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Delco and other Low Voltage DC Light Plants Antique Generators, Light Plants, Typically 24, 32 or 48 volt although some are 110 volt. DC Lamps, Motors and appliances.

Delco and other Low Voltage DC Light Plants

Calculate the correct resistor to use?


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Old 10-14-2018, 03:21:47 PM
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Mike Schweikert Mike Schweikert is offline
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Default Calculate the correct resistor to use?

My brain hurts....

This is the wirewound resistor off of Western Electric light plant #2 that I have. It is still intact, where the one on #1 light plant is a mess of broken magnet wire ( cannot save it period).

The purpose of it is in the charging circuit, when in battery charge mode, the current is limited through this to the battery. When in overcharge mode, the small portion of the resistor is used via a knife switch to go from one to the other. I get it changes the resistance to either charge at a reasonable rate or overcharge to equalize the battery set.

I need to figure out what wirewound power resistor to use. So far I have tried to meter it from lead to lead on both sections. Perhaps 175 milliohm for the large side and 50 milliamps for the small one. That does not make sense to me..

Ok, then tried to figure out amps. Hooked a 12 volt battery on one lead and a meter lead on the other and one on the battery. Large section dropped from 12 volts to just under one. Small section dropped to 1.5.

Then I put a 12 volt bulb in series with it and measured at battery. About half an amp for both. I am going to use 3 12v garden tractor batteries.

The specs for the plant are 1500 watts at 40 amps at 40 volts. It uses a charging relay in series with this resistor. The literature suggests to adjust it so the relay opens when the current drops to about 14 amps, stopping the charge. The literature says the batteries are fully charged. It supposedly takes 7 hours to charge the 180 amp batteries.


I've been on those sites with the calculators for watts and power resistors The literature does not specify the ohm or watt rating for this resistor. I am not coming up with a plausible replacement.


Could someone hurt their brain too and let me know what you think??


Thank you
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Old 10-14-2018, 03:49:38 PM
Zephyr7 Zephyr7 is offline
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Default Re: Calculate the correct resistor to use.....

Hmm. You need to measure the resistor you have correctly to be able to get a replacement. To get the value, measure resistance across the two ends (the tap in between doesn’t matter for this test). You should see some number of ohms, although there are fractional ohm resistors out there. You mention seeing milliohms and also milliamps so I’m not sure what you were measuring before.

If you can’t measure the resistance directly, try connecting the resistors ends (ignore the tap again) in series with a lightbulb and an ammeter. Connect 12v dc or whatever the supply voltage is. The ammeter will tell you the current the lamp is drawing through the resistor. Measure the voltage across the ends of the resistor with a volt meter. From this info, you can calculate the value of the resistor using ohm’s law like this:

Volts you measured = current you measured * R
Solving for R gives:

Volts you measured / current you measured = resistance

This method will work reliably for even very small resistance values.

You’re going to have to guess at the power rating. I’d get a resistor that is close in physical size to the one you have and try that. You need a variable tapped wire wound resistor which will have a similar sliding tap to what you have now.

I hope this helps you with your restoration work.

Bill
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Old 10-14-2018, 03:59:29 PM
Vanman Vanman is offline
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Default Re: Calculate the correct resistor to use.....

Does that resistor actually go in series with the battery and generator armature? Or in series with the generator shunt field?
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Old 10-14-2018, 07:35:52 PM
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Default Re: Calculate the correct resistor to use.....

Thank you Bill. I did connect one end of it to the 12v battery, and had a 12v light in series with a meter. I had measured half an amp. That would give me a 24ohm resistor. I think the half an amp will be the same though 36v?


I believe the resistor is in series with the battery and series field (reality check welcomed...) Here is the diagram for it.
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Old 10-14-2018, 10:35:36 PM
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Default Re: Calculate the correct resistor to use.....

Zephyr7 looks like from the diagram there are two separate resistances. I would try measuring each resistance using sharp tipped probes. May have to measure from each end of each resistance in several places if resistance wire is broken in several places. Should get an approximate resistance for each resistor. I think I even confused myself... Allie
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Old 10-15-2018, 10:13:21 AM
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Default Re: Calculate the correct resistor to use.....

There are 2 separate windings of wire on the tube. You can see it in the picture. The wire is wrapped around a piece of conductor at each "end" of the resistance tube wiring for a total of 4 wires coming from it.

I have been able to pass current through it at .5 amp so far at 12 volts.

I am going to get a different meter to check what I had seen earlier. Milliohms just does not seem right at all....
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Old 10-15-2018, 11:03:49 AM
Zephyr7 Zephyr7 is offline
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Default Re: Calculate the correct resistor to use.....

Quote:
Originally Posted by Mike Schweikert View Post
Thank you Bill. I did connect one end of it to the 12v battery, and had a 12v light in series with a meter. I had measured half an amp. That would give me a 24ohm resistor. I think the half an amp will be the same though 36v?
It’s unlikely the current would be the same at a higher voltage. Assuming the lamp and resistor don’t change, 36v would give a higher current. If you are connecting the resistor directly across a 12v supply to see that 1/2 amps current, then the same resistor connected across a 36v supply will give 1.5 amps.

Quote:
Originally Posted by Mike Schweikert View Post
I believe the resistor is in series with the battery and series field (reality check welcomed...) Here is the diagram for it.
It looks like that resistor is actually two seperate resistors in a single package not connected in any way. That’s probably a custom resistor, you you’ll have to replace it with two seperate units. If you use the hollow cermic resistors, you can put a mica washer or two (these are insulating washers that can take some heat) between them, you can mount both with one long screw and maybe install it the same way the original resitor is mounted.

The sharp-pointed probes for the meter are a good idea. The points will poke through the oxide layer on the terminals to get a more reliable measurement. Poke them with some pressure, wiggle them around a little while keeping the point in the same spot, then hold them tight and take a reading.

If the resistance wire is broken you’ll need to measure the length of the wire and the resistance of the longest piece. Then you can calculate what the total resistance should be for the resistor so that you can order a new part.

Bill
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Old 10-15-2018, 01:31:08 PM
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Default Re: Calculate the correct resistor to use?

I've reread the original post. You have two power plants. Are the #1 and #2 referring to the model numbers or #1 refers to one of your plants and #2 refers to the other. If both plants are the same model, I would try measuring the resistance of the undamaged resistor and procure a couple standard value power resistors closest to the measured resistance. Allie
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