Voltage regulator question for you electronic guru's

[Banks]

Finally going to do this but want your input on the devices. are the following ok?

2N5880  transistor

LM7908CT regulator

 

Thanks for our help

 

 

 

 

 

[Banks]

The existing  regulator will stay in place.  I'm going to set it high enough to keep u3p with the headlights and put the electronic regulator between the generator and battery. Depending on engine RPM it will see approximately 9 to 10 volts no load

[servoguy]

One of the potential problems with the electronic regulator is it has no current limiter.  The 12 volt regulator has an internal current limiter, but the power transistors will increase that current limit by the transistor gain hFE.  

If you use two power transistors in parallel, you should add a couple of emitter resistors to balance the load between the two transistors.  Two 0.02 ohm resistors in series with the emitters should be OK.

I think I would opt for the 6.7 volt output.  7.3 volts sounds a bit high.

[Banks]

Hello there, 

1st time the font thing came up on my phone. 

I agree that an alternator would work better. I'm trying to keep it original. It looks like a little time and money will install the regulator circuit I posted. Probably less than buying an alternator converted to 6 volt and definitely less work. 

[servoguy]

Considering that there a gazillion alternators in cars, busses, and trucks, diode failures are extremely rare.  The most common failures I have seen are bearing failures, regulator failures, and brushes wearing out.

[phillyreading]

An alternator would probally be better then a generator as an alternator has more output at an idle speed. An alternator has three fields of current compared to a generator's one field of current, however an alternator can give you other problems(diode failure; when that happens it puts out AC instead of DC)if it goes bad that a generator(only puts out DC) won't do.

[servoguy]

Here is a data sheet for the 2N5881 transistor.  On the data sheet are the complementary PNP transistors.

http://pdf1.alldatasheet.com/datasheet-pdf/view/2489/MOSPEC/2N5881.html

[Banks]

The regulator is definitely working. The voltage responds to slight increase or decrease in pressure on the spring while running. 

I understand about turning the components around. Can you recommend a PNP transistor? 

[servoguy]

If you make the regulator circuit, you have to turn everything around.  Get a pnp transistor instead of an npn.  Turn the capacitor around as it is polarized.  Turn the diode around.  Use the 7908 regulator as you have stated.  Are you going to convert to 12 volts?  Or just use this regulator to charge the battery.

I am not sure you understand what I suggested you do to test the regulator.  When the regulator is limiting the generator voltage, the contacts on the voltage regulator relay should be sparking as the contacts are opening and closing rapidly to limit the voltage.  If the contacts are not sparking, the regulator is not regulating and not limiting the generator output voltage.

[Banks]

From my research I know my regulator controls only voltage. It is a 2 coil unit not 3..

I know it will run the lights for hours at a time with no sparking of the brushes or other ill effects.

I'm thinking of trying this circuit but with negative 7908 regulator and the appropriate transistor. Maybe even two for more current.

 

Thoughts???

[servoguy]

BTW, I have a Masters Degree in Electrical Engineering and 52 years of engineering experience.  I got my first car in 1957.  It was a 48 Ford coupe with a '53 Chrysler hemi in it.  I did all my own maintenance work.  In 1958, I put the motor into a '39 Ford coupe and sold it in '59 to go to engineering school.  I have done almost all of the maintenance work on my personal cars since 1962.

[servoguy]

"it depends on the state of the battery."  Just after starting it will charge 5 or 7 amps. After that it will hover on the + side of 0.  This sounds like proper operation.  After starting, the battery voltage will drop, and the regulator will increase the current until the battery voltage comes up to the proper voltage which should be about 7 volts.  

"Reving the engine wide open reduced discharge to near 0."   This indicates that the generator is too small for this application, and/or that the RPM of the generator is too low.  When you speed up the motor, and the generator still can't quite keep up with the load, it indicates the generator is too small.  

Try this.  With the engine running and the voltage set to 7 volts with the lights off, turn the lights on and see if you see arcing of the contacts of the voltage regulating relay.  You can also use a screwdriver to short the contacts and see if the voltage comes up.  If the voltage doesn't change with the contacts shorted, the generator is putting out all it can.  You can then rev the motor up and short the contacts again to discover what happens with more RPM.  

Let me explain the limitation of these mechanical regulators.  Realize I am an expert on servos and feedback control devices.

The regulator does not have infinite gain, so as you increase the load on the generator, the voltage will drop somewhat.  It appears that the generator is too small for this application, and so when the current command increases, the generator and regulator cannot keep the voltage constant.  

[Banks]

it depends on the state of the battery. Just after starting it will charge 5 or 7 amps. After that it will hover on the + side of 0. Lights on it goes to +/- 5 volts and discharges 5 or 10 amp. Reving the engine wide open reduced discharge to near 0. Bump the voltage  to keep up with the lights and it boils the battery without them. 

[servoguy]

When the engine is running, and the voltage is around 7 volts, what does the ammeter show?

[Banks]

This generator isn't capable of much.  The ammeter is 30 0 30 and I've never seen it past 20

[servoguy]

Years ago, I had a '55 Chrysler hemi in a '40 Ford convertible.  I converted it to 12 volts but kept the 6 volt starter and never had problems with the starter.  However, the engine started easily so the starter was never on very long.

Since the tractor didn't have lights originally, it is possible the generator is just too small.  There are two relays in the regulator that regulate.  One regulates voltage, the other regulates current.  When the voltage relay is regulating, you should see a little sparking of the contacts as the relay is opening and closing the contacts to regulate the voltage.  If the current relay is regulating, you have a load that the generate cannot cope with.  The voltage relay has a coil of fine wire, and the current regulator has a coil of heavy wire.

I don't think you can find a regulator to regulate the high current coming out of the generator.  The existing regulator controls the field current which it fairly low compared to the armature current.  The armature current can get to 60 amps or more which is much more than the 2-3 amps of the field current.  

[Banks]

Thanks for the reply. .

 it has the original regulator. My research in some 1940's Popular Science magazines and the local guy refereed to me by several old tractor owners agree. The regulator on my tractor was used when tractors didn't have lights and the load didn't vary much . He installed a smaller pulley which improved low RPM performance. 

I hesitate to convert to 12  volt because one of my projects in VoTech was repairing a 6 volt starter run on 12 volts. The solder was out of every commutator bar and the  wire ends got mangled. 

I know there is an internal regulator available for AC Delco alternators to make them 6 volt. We had the local guy do one and put it on son's tractor. The only thing we had to do was reverse the polarity of the coil because it was originally positive ground. 

I'm trying to keep this original and was hoping to find an electronic regulator to install between the generator and battery. 

[servoguy]

I have some experience with generators and mechanical regulators.  When you set the regulator to 6 volts with the lights on, it apparently is not regulating because if it were, the voltage would rise only about 0.2 volts when you turn the lights off.  If the generator is not turning fast enough, you will get the symptoms you describe.  One of the things you have to do with a generator is magnetize the field by momentarily connecting the field terminal to the battery.  In your case, that would be done with the negative terminal of the battery.  

If you increase the RPM of the motor, does the generator still exhibit the problems?  It is possible someone put the wrong pulley on the generator, and it is not spinning fast enough.  Back in the days of generators in cars, the engine had to be turning 800 RPM or more to charge the battery, particularly if the lights were on.

Unless you want to keep the tractor original, you might consider converting it to 12 volts, neg ground, and install an alternator.  Most alternators these days have internal regulators and so there is only one wire to connect:  The wire to the battery + terminal.

[KRM]

Banks, My neighbor Jim next door has been seeing the same problem with his 1957 AC D-14. He has found that most of it was with the China knock off replacement regulator he has been using. There is a guy who set everything up for him who does old generators and set the original OEM regulator settings. Look around where your at, you are not alone. the other thing is generators need to be run all of the time or they screw up from non-use.

[Banks]

This is not a train question but an old tractor question. It's still using a 6 volt generator and a voltage regulator. If I adjust the regulator to keep up with the headlights and run a hair over 6 volts it charges well over 9 volts and boils the battery when running without the lights. If adjusted to 7.5 volts without lights it will dischage 10 + amps with them on.

Do you know of an electronic voltage regulator I could put between the generator and battery?

PS...this is a positive ground tractor