Fixed my porch light, not sure how

[TimR]

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?

[Sjouke Burry]

TimR wrote in news:2fad4ded-a0ce-4fae-9166- [e-mail]:

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.

[TimR]

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.

[gregz]

An old analog would likely show different voltages with each range. Carbon in switch ??

Greg

[HeyBub]

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.

[Ralph Mowery]

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.

[clare]

80 volts terminal to terminal on the switch means 80 volt drop across the bulb. Same with switch o or off means the switch was no good. Feed to ground would read approxemately 120 volts. With bad switch load to ground would read zero. Good switch, load to ground 120.

[Stormin Mormon]

Sometimes, with no load, a digital meter will pick up a "ghost load" reading. Or ghost voltage, can't remember. The two wires next to each other have a very slight transformer effect. Not enough to light a bulb, but enough to read with a DMM.

Christopher A. Young Learn more about Jesus

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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?

[clare]

Just learn how to use the meter.

[clare]

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.

[clare]

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.

[clare]

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

[Metspitzer]

Don't you have a vacuum cleaner in your tool kit?

[HeyBub]

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"?

[HeyBub]

Huh?

By definition, with a functioning switch energized, both sides MUST read the same and that "same" must be zero. With the switch in the OFF position, and using a digital voltmeter, you can get any reading from zero to 120.

[N8N]

If you measured across the two wires that were connected to the switch, you're measuring the potential from hot *through the bulb* to neutral. If you want to sleep better, pull the switch off again and measure from hot to ground, you should then read 120V or thereabouts. If the bulb is a CFL there may be some odd effects causing an incorrect voltage reading the way you measured it.

nate

[bud--]

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).

[TimR]

I started with the assumption the switch was bad, did the checking mostly out of curiosity (and of course, when trying to figure out which breaker controls the circuit and the light is NOT working, it's handy to have a way to know).

And yes I've had trouble with digital meters and phantom results on no load circuits, that's why I asked (and why I have a Simpson on the shelf, but unwinding all those probe wires is a pain, not as bad as winding them up to get them back in the case though).

But on 120 V circuits normally that meter reads okay or floats a bit.

I left out a couple of things. The fixture had no lamp in it, so that circuit should be open. The kitchen lights are on the same circuit, they went out when I got the right breaker (if I'd known that ahead of time I wouldn't need a meter, technically).

The 80V terminal to terminal on the switch was with the switch off AND no lamp in the fixture. With switch on, it went to 0 as I'd expect. It was consistent with several measurements and was the same as wire to wire with the switch removed (and still no lamp in the fixture). That surprised me. And then when I went outside and put the same meter on the fixture and got 120 V I was suprised again.

After work tonight I'll put the analog on it and see.

[trader4]

I would have tested for 120V using one test lead on the light side of the switch and the other held to ground. I'd use ground because that's usually there and available without taking anything apart. Or he could have tested from the light side of the switch to neutral.

[Ralph Mowery]

One thing that you can also try with the analog is to set it to the highest voltage scale. If you show anything that is less than the next scale down, switch to that scale, the meter will stay almost the same physical place if a 'phantom voltage', you can then go to the next lower scale and the meter will not move much from the same physical point. The actual voltage the meter reads will be differant, but the physical point will not move much.

This digital vers analog comes up almost as much as the WD-40 being oil or not.