Is it possible to vary the output of a 3 wire 10SI by putting a resistor inline with the F terminal? I think this would lower the output voltage? Or is this not possible with the internally regulated alternators?

Thanks!

Only way to control what the regulator puts out is to alter what is seen on the S or sense wire. The internal reg tries to keep the voltage on this wire at ~14.2v. Putting a resistor in this wire would cause it see a lower voltage and put out more. Are you trying to get more or less out of your alt? Where is the S wire hooked to now?

I have it wired like the top example:

http://www.chevelles.com/showroom/data/516/laltwire.gif

I'm trying to lower it's output.... I have it hooked to a 3hp Briggs and when the battery is really low, it requires too much power to turn. If I could lower the output, I think it would work. Ideally, if I could use a variable resistor to adjust the output, I'd be happy as a clam :)

I thought that it could be wired into the F terminal? But now that I think about it, maybe you're right. That would increase the output... nor reduce it?

On the plus side, it seems to work nicely if the load is within the engines range. Tops off a battery in no time! I've been able to support about 40A. any more and it snuffs the engine, hehe!

One other quick question if you don't mind.... :)

The switch on the R terminal supplies 12v to the R teminal to turn the alt "on". One it's running, turning the switch off again has no effect. Would grounding the R terminal turn the alternator "off" again and remove the load from the engine?

Thanks!

I don't think the R terminal is supposed to be grounded. It's either open, no connection, or has voltage to tell the alt to turn on.

Lowering the output won't be easy. You can make the cable from the alt output to the battery a very long cable so there is a voltage drop. That will reduce the current as the alt is sensing voltage at it's output not the battery. You could also put a resistor in line between the alt and battery but it would have to be something that could handle a lot of wattage, good luck finding something that size.

Put a smaller pulley on the alt or larger on the engine.

Thanks Hank! Further experimentation today showed me just what you said. I used a really long wire for the charge wire. It was a 100ft spool of 14ga, and had enough resistance to lower the output quite a bit, from about 35A to 5ish Amps). Maybe too much, but still showed me that a properly sized resistor could work nicely. (I also figured out that tuning the mixture on the Briggs carb helps too, hehe!)

You're right though, finding one to handle the load for extended periods might be tough. I bet I'll try to find something though. I don't know why, but useless projects really capture my attention, and I can't give up :)

First, are you sure its a internally regulated alternator? What kind of voltage readings are you getting??? During the transitional yrs, GM alternators were marked with both 1 and 2 and "F" and "R". You can usually tell the type of regulator by looking at the terminals. Heres a pic of a 1972 externally regulated alternator
http://www.chevelles.com/showroom/data/500/Dsc00886.jpg
If the terminals are turned the other way, like this its internally regulated.
http://www.chevelles.com/showroom/data/532/Alternator1.jpg
You pic showing the wiring diagram for the external mechanical regulator really won't work. The "R" terminal controls the points in the regulator, also needs 5 connections, Switched 12 volt, BAT, field, relay, and ground. The best way to control output would be by gearing. Try changing the gearing to 1 to 1 or even underdriving some. I know I said 2 to 1 in my first post but didn't realize that the alternator might see a heavy load even at slower speeds. At idle the alternator output will be low and might not stall the engine. When you speed up the engine, output will increase and the engine will be more in its power band. I have never built anything like this, only trying to help.

I had that backwards in my previous post, smaller pulley on the engine or larger on the alt should help. Think that's what Greg was saying. It will slow down the alt but the engine may be handle the load then.

The wire on terminal #2 measures the voltage for the regulator. So, make it see 14.4V when it's not really 14.4V and you'd limit the output voltage. But, you'd need some type of external voltage source to do this.

Your wire idea should work as long as you allow for air movement over the wire so it doesn't overheat. Or, maybe try using some headlights between the alternator and the battery.

Peter

Greg - It's definately a 10SI internally regulated alt. Here's a pic:

http://www.chevelles.com/showroom/data/500/Picture_010.jpg

The voltages while it's running on a dead battery is about 13.75 at the battery, and 14.0 at the alt batt stud.

I understand what you guys are saying about gearing. I had wanted to keep the alternator at 5000 rpm or so, so I geared it up with a 4" pulley on the engine. Now I have a 3" which is about 1:1 with the alternator. (I assume that the briggs is about 3600 rpm) I might try using a smaller pulley yet on the engine and see what that does.

Peter, funny thing you mentioned headlights inline with the batt. My buddy was trying to get me to rig up the same thing! I understand how it works, but it's really counter-intuitive.... the alternator is taking too much power to charge the battery, so run a headlight at the same time and it won't load the engine so much? But, yep that's the way it works! :clonk:

HEre's a couple more pics.... I think it's pretty neat looking, even if it isnt' very useful :)

http://www.chevelles.com/showroom/data/500/Picture_013.jpg
http://www.chevelles.com/showroom/data/500/Picture_009.jpg
http://www.chevelles.com/showroom/data/500/Picture_014.jpg

Cool, you really need to get a ceramic coated header for that baby, then it would be totally awesome. Chevy orange on the engine too.

The headlight is another resistance in the circuit, just like 100' of wire. The more resistance you add in series on the output the more the current ouput will drop.

If the alt is putting out 60 amps at 14v that's 840 watts or 1.1256 hp. If the pulleys are 1:1 then the engine should be able to handle that. Even if it's 120 amps that's only 2.5 hp. Anyway reduce the pulley ratio, the alt may not put out at full capacity but it will charge the battery and eventually get to 14.2 volts. Obviously the engine can't handle the alt when it's putting out max amps. A headlight, resistor, or different pulley ratio will all give the same results. Changing the pulleys will just keep it a cleaner setup.

I would put a K&N air filter on that sucker too. :D

You should probably have a light bulb between the wire on terminal #1 and the power where you're connecting it. I'm not sure what's inside the alternator connected to that terminal but I know it won't like being connected to power without the alternator running. If you're not going to put a bulb then just put a momentary button on it because when it's running and you just touch power to the terminal the regulator turns on and doesn't need power there anymore.

I'd think the alternator starts working fairly well at about 3000 rpm so once you get to that rpm anything more is just loading the engine more. In the car they usually use about a 1:3 pulley ratio. Most alternators will put out full voltage into a fairly heavy load at about 1000rpm engine speed which equates to about 3000 rpm at the alternator.

Peter

HEre's a couple more pics.... I think it's pretty neat looking, even if it isnt' very useful :)

Cool, you can make it more useful if you add a 12 DC to 120 AC inverter. You can buy them of there are plans on the web to build your own.