson, PS, the tractors you showed me were all gas as I recall, them neat old Cockshuts.

Were you needing to rig up some Diesels?? (I was thinking only gas tractors in my earlier post) Of course if so, there's no ignition coil backfeed problem which could keep them running due to an ignition coil backfeed like with a gas tractor (Well DUH you say lol).

On diesels I've seen many various wiring configurations (was a used tractor dealer and farmer) used to initially excite that GM 3 Wire Alternator.

I've seen momentary push switches,,,,,,,,,,,,I've seen toggle switches,,,,,,,I've seen oil pressure switches,,,,,,,,,,,I've seen relays or solenoids used, either factory ones already in place or ones that were farmer added. THERES LOTS OF WAYS TO DO IT ON A DIESEL and what's already in place (solenoids, relays, etc etc) can determine the method you choose to excite the alternator.

While you don't have to worry about backfeeding an ignition coil on a diesel and needing a resistor or diode to prevent that, depending on the wiring and system and what if any factory or farmer added relays are used for OTHER purposes, you may have a situation where a diode becomes necessary to prevent backfeeding a relay or solenoid if such is a problem on your particular diesel tractor and depending on how you wire your excitation circuit.

So Wilson, are you ONLY asking about gas tractors ??? As I posted, I've seen and used three different methods on those (Here's a repeat for your convenience):

A) Resistors (while a few to 10 or more ohms works, higher wattage units have bigger stronger easier to wire leads which is why some prefer them). The resistor method is very cheap (subject to type and quality), very easy to wire with no polarity issues, and reliable.

B) Diodes (while only a few amps suffices fine, again somewhat higher rated units have bigger stronger easier to wire leads). May cost more then a resistor (depends on size and type and quality etc), but still relatively cheap and a lot of farmers, including yours truly, have used them.

C) Idiot Lights which can serve a DUAL purpose as a charge indicator PLUS prevent run on caused by ignition coil backfeed. A lot of people liked that method for the idiot light charge indication feature PLUS it also prevented coil backfeed.

One last note, while I never experienced a resistor problem where they were used, I did see a few diodes go bad HOWEVER that may have been caused by incorrect sizing or poor quality or other reasons. If sized and installed correctly they usually cause no problems. The ones I personally installed worked fine as well as when I used a resistor. EITHER (or the idiot light) CAN WORK FINE

There ya go neighbor, this post regarding diesels and that below about gas engines should help you make your choice.

Keep in touch, will I see you this winter in Florida?????

John T
 
I just take an old rectifier bridge and knock the diodes out. Solder a lead onto each end. Don't want to over heat it though. They will carry several amperes.
 
I dont like using a diode when wiring a 3 wire alt to an older machines ignition supply circuit.
A diode will full field the alternator, adding 2-3 amps load to what may already be a marginal power supply to the coil. The added 2-3 amp load on that ignition circuit can cause voltage drop and poor spark for starting.
Substitute a charge warning light for the diode.
A charge light only adds less than 1/4 amp load on the ignition circuit, yet will excite your alternator at idle. The charge light is also a great indicator to tell you that the alternators field circuit is functioning and if the alternator is charging. Also a warning to remind you that you left the ignition switch turned on with the engine stopped.
A #1815 lamp and two wire base works good for most applications and can be hidden or exposed.
 
Good post, you're sure NOT the only one who likes using an incandescent idiot charge indicating light (versus a diode or resistor, either of which still work and used by many) for that DUAL PURPOSE of preventing ignition backfeed PLUS charge indication. A resistor (that lights a decent sized resistor) can provide some degree of current limiting as you stated.

Fun chat fun topic, to each their own method, we gave Wilson several good choices.

John T
 
(quoted from post at 13:59:17 11/22/14) I just take an old rectifier bridge and knock the diodes out. Solder a lead onto each end. Don't want to over heat it though. They will carry several amperes.

You will have plenty of capacity there. The smallest ones you will find will be about 25 amps.

IH first used a 100 ohm resister but it has been changed to 25 ohm to get the alternator to start charging at a lower rpm.
 
Bill, I'm not sure of diode #. I just go to my box and grab a used ones. I may even have one I removed from my delco alternator and replaced it with an Idiot light. I like the light over the diode because I have all my tractor circuits fused. I can look at light and tell that alternator is powered up. One tractor I use same circuit to power up ballast resistor to coil. I would think a 2 amp diode would work. 5 would be plenty.
The problem with using an alternator on slow running engines is you have to use the large B width pulley off old generator. Recently replaced 2 alternators, one on Jubilee and one on Farmall C. The newer alternators seem to have a built in delay. And both need a wake up call to charge. Wake up call is reeving the engine for a few seconds. Sometimes slowing the engine, alternator stops. The older style delco would begin charging, no delay, at an idle.
George.
 
Thanks to all replies. John T the tractor is a Cockshutt 30. It and Case DC with loader are only two I would use an alternator as I like my junk to be as stock as possible. I really NEED a Cockshutt 50 as have 20 two 30s, 40 diesel. A guy just must have the whole set!!
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JohnT,
Both my alternators have a few ma drain, so I run both the wires through the switch to prevent any alternator drain especially during the winter my farmall gets little use.
George
 
The specifications for field draw on a 10SI alternator is 2.2 - 2.6 amps. Once the alternator starts to charge there will no longer be any draw on this circuit. That leaves a 2 amp diode a bit small for the job.
 

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