Diode

B

Bill H.

Member
The replacement diode I've used to replace the generator cut-out has blown, but has no markings on it. Which diode do I use, and where can I buy it?
 
Brattons also has one , part number 15600 price $8.95. This part comes with simple to follow instructions. The problem that some have is a poor solder joint. get some rosin core solder and an electric solder gun. Your best bet would be a Fun Projects can style voltage regulator. This part installs in place of the old cut out and gives voltage regulation. No mods to the generator is needed. Just install the regulator as you would the cutout and set the adjustable brush. These units can be ordered in 6 ,8, or 12 volt.
 
If you have an alternator shop around check with them. A couple of bucks. I'd give you one for nothin if you want to drop by...The old Deco 10DN has 6 of 'em with flag leads. You just knock 'em out. At one time I had so many I'd just give the rear housing away with three diodes in it..
 
Willie Krash said:
If you have an alternator shop around check with them. A couple of bucks. I'd give you one for nothin if you want to drop by...The old Deco 10DN has 6 of 'em with flag leads. You just knock 'em out. At one time I had so many I'd just give the rear housing away with three diodes in it..
Click to expand...

I put a diode in my cutout some 35---40 years ago, a used one punched out of a gm alternator, i recently found the remains of the housing in my core pile, and the other 2 negative side diodes in my old toolbox with the flag leads, on my cutout I gutted the reley and drilled a hole in the frame to press the diode in, very simple, and has survived my abuse
 
PV Antique & Classic Ford, Inc. has them. Part # A-10150-295 Price is $2.95 and they will ship First Class Mail for economy.
Vic
 
The primary reason for diode failure is heat. Alternator diodes are pressed into a heat sink with a fan-over for a reason. The amperage rating of a given diode is based on proper thermal heat sinking. Most cutout diode conversions have little, if any, heat sink attached to the diode case.

At 15 amps the diode will need to dissipate 10 watts of thermal energy or it will overheat. (P=VA, 0.7V diode drop x 15A = 10.5 Watts) You could put a 100A rated diode in a cutout can, but if the can is only capable of dissipating 5 Watts of heat that 100A diode will fry and fail with an 8 amp load!

The original magnetic relay design of the cutout does not suffer this thermal dissipation limitation, as the points within do not present a voltage drop as a diode does.

To me, diode cutouts are just another case of newer not necessarily being better. Here's a few numbers in a scenario to illustrate why:

You're running and the battery is limiting voltage to 6.7V. You have "low draw" Halogen headlamps, 35W each, 3 amps to the ignition, 2 amps to the tail lights, and your battery is charging at a relatively low 3 amps. The total draw is 18.5 amps. (That's the smokin' hot limit for an A generator!!) The generator is producing 6.7V x 18.5A = 124 Watts of power.

NOW, you replace the stock relay cutout with a diode. @18.5 amps, the diode will need to dissipate 13 Watts. Since the battery regulates voltage in this system, the generator itself will now deliver 137 Watts to maintain the 6.7V at the battery. Your A generator will now produce that elusive, magical electric smoke. Lucas, move over, you've got company!

I'll keep my stock cutout. If you drive daylight only, or are OK with stock 21cp or 32cp lamps, and do not try to drive with a 1/2 dead battery that is drawing 15 amps, you will probably do fine with the diode modification.
 
Thanks Mike, and that's why I've always prefered the original cutouts.

Many electro-mechanical repro cutouts are so poorly made that the contacts overheat and fail. I don't ever recall an original cutout with burned contacts to the point of that being what made the cutout fail. I've had to clean contacts and sometimes solder the fine pull-in winding ground wire, due to a cold solder joint, and I also have had to adjust original cutouts pull-in voltage, but once restored to original specs I find them extremely reliable.

BTW, A&L does use a good size heat sink on the diode cutouts they sell. I'm not sure which other dealers also sell the A&L diode cutout.
 
I 100% agree with the need for a heat sink. If I was to make a diode cutout I would use the proper diode type with a case that can be electrically isolated, but thermally heat sunk to the "can". Without a heat sink, you will let the magic smoke out of it, if not right away at some point it will happen. The fan will help keep the can cool, so the can will be able to dissipate more heat than it could if there was not air blowing on it.

The bigger question is why not just use the original style, they are fixable, or put in the "regulator in a can" style and gain voltage regulation. Seems to me a diode cut out is kind of a "half of the way thing, if modern electronics is wanted use a regulator".
 
I agree the can style voltageregulator is by far the better choice. For a low cost way to go is the Brattons diode. The diode doesn't give voltage regulation but if a person doesn't do much night driving seems to work good. There is no points to stick. The one in our 31 tudor had give trouble free service since 2000 . We seldom or almost never drive at night. We did once make about a 50 mile trip at night after a car show with 50 candle power bulbs with no problem but it was in cool november weather.
 
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