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Q? for John T - field resistor ohms

Bob M

Bob M

Well-known Member
John T - Do you happen to know the correct value for the "low charge rate" field resistor for a Delco 6 volt, 3-brush generator+cutout charging system?

I seem to recall 4 - 5 ohms - but cannot recall now where I saw it.

Thanks!! ....Bob M
 
Bob, I think thats a little high and Ive read (cant recall where) numbers more in the 3 ohm range.

Heres my reasoning FWIW why 3 is about right.

Typical field currents (for some Delcos Im aware of) are like 2 amps which means in a 6 volt genny the total resistance of the two field windings is about 3 ohms sooooooooo if you add another 3 ohms that reduces the gennys output significantly but I think 4 to 5 ohms would reduce it wayyyyy toooooooo much.....

Thats my story and Ima stickin to it

John T
 
Thanks for the speedy response guys - that's the info I need!

----

Jim - I believe your power disipation is off by about factor of 20(!). Calculation:

Genny field resistance = 3 ohms (thanks John T!); field resistor = 3 ohms. Total resistance field circuit resistance is thus 6 ohms.

Assume the system voltage is 7.5. New using E = IR (rearranged to I = E/R) field current is:

I = 7.5 / 6 = 1.25 amps

Power dissipation across the series resistor (I squared R) then becomes:

1.25^2 x 3 = 4.9 watts(!)

Therefore 5 watt resistor will be more than adequate.

Bob M
 
Hey Bob, John, Jim.

Went to the shop and measured two different ones and got 2.8-3.2 ohms. Checked my IH shop manual and the spec is 2.8 ohms.
FWIW the spec for the MD 12 volt is 8.0 ohms.

Right on I would say!!!
 
Bob,
In the military we had to have things made easy to remember - relating to either females or food.

P=IE Power(watts)= I(current)X E(volts)
 
One other titillating tidbit

In the military, DC Current flows in the opposite direction than civilian current.

+ to - instead of - to + (or was it the other way around)
 
Calculations are a good thing, I agree. The original is a wire wound exposed mica slab that is much closer to 40 or 50 watts dissapation than 5. I think a 5 watt color banded resistor would turn black and be toast. the high watt resistor also allows it to be cooler (surface heat transfer to other wires in the box.) Jim
 
Based on where the third brush is, voltage in the field circuit has to be close to the voltage at the A terminal. 5 to 6 volts seems like a good guess. Dividing that by 2.8 ohms gets about 2 amps. Multiply volts by amps and you have 10 to 12 watts.

Based on what resistors are easily obtained, I have suggested getting a pair of 5 ohm resistors rated for at least 10 watts. Connect them in parallel, giving 2.5 ohms and 20 watts. Leave the leads long enough that the resistors can be separated a bit from each other and the switch. Keep other wiring separated from them.
 
Thanks Geoff. However I need only a temporary resistor in order to run my Minnie-Mo U at the Christmas tree farm this weekend without boiling out the battery. (Will be replacing the original cutout with a voltage regulator when time permits.)

...Bob M
 
For temporary use, just connect the generator field to the unused F terminal of the cut-out. That will give you low charge. Add a short jumper wire with an alligator clip to either terminal and clip the other end to a ground for high charge.

Better yet: Add the above mentioned jumper from F to F, remove the resistor from the switch. The switch will control charge rate normally.
 
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