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Old 08-21-2012, 11:56 PM   #1
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Hoping I can use this gizmo I found. help figuring it out? variable resistor?


My brother is into scrapping. pop let him scrap a couple dozen old "film projectors" we have had for years and are worthless :-)

anyway they are YELCO film projectors. they had this VARIABLE speed control knob marked slow fast

I AM REALLY hoping I can use these as low power "variac's" (I use variac's to lower the voltage to chines 120v LED bulbs that are run "too hot" lower the voltage and you lower the wattage and they stop "burning up" :-) problem is they are EXPENSIVE and BIG

these kind of look like variac's so fingers crossed.

what are they? do they run at 120v AC ? Can they ? if not what do they run at ?

Here are some pictures.






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Old 08-22-2012, 06:22 AM   #2
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Hoping I can use this gizmo I found. help figuring it out? variable resistor?


These are heavy duty potentiometers, used in the projectors as rheostats and they can be used as dimmers also.

However they are energy inefficient, turning a lot of electrical energy into heat.

Some electrical engineering math is needed to use them as dimmers, and they will probably not work with anything other than incandescent and LED lamps. (The contour of bright to dim as you turn the dial will vary depending on the lamp wattage.)

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Old 08-22-2012, 08:12 AM   #3
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Hoping I can use this gizmo I found. help figuring it out? variable resistor?


so these will act as a variac? ie I can plug 120v AC into it and it will "lower" the voltage of electricity passing through it?

I am talking running 3-6 3or4 watt led bulbs watts of LED bulbs through it. Will it handle that?

I know it won't be efficient but at 4-16 watts the inefficiency won't amount to very much. typically I need to drop the voltage to around 90-100 volts to get the LED's into optimum power bands so they don't over heat.
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Old 08-22-2012, 09:25 AM   #4
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Hoping I can use this gizmo I found. help figuring it out? variable resistor?


These potentiometers will work equally well on AC or DC.

You will need a minimum of 20 watts of total load to drop the voltage under 100 volts. Ten watts will allow dropping the voltage to around 108.

I am guessing that the maximum wattage for the load is around 75.
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Old 08-22-2012, 10:41 AM   #5
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Hoping I can use this gizmo I found. help figuring it out? variable resistor?


108 is not bad. not bad at all I can live with that. especially since they are free :-)

how do I wire them up? it has 3 wires. I assume the brown and the white are in and out hot

what is the black? do I use that on AC ? is it both an in and an out ?

now is this actually lowering the voltage or is it enlarging the "null zone" in the AC cycle like a regular dimmer does?

LED don't like those kinds of dimmers (they flash or don't work at all)
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Old 08-22-2012, 10:49 AM   #6
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Hoping I can use this gizmo I found. help figuring it out? variable resistor?


It's simply lowering the voltage. Use the black and brown so turning it counter clockwise will lower the voltage. I believe your rheostat is rated at 25 watts BTW.

White is not used.

Last edited by zappa; 08-22-2012 at 10:53 AM.
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Old 08-22-2012, 11:15 AM   #7
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Hoping I can use this gizmo I found. help figuring it out? variable resistor?


does it matter which is which? does one need to be source side and led side?
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Old 08-22-2012, 11:21 AM   #8
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Hoping I can use this gizmo I found. help figuring it out? variable resistor?


Quote:
Originally Posted by nerys View Post
does it matter which is which? does one need to be source side and led side?
I think you have it. Wire it in series, source and LED. DO NOT wire it across your line voltage. It doesn't matter which color wire goes where in series.
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Old 08-22-2012, 11:32 AM   #9
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Hoping I can use this gizmo I found. help figuring it out? variable resistor?


excellent when my next batch of LED's comes in going to give it a try. I like to use twice as many led's as I need and then lower the power so they are as bright as half. dramatically increases their lifespan !!

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