47 VAC at ceiling box

Sep 07, 2006 4 Replies
J

I have the ceiling box open for my kitchen light fixture (getting ready to paint.) With no load across the leads that feed the ceiling light and the switch off I measure 47 VAC. If I put a 60W bulb across these leads the voltage goes to zero. When the switch for this fixture is turned on I get a normal 120 VAC.



Can I assume the 47 VAC is just some coupling from an adjacent circuit into a high impedance load?



FWIW, this light fixture is serviced by two three way switches and the color coding does not follow any standard I have seen (there is no red wire that I have found yet.)


John Keith snipped-for-privacy@juno.com


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N

Yes - especially if you are using a digital meter

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J

I am using a digital meter, and I've heard that comment before but I've never understood why a digital meter responds differently than an analog meter. Both present a high impedance (11 Mohm?) to the circuit under test. What is the difference?

John Keith snipped-for-privacy@juno.com

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D

If they both present the same impedence, then there is no difference. But a really good analog meter will top out at 20Kohms/v and the really cheap ones are much less than that. Any old timers will remember VTVMs (vacuum tube voltmeters) that presented impedences above 1Mohm/volt. Today's digital meters are all above 1Mohm/v which is significantly above that of an analog meter.

dickm

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M

The difference isn't about analog or digital. It's about high-impedance input circuitry. Most (if not all) digital meters have it. Some analog meters have it.

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