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Old 04-17-2010, 06:50 PM   #1
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ceiling fixture wires - neon tester glows when off


Replacing an old ceiling light with a new one. It is controlled by two switches. A neon test light DOES NOT go completely out. It glows bright at one switch setting and dim at the other. What is wrong? (perhaps hot/neutral wrong on one of the two switches?)

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Old 04-17-2010, 08:50 PM   #2
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ceiling fixture wires - neon tester glows when off


Is either switch "illuminated" to make it easer to find at night when the light is off? Switch lights form a complete circuit together with the light being controlled for just a small current flow.

Also could be phantom or induced current sufficient to light the neon lamp; this is more likely with 3 way switch setups as opposed to single switches since we have a dead wire connected to the light juxtaposed with a live wire (the two travelers) over a fairly long distance.

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Last edited by AllanJ; 04-17-2010 at 08:57 PM.
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Old 04-17-2010, 09:46 PM   #3
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ceiling fixture wires - neon tester glows when off


No.. standard on/off three-way switches. A really cheap analog meter shows slight voltage. Strange, no matter what scale the the meter is on it deflects about the same so I can't give a voltage reading. But it low. Yet the bulb tester glows slightly.... nothing to worry about?
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Old 04-18-2010, 08:38 AM   #4
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ceiling fixture wires - neon tester glows when off


I would not worry about it.

A voltmeter requires a minute current flow (must consume a minute amount of electricity) to work. In an induced current situation the amount of current that can possibly be drawn is very small. One of the consequences of this is that the voltage actually measured by the meter cannot be easily predicted. Thus the needle movement you see does not appear to make sense.

The voltmeter represents a resistance, each range (10 volts, 100 volts, 1000 volts, etc.) represents a different resistance. Voltage across any two points (for example the test prods) always equals current flowing from one point to the other times the resistance between those two points (Ohm's Law). The concept of Ohm's law is the same although the math is different and more complex when dealing with alternating current and current induced through the insulation of juxtaposed wires.

In some cases the night light in a switch consumes sufficient power (allows sufficient milliameters to pass) that a compact fluorescent light (doesn't need that many watts, or draws that many milliamperes, itself) being controlled by the switch glows a little.
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Last edited by AllanJ; 04-18-2010 at 08:44 AM.
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Old 04-18-2010, 09:07 AM   #5
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ceiling fixture wires - neon tester glows when off


Quote:
Originally Posted by 1611mac View Post
Strange, no matter what scale the the meter is on it deflects about the same so I can't give a voltage reading.
This is a phantom voltage.

The reading changes little because you are measuring the output of a current source, the current source being 120vac in series with several hundred kilo-ohms of capacitive reactance.

If your meter has a sensitivity of 1000 ohms/volt and the meter reads half scale, then 0.5 mA is flowing, barely enough current to feel in your fingers. The capacitance of 110' of Romex at 60 Hz will give you about 0.5 mA with 120vac applied.

A DVM with constant, high input impedance regardless of the scale selected should read a fixed voltage close to 120vac.

Wiki talks about current sources and the "dual", a voltage source, but it's not written in a way that's useful for this kind of hands-on stuff.

Last edited by Yoyizit; 04-18-2010 at 09:17 AM.
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