Diode for 12V conversion

Dean Olson · Dec 20, 2010

I'm converting a 300 to 12v and have everything done except for installing the "diode". I visited the local Radio Shack and got no help. They have a whole drawer full of diodes. Diodes went from 12-600 volt and everything in between. I assume I need the 12v but got lost again when it talked about anode and cathode on the package.

Alternator is a Hitachi 14231. S is to battery stud on alternator and L is going to be going to the ingition with the diode.

What diode do I need and what is the direction of the current flow?

Bob · Dec 20, 2010

With a Delco, I use 3 Amp, 1000 PIV diodes (1N5408) which are WAY overkill, but I've never had a failure, as opposed to using, say a 1 Amp, 100 PIV diode which WILL occasionally fail, apparently from voltage or current spikes.

I buy the 1N5408 iodes in bulk, typically for about $.05 so "overkill" doesn't cost much here!

The PIV (Peak Inverse Volts) rating has to do with the highest reverse voltage spikes the diode can live through, in general, the higher the better, at any rate you'll want to be WELL above 12 Volts.

The AMP rating is the amount of current the diode can handle in the forward direction, when exciting the alternator, which can take perhaps around 2 Amps.

NO reason a similar diode wouldn't work with about ANY internally-regulated alternator.

For a (-) grounded system, the CATHODE (white-banded) end goes to the alternator, as shown here with a Delco alternator.

<img src = "http://www.gondtc.com/~blweltin/Bob/AlternatorHarness3.jpg">

Dean Olson · Dec 20, 2010

Thank you very much!

John T · Dec 20, 2010

Dean, Im unfamiliar with that Hitachi alternator, not sure if it requires initial external excitation like the GM 10SI family or not?????? If so, see below:

On the GM I prefer the current limiting (a bit of a safety factor) of a 10 to 47 ohm 5 watt resistor instead of a diode. (5 watts isnt required but has bigger tougher easier to wire leads) It can BOTH limit current PLUS prevent ignition backfeed if required such as on the GM 10 SI family of alternators (again unsure on the Hitachi)

Another option instead of a diode or resistor is the use of a small incandescent "idiot light" which serves THREE purposes,,,,,Gives you "some" indication of charge (on when not charging, off when charging),,,,,,,,,,Excites the alternator if required,,,,,,,Provides some degree of current limiting protection.

As far as Polarity on the diode, for it to work, for Neg ground its cathode must be on the alternator side NOT from the ignition

Bob has ya well covered as far as size and a neat picture, if you use a resistor or idiot light instead, it simply replaces the diode

Merry Christmas

John T

JDemaris · Dec 20, 2010

Next time you go to Radio Shack, just grab some 3 amp rated diodes, like their # 276-1143. You'll get a pack of three for less then $2. I usually wire in two in parallel, for redundancy. That way if one goes bad, it will still work fine. It also increases the amp rating a bit. Some alternators can spike at a 3 amp draw on the field circuit.

I use more Hitachis then Delcos, mainly because they built better with ball bearings on both ends - and they are more compact then Delco 10SIs or 12SIs.

greyfoxxca · Dec 24, 2010

Bob
As I just bought a 300 utility with a 12 volt conversion done previously. The alternator is a delco and was hooked up one wire and a rubber block stuck in the 1 and 2 connectors. It had a tab you pulled off the top and instructions on placing to lower and raise voltage.
As it was not working I followed your excellent instructions and rewired it and it is now charging like a champ.

George

Diode for 12V conversion

Dean Olson

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Bob

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Dean Olson

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John T

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JDemaris

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greyfoxxca

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