Fixed my porch light, not sure how

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The back porch light stopped working, a new bulb didn't help.
So. I put a meter on the socket. 0 V.
I pulled the switch plate off (just a normal single pole). 80 V terminal to terminal. Hmmm?
Killed the power, put an ohmeter terminal to terminal, infinite resistance at both switch positions. Diagnosis bad switch, supported by the fact that before it died completely, flipping the switch several times made it work. Besides I've always had a CFL in that one and I suspect the capacitor shortens the switch life due to arcing.
Okay, a new switch is $1.29, no big loss if wrong. Took the old switch off, turned the power back on and checked wire to wire just for grins, still 80 V. Uh oh.
Put the new switch in. Turned power on, checked the socket, 120 V. Put bulb in, (CFL), lights up fine.
Well, I have a working porch light again, but I'm left with the 80 V mystery. I don't know any way to get 80 V on a normal residential power setup. The meter was a digital Radio Shack multimeter. If I'd had time I'd have checked again with the Simpson analog, digitals sometimes give funny readings, but I've never seen 80 V. What am I missing?
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A bad contact somewhere? Put a heavy load, an iron or vacuum cleaner on it to make sure there is a solid feed. If those work, stop worrying. If not, start worrying, and start checking backward along the feed. A lousy job, but loose contacts can cause a fire.
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Not sure that makes sense.
Pulling a current across a resistance, like from a bad connection, should cause a voltage drop. But no-load open circuit should be full voltage.
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wrote:

OP said they measured across the switch.
Learn how to use a meter for troubleshooting, and what the reading mean.
The problem IS FIXED. There was a BAD SWITCH.
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Just learn how to use the meter.
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On Tue, 15 May 2012 21:44:22 -0400, snipped-for-privacy@snyder.on.ca wrote:

Don't you have a vacuum cleaner in your tool kit?
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An old analog would likely show different voltages with each range. Carbon in switch ??
Greg
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TimR wrote:

You cannot believe the accuracy of a multimeter with no load on the circuit. The reason is too complicated to divulge here, but it's a fact.
If your meter reads anything above about five volts - when it should be 120 - it really IS 120.
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wrote:

The OP stated "I pulled the switch plate off (just a normal single pole). 80 V terminal to terminal. Hmmm?"
What's wrong with this situation? What is being measured? What is indicated by the fact there is no voltage at the socket and 80 volts across the switch? Don't need to be a member of MENSA to figure this one out. The voltage reading is likely accurate - accurate enough, anyway. Just so happens it is immaterial because it is not a valid measurement. The voltmeter was being used as an ammeter instead of a voltmeter.
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It is often normal to get strange readings with a digital meter, as someone said, too much to go into here. Short answer is the wiring is acting like a capacitor or transformer and the high impedance of the meter is picking up voltage with a very small current. Almost any load and the voltage will dissapear.. If the bulb was still in the circuit, it may be a small ammount of feed through in the CFL.
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On Tue, 15 May 2012 21:14:57 -0400, "Ralph Mowery"

Short answer is the voltmeter was not being used properly, and the longer answer is nobody else here caught it. Nothing to do with inacuracies of digital meters - totally a case of not knowing how to use a meter to troubleshoot - or knowing how to interpret the readings when the meter is mis-connected.
The readings are accurate
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snipped-for-privacy@snyder.on.ca wrote:

The readings are NOT accurate. The indicated voltage is a phantom.
Consider your assertion that the meter is "mis-connected:" The meter has two probes. There are two wires. There are two and only two possible ways to connect the meter and the wires. Which of the two, according to you, is the correct way to connect all this stuff? If both of the two possibilities yield the same result, where is the "mis-connection"?
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On 5/16/2012 5:48 AM, HeyBub wrote:

The OP was measuring across an open switch with no load (no bulb). One might expect zero volts. What was measured was a "phantom" voltage, as Hey says, caused by capacitance between switched and neutral wires to the light and the high impedance of the meter as Ralph said. It is a rather well know quirk (inaccuracy) of digital meters.
The measurement across the switch would be meaningful with an incandescent bulb installed. Or a low impedance meter would measure zero volts (which would tell the OP nothing).
--
bud--




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wrote:

Except the OP had a CFL in the socket when they tested the voltage across the switch.

Because the meter was, as I said, misconnected for the rquired test.
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snipped-for-privacy@snyder.on.ca wrote:

Again I ask, in a slightly different form, what, in your expert opinion, is the proper way to connect two probes to two wires?
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wrote:

Each wire in turn, to neutral or ground. VoltMeter across a switch is a pretty lame attempt to troubleshoot the problem, because no matter what reading he gets, it doesn't mean SQUAT.
You are obviously unable to grasp the concept.
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On 5/16/2012 9:29 PM, snipped-for-privacy@snyder.on.ca wrote:

I read the original post as not having any bulb in the socket. From the OP's post yesterday that is correct.

Bullcrap. There is nothing wrong with measuring the voltage across the switch if there is an incandescent lamp in the socket.
From the OP's post yesterday there is no neutral or ground at the switch. It makes even more sense to measure the voltage across the switch (with a lamp). Repeating Hey's question - how do you connect two probes to two wires, no ground, no neutral.
I have, for instance, measured the voltage across cartridge fuses. If there is voltage the fuse is open. If there is no voltage further checks are required. (But then the US does not have fuses that "weld themselves back together and work for a while".)
--
bud--



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wrote:

OK. EXACTLY what ic onnecting the meter across the switch telling you? If you get a non 120 volt reading either a)- there is no power to the switch B) the switch is shorted or closed, C) the bulb is open, D) there is a bad connection between the switch and the bulb, E) there is a bad connection between the bulb and the neutral buss on the panel, F) the meter is defective.
Take your pick - you really have not learned anything of value.
If on the other hand you get a 120 volt reading, you know the switch is open and there is a measure of continuity between the switch and the neatral buss, as well as between the switch and the fuse/breaker, which is supplying power.
If the test is done with no bulb in the socket - (no load) there is no valid reading that tells you ANYTHING.
And that's no bullcrap.

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On 5/17/2012 6:52 PM, snipped-for-privacy@snyder.on.ca wrote:

You may not have learned anything.
A competent technician can evaluate what is happening. The information is combined with the test below - if there is 120V with the switch open it leaves B.
If there is 120V in both positions the switch is bad. That is what the OP would have found if he had an incandescent bulb in the socket. A simple test.

You might not figure out anything. But a competent technician would evaluate no bulb and 80V in both positions as an open switch. I believe that is what the OP found.
I said 2 times have an incandescent bulb in the socket.

Only if your grasp of electrical troubleshooting is defective.

You have still not answered Hey's question. The OP said there is no neutral or ground at the switch.
You are obviously unable to grasp the concept.
--
bud--


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wrote:

Forget it bud. I do this diagnostic stuff a lot, and have for almost 50 years.
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