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Old 10-19-2011, 05:58 AM
TimCollins TimCollins is offline
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Default How to determine AC/DC

I have a question. I have a small electric motor out of an old jukebox that I think is bad. I found the wiring diagram telling me the motor circuit is 28VDC. However, before I found the diagram I was using a voltmeter testing the plug into the motor and I got about 30VDC however, when I switched the voltmeter to AC I also got a reading of about 13.5VAC. So... my question is, if I did not have the wiring diagram how would I know whether the circuit was AC or DC because I am getting what looks like valid readings from both on my meter?

Thanks in advance for any help...
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Old 10-19-2011, 06:06 AM
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SgtWookie SgtWookie is offline
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Do you have a couple of LEDs and a 1/4W 5.6k resistor?
If so, wire the LED's back-to-back (anode to cathode, cathode to anode) in parallel, and the 5.6k resistor in series with them.
Then connect that across the voltage.

If both LEDs light, then it's AC. If only one lights, it's DC.
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Old 10-19-2011, 06:06 AM
K7GUH K7GUH is offline
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You could look at the waveform at various points in the circuit, using an oscilloscope. Otherwise, you may have to do a lot of guesswork. I should think that a pulsating d.c. current would give you both d.c. and a.c. readings on some types of meter.
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Old 10-19-2011, 06:38 AM
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Oscilloscopes are great - if you happen to have one. Since this board is oriented towards beginning to intermediate level hobbyists, most of the members won't have an O'scope handy.

A couple of LEDs and a resistor is pretty low tech, but it's cheap, easy, and will get the job done.
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Old 10-19-2011, 01:32 PM
TimCollins TimCollins is offline
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Thanks for your answers. I was hoping there was an easy way that maybe I just didn't know about. But, you have answered my question. Thanks!
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Old 10-19-2011, 05:32 PM
colinb colinb is offline
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Maybe the ac voltage is actually a result of the back emf produced by the motor.

Did you measure the 30 V dc / 13.5 V ac with the motor connected and running? Try measuring the voltage with the motor disconnected so that back emf is not a factor. And/or try measuring the voltage with the motor connected but stalled (hold the rotor stationary).
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Old 10-20-2011, 12:17 PM
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The "old jukebox" may be running the DC motor from rectified AC but with no filter caps. That was common in some 1950's equipment, I have a 1950's battery charger that does the same thing.
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