Model A Electrical Inquiry re: cut out

[hankirk]

I have a 1929 Ford Tudor. With engine running, I notice that voltage polarity on the generator post of the cut out is reversed from the polarity on the battery side of the cut out according to the volt meter. The generator post shows about 15v and the battery side shows 6v when the engine is running. The ammeter does not register but shows discharge when horn is used. Is this polarity reversal normal and if not, what am I doing wrong? I am using the diode style cut out with positive ground.

[rotorwrench]

The diode is only there to prevent a back current feed to the generator after the engine shuts down. It keeps the generator from motoring and burning itself up kind of like an electrical check valve. The generator flows current one way to the output terminal from the number 1 brush inside there in order to charge the battery. Depending on how much the amps are set to on the number 3 brush adjustment, the output amperage and voltage for that matter (Ohms law) will be at some level above what is flowing from the battery. After the current flows through the diode, it's going to equalize with the battery since it controls the voltage in the system. I'd be more inclined to find out what the output amperage is more than the voltage. The number 3 brush is set with an amp meter.

[hankirk]

Thank you rotorwrench for your reply. If the output from the diode is equal to the output of the battery am I correct to assume that the ammeter will read at zero? In other words, will or will not the ammeter show a charge when the battery level matches the output from the cut out? I am trying to establish if the generator is servicing the battery even though the ammeter is at zero. I hope this is making sense.

[wmws]

If you have a standard system 6 volt positive ground, the system has no voltage regulator. The generator should always be putting some amps into the battery. How much depends on the position of the third brush. You might try lowering the position of the third brush which will increase the output of the generator. Then see if you have an amp reading on the ammeter.

[Tom Wesenberg]

Sounds like you have an open inside the cutout. Either the diode is burned out, or has a bad connection. If it is a voltage regulator cutout, then the output voltage should be 7 volts or a hair more. A&L makes a quality diode cutout, and Bert's sells a good rebuilt original cutout.

[hankirk]

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Thanks for the suggestions. I'll give them a try tomorrow.

[redmodelt]

Without knowing at what RPM he is taking the reading, 15V's seem high. At about 1425 to 1500 RPM on the Wilson test stand, the generator I checked today was getting around 7V's output and set for about 6-8 amps. What is your amp reading? If your amp reading is higher then that you might need to adjust the 3rd brush esp if the reading was at idle or a little above.

[rotorwrench]

If the car has a reproduction amp meter, it should be checked with a known accurate test meter to read the actual current output. I have no trust for the reproduction amp meters and I certainly wouldn't use it to set the output for 6 to 8 amps. That generator certainly could be putting out too many amps which would make the voltage read high too. Be sure and check all your connections.

The Fun Products solid state regulator that looks like a cut out, will actually regulate the current output which in turn regulates the voltage. This is the only recourse to have a 3-brush generator that will actually regulate output to the needs of the battery and system. It keeps the generator from constantly generating unneeded current which lowers the life span of the generator. I just don't know how the availability is on them. John sold the business a while back and I don't know how they are doing.

[Badpuppy]

The polarity will be the same on both sides of the cutout, regardless of type. The voltage difference across it is the concern. Your readings would not be unusual for a regulator type, but the output at 6 volts is too low; it should be about 7.2V, as Tom says. A power diode such as used in the diode type will drop around 1.2 V at any current. Measure between the cutout contacts for the reading. Also check for any ground resistance between generator housing and battery ground post; there should be none.

[Benson]

From 1967 as taught in my IBM tech school:


Voltage drop across a forward-biased diode depends whether it is a Germanium or Silicon diode.



Silicon diodes require 0.7 volts to become forward-biased, whereas germanium diodes require only 0.3 volts to become forward-biased.

[hankirk]

Thanks to all for your helpful comments. All sorted itself out after we changed the cut out and generator. All is running correctly now.

[Badpuppy]

Originally Posted by Benson

From 1967 as taught in my IBM tech school:


Voltage drop across a forward-biased diode depends whether it is a Germanium or Silicon diode.



Silicon diodes require 0.7 volts to become forward-biased, whereas germanium diodes require only 0.3 volts to become forward-biased.

Benson, read the specs for a stud-mount diode as used in these cutouts. Typically 1.2-1.5V drop at rated current. Germanium diodes are for small signals, not power.