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rheostat for old walthers turntable

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rheostat for old walthers turntable
Posted by banjobenne1 on Wednesday, June 27, 2018 7:03 PM

Hello I have an old (non digital) Walthers turntable, what type of rheostat do I need to control the speed?

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Posted by richg1998 on Wednesday, June 27, 2018 10:40 PM

banjobenne1

Hello I have an old (non digital) Walthers turntable, what type of rheostat do I need to control the speed?

 

You don't want a rheostat. A regular old DC power pack will work just fine.

That is what I use to use.

Rich

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Posted by banjobenne1 on Thursday, June 28, 2018 10:22 AM

Yes I aggree old DC power pack will work, however I have built a control board for the roundhouse, and turntable useing hard board, and have left space for this rheostat so it all looks good and uniform. So I still need the specs for this rheostat. Thank you for any help you may offer.

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Posted by RR_Mel on Thursday, June 28, 2018 1:06 PM

We need the motor specs to calculate the resistance of the Pot.
 
 
Mel
 
 
My Model Railroad   
 
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I'm beginning to realize that aging is not for wimps.
 
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Posted by richg1998 on Thursday, June 28, 2018 7:13 PM

Are you looking for a rheostat or potentiometer? Those are two different animals.

I suspect the motor will be in the 500 ma range. We never did measure the current. If you have a power pack, measure the current first, then buy a rheosat. I have not bought a rheostat in years. They are tough to find. potentiometers are not high current. Our club had one of those TT's but we used an old power pack. We saw no reason to go any other way.

Rich

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Posted by banjobenne1 on Thursday, June 28, 2018 8:51 PM

Yes, I suppose I should be talking about a potentmeter, anyway the motor is 24ohms just sitting dead and works with no more than 12 volts.

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Posted by richg1998 on Thursday, June 28, 2018 9:10 PM

I doubt a pot will work as they do not handle much current by them selves. Most Pots are rated at two watts from what I recall. Do the math.

They ususlly are used with a power transistor and you need a switch for reverse which I am sure you know. Years ago I made my own DC power packs with a 10k pot, low power npn transistor, and 2N3055 power transistor.

Connect your multimeter with a power pack and TT and turn up the pack and watch the meter. Easy enough to do. Every model railroader should have a meter.

Rich

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Posted by RR_Mel on Thursday, June 28, 2018 9:29 PM

You might try a 250Ω 5 watt wire wound potentiometer.  I would keep a meter in series with it to make sure the current doesn’t exceed the 5 watt rating.  You can remove the meter when your satisfied with the operation of the bridge and speed control and sure the current doesn’t run more than the rating of the potentiometer.
 
Mel
 
 
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I'm beginning to realize that aging is not for wimps.
 
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Posted by banjobenne1 on Friday, June 29, 2018 6:44 PM

So at 3.6 volts the amps are 45 to 46ma.

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Posted by RR_Mel on Friday, June 29, 2018 7:44 PM

Not with a 24Ω motor, 3.6 volts in a 24 ohm load is .15 amps or 150ma.
 
Why 3.6 volts?
 
 
Mel
 
 
My Model Railroad   
 
Bakersfield, California
 
I'm beginning to realize that aging is not for wimps.
 
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Posted by banjobenne1 on Saturday, June 30, 2018 8:35 AM

Of a total of 12 volts the table moves at the right speed 3.6 volts 45 to 46 ma.. I am reading whats shown on the meter. I guess I could take a photograph if you like. 

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Posted by RR_Mel on Saturday, June 30, 2018 11:36 AM

An 8.4 volt drop from 12 volts at 45 ma would require a 186.66 ohm resistor at .378 watts, I’d go with a one watt resistor instead of half watt.
 
180Ω would be the closest standard value.  180Ω should be about 3.8 volts.
 
 
Mel
 
 
My Model Railroad   
 
Bakersfield, California
 
I'm beginning to realize that aging is not for wimps.
 
 
 
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Posted by banjobenne1 on Saturday, June 30, 2018 12:52 PM

How about we back up a little bit. Since the decimal point on my meter doesn't move lets say the amps are a little less 0.5 amps.. Are you suggesting I use a one watt resistor instead of a potentiometer?

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Posted by RR_Mel on Saturday, June 30, 2018 1:27 PM

.5 amps is a lot of current at 3.6 volts.  That would mean the motor has 7Ω resistance.  If you need to change the speed you will need a potentiometer.
 
Assuming the motor DC resistance of 24Ω at 3.6 volts the current should be 150ma.  To drop 12 volts to 3.6 volts at 150ma the resistor would be 56Ω at 1.26 watts.
 
 
Mel
 
 
My Model Railroad   
 
Bakersfield, California
 
I'm beginning to realize that aging is not for wimps.
 
 
 
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Posted by banjobenne1 on Sunday, July 01, 2018 8:20 PM

I don't think we are talking about a large sum of money here, so I would really like to get both a pot and a resistor. Try them both see which works best. The specs for the pot would be............ ? Thank you, you have been most helpful.

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Posted by RR_Mel on Sunday, July 01, 2018 9:11 PM

Assuming the motor resistance is 24Ω: The low end of a 500Ω pot would be 22ma.  The low end of a 250Ω pot would be 45ma.
I would expect that the motor at 3.6 volts will draw 150ma that = .54 watts.  The higher the voltage the higher current through the pot.  I would try to find a 5 watt pot to be on the safe side.
 
 
 
Mel
 
 
My Model Railroad   
 
Bakersfield, California
 
I'm beginning to realize that aging is not for wimps.
 
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Posted by wjstix on Monday, July 02, 2018 4:15 PM

In the old DC layout days I used a 25-Ohm / 3 watt rheostat and DPDT-center-off toggle switch to run trains with a homemade tethered throttle. I would think it would work for a turntable too.

http://www.hollandcomputers.com/store/pc/25-Ohm-3-Watt-Rheostat-5-Tolerance-Wirewound-Variable-Resistor-Potentiometer-p5991.htm

Stix
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Posted by RR_Mel on Monday, July 02, 2018 5:52 PM

A 25Ω would work with a .5 amp or more motor but not with a low current motor at 150ma.
 
 
Mel
 
 
My Model Railroad   
 
Bakersfield, California
 
I'm beginning to realize that aging is not for wimps.
 
 

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