Diode cutout conversion technical question

[Sparky]

I'm planning to convert an old cutout I have with a diode. I have the instructions and it looks simple enough.

My question is about the ready-made diode cutouts sold by the parts houses. Some of them say that the output current is "factory set to 5 amps." How is this done with a simple diode? My understanding is that with the diode cutout the generator third brush still determines the output current.

[jhowes]

My understanding is the same as yours They do have a 1/2 or so volt drop across them. Is that what is being reffered to?

[SeaSlugs]

yea diodes do require a .7V drop across them to function.

[Cool Hand Lurker]

I don't understand the "factory set to 5 amps" statement either. They do have a maximum load rating, like "Max load is 15 AMPS" but that means the diode could burn out if you set your generator output above 15 amps. Check the sheet again and give us a link to where you got the diode. Maybe this calls for a contact with the supplier if it is a misprint.

[Sparky]

Originally Posted by Cool Hand Lurker

I don't understand the "factory set to 5 amps" statement either. They do have a maximum load rating, like "Max load is 15 AMPS" but that means the diode could burn out if you set your generator output above 15 amps. Check the sheet again and give us a link to where you got the diode. Maybe this calls for a contact with the supplier if it is a misprint.

It's this one in particular:

http://www.macsautoparts.com/ford_mo...ve-ground.html

I wasn't planning to buy it, it just piqued my curiosity.

[Kevin in NJ]

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That is some kind of error in Macs.

I would ignore it.

I would like to correct a concept that was stated earlier.

This statement is not said correctly:
They do have a maximum load rating, like "Max load is 15 AMPS" but that means the diode could burn out if you set your generator output above 15 amps.

You can set your generator to be able to supply 1000 amps, the load is what makes the draw. So if they battery or lights will take 15 amps to run then the generator is capable of supplying that current.

If the car needs more then 15 amps then the gen has the ability to supply the current. The problem is when the wires or other things on the wires like connectors can not handle the current.
They will get too hot and fail or become a fire hazard. This is why the fuse is supposed to be sized to protect the wire from getting too hot.

Extra Credit.
Heat is a function of current and time. Too much current for a short time is not a problem as the thing passing the current may not get warm enough to do anything. So a 10 amp fuse may be able to pass 20 amps for 1 second (depending on the design of the fuse and there are a lot of designs). The starter cable can drive the starter to start the car, but too long and something will not be happy.
This is why the idea of a on the starter is not good. It does not protect the wires cause it is actually too large for the wires used on the car. As I like to point out, the toaster can start a nice fire and not blow the fuse.

[Mikeinnj]

Kevin- What does this mean ?????

"This is why the idea of a on the starter is not good. It does not protect the wires cause it is actually too large for the wires used on the car."

[eagle]

The diode cutout listed on the Macs link has a 5 amp limit. It would be fine for daytime driving only, or very limited nightime driving. The instructions say to set the generator (3rd brush) to 6 amps max. I have the Voltage regulator from Macs on my A, has been on for 3 years, and it works great. Haven't touched my third brush since. If you are using your own diode to make a cutout, go by the ampere rating of that diode. I would go less than 1/2 of stated rating to be safe. In other words, if you are going to to a lot of night driving set your generator to 10 or 12 amps, and buy a 25 amp diode. etc.

[Tom Wesenberg]

Be sure your diode has a good heat sink because the diode will heat up and it has no air flow to cool it. A&L makes a quality diode cutout.

As eagle said, the Mac's unit appears to be a light duty voltage regulator, not meant to handle more than 5 amps.

John Regan makes a cutout style voltage regulator that can handle more amps.

[Cool Hand Lurker]

Originally Posted by Kevin in NJ

That is some kind of error in Macs.

I would ignore it.

I would like to correct a concept that was stated earlier.

This statement is not said correctly:
They do have a maximum load rating, like "Max load is 15 AMPS" but that means the diode could burn out if you set your generator output above 15 amps.

You can set your generator to be able to supply 1000 amps, the load is what makes the draw. So if they battery or lights will take 15 amps to run then the generator is capable of supplying that current.

If the car needs more then 15 amps then the gen has the ability to supply the current. The problem is when the wires or other things on the wires like connectors can not handle the current.
They will get too hot and fail or become a fire hazard. This is why the fuse is supposed to be sized to protect the wire from getting too hot.

Extra Credit.
Heat is a function of current and time. Too much current for a short time is not a problem as the thing passing the current may not get warm enough to do anything. So a 10 amp fuse may be able to pass 20 amps for 1 second (depending on the design of the fuse and there are a lot of designs). The starter cable can drive the starter to start the car, but too long and something will not be happy.
This is why the idea of a on the starter is not good. It does not protect the wires cause it is actually too large for the wires used on the car. As I like to point out, the toaster can start a nice fire and not blow the fuse.

The above statement was mine. I think Kevin misunderstood it. Everything he says about the generator being able to supply lots of amps and about too much current going through wires and about other things on the wires not being able to handle too much current is right.

But the diode cutout is one of those things on the wires. If the diode is rated at only 6 amps, then it's capacity to handle more than 6 amps is the problem. If the draw from something is greater than 6 amps, like for headlights, then the diode cutout isn't guaranteed to handle it and has a danger of burning out. It is "one of those things", just like a fuse or wire that is gauged too small.

I don't think I would buy a diode cutout that is restricted to only 6 amps.

[marc hildebrant]

If you read the instructions for the diode cutout, it says to set the third brush for 6 amps during the day.

The "5" is some sort of misprint as the cutout is "just" a cutout and not a regulator. If it was a regulator, it would be a "shunt" type of regulator where you set the third brush for a high value and the regulator keeps the voltage constant by "shorting" the output for various amounts of time.

Marc

[Larry Jenkins]

Way back when the diode idea was available for the Model A cutout, the diode specified was a HP 151. (Maybe HEP 151?) Gettin' old..

You might review those specs and see if anything is different.

Larry

[Kurt in NJ]

I made my diode cutout some 40 years ago using "free" parts, a very simple design---i removed the original relay core, drilled the side of the frame to fit a press in diode from a 60s GM alternator, no special heat sink, ---for the last 15 years the generator has been set to 15 amps, and at times when I was using 50cp bulbs 18 amps so it must be good enough ---

[Mike V. Florida]

Originally Posted by Sparky

I'm planning to convert an old cutout I have with a diode. I have the instructions and it looks simple enough.

My question is about the ready-made diode cutouts sold by the parts houses. Some of them say that the output current is "factory set to 5 amps." How is this done with a simple diode? My understanding is that with the diode cutout the generator third brush still determines the output current.

http://www.ebay.com/itm/121244598283 under 6 bucks -25 amps