no voltage in switch and lighting fixture wires

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  #1  
Old 09-27-12, 09:02 AM
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Question no voltage in switch and lighting fixture wires

I have a lighting fixture, which was hooked up to a dimmer switch. Whenever
you turn the dimmer switch on and to full, it would not light up. This used to
work. I dont know how long this has not been working for and the tenant has moved out of the place as well.

I took the light fixture out, and connected a multi-meter to it, one end to the
black wire and one to the white wire. I was expecting to see 120V (or close to
it) when I turned on the dimmer, but what I get is 18.1V instead.

That doesn't sound right to me, so I took out the dimmer switch, disconnected it, and put the multimeter between the black and white wires of the cable coming out of the wall, and that only showed 46-47V. I would have expected
that to be 120V as well.

This is in the dinning room and the light fixture in the kitchen, which is next to the dinning room and I think on the same breaker, is also showing the same signs.

How do I debug this problem and find out why I'm getting low-voltage ?
 
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  #2  
Old 09-27-12, 09:23 AM
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Welcome to the forums!

It sounds like you've already got a start on it. For additional tips, see Troubleshooting a dead receptacle or light, Basic Terminology & Other info. The part you're looking for is midway through poat #1 - just scroll down to find that.

I took the light fixture out, and connected a multi-meter, one end to the
black wire and one to the white wire. I was expecting to see 120V (or close to
it) when I turned on the dimmer, but what I get is 18.1V instead.

I took out the dimmer switch, disconnected it, and put the multimeter between the black and white wires of the cable coming out of the wall, and that only showed 46-47V.
Those sound like induced voltage readings. If you're using a digital multimeter, changing to an analog one should help you get more accurate readings.

Have you switched the breaker for this circuit all the way off and then back on?
 
  #3  
Old 09-27-12, 09:34 AM
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Yes I've switched the breaker on and off for it. At least I think I did. I'll give it another shot.

What is "induced voltage" readings ? I do not have an analog on, its a digital one, that gives me correct and accurate readings for everything else. I used the digital multi-meter to take readings for a receptacle and it gave me the correct readings.
 
  #4  
Old 09-27-12, 09:39 AM
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No voltages in SW. and lighting fixture wires

Ok. my dear friend, the Black and the white wire out of the box of the wall are the same. the swith is interrumping the black wire on the off position and then go on the ON position thru the white wire to the fixture. the fixture at the other hand has a direct white wire coming from the source.

Mod note: The above post refers to a switch loop where power comes in at the light. If power comes in at the switch you would measure the black and white of the hot cable.

Mod note: The diagram in this post shows a light and a switch connected by single conductors which appear to run in series. That is not how AC electrical systems are wired, and it is not an accurate representation of the wiring that the OP is working with. It is useful, though, as a single-line schematic, which can often serve to clarify thinking.
 

Last edited by Nashkat1; 09-27-12 at 01:30 PM.
  #5  
Old 09-27-12, 10:09 AM
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Yes that is correct, and if I were to replace the switch in your diagram with the multi-meter, it should show 120V which it doesn't. It only shows 46-47V.
 
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Old 09-27-12, 10:28 AM
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and if I were to replace the switch in your diagram with the multi-meter, it should show 120V
No, it wouldn't because the the light bulb would be a series resistance dropping the voltage.

its a digital one, that gives me correct and accurate readings for everything else
Not always because the impedance is too high on cheap digitals to cancel out induced voltage. Any time two AC wires are parallel they act like a transformer with voltage in an energized wire creating voltage in the wire next to it. The current is small and a low impedance device such as an analog meter will drain the current off with out giving a significant reading.

To test a switch circuit with a switch loop you measure the voltage at the light* not the switch because the reading must be between neutral and hot. The white in a switch box if connected to the switch (and it is not a special switch) is not a neutral.

*Note dimmer must be in full on position to test at light.
 
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Old 09-27-12, 01:41 PM
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the line above is the black wire interrupted by a switch then go to the light,
the other line is a white wire directly to the light.
this is a AC two wires source for a fixture.
 
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Old 09-27-12, 01:44 PM
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if I were to replace the switch in your diagram with the multi-meter, it should show 120V
No. If, when you said in your first post, that you
put the multimeter between the black and white wires of the cable coming out of the wall,
that there is only one 2-conductor cable in the switch box, then that switch is wired with a switch loop and there is no neutral to connect your meter to.

When you go on to say
and that only showed 46-47V,
that is also consistent with an in-line measurement taken with a resistant load in the line. This is where I find the single-line schematic to be useful - the light is that resistant load, and it shows that clearly in that drawing. If you test from black to ground and from white to ground in the switch box, one of those tests should show 120V. If it is wired properly, the 120V should be the reading between the white wire and ground.

We can help you resolve this. Just tell us what wires you see in the box above the light, and how they're connected.
 
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Old 09-27-12, 01:47 PM
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the line above is the black wire interrupted by a switch then go to the light,
the other line is a white wire directly to the light.
this is a AC two wires source for a fixture.
It's an interesting drawing (see my other posts) but does not appear to represent the situation the OP is asking about.
 
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Old 09-27-12, 03:19 PM
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Sorry could not post anything since I was at the townhouse, where I've got no internet connection.

In the switch box there was a 2-wire cable. In the light fixture, there were 3 cables. Everything was connected accept for a single white wire and a single black wire which were going to the light fixture.

Every receptacle in the living, dining, and kitchen area was giving out 120V. They are all on the
same floor, though there are 2 or 3 different breakers that they are on.

The problem turned out to be at the panel in the basement. The breaker for the lights in the dining and kitchen area was also connected to the transformer for the doorbell. The doorbell had been busted and probably shorted. I ended up getting a new 15A tandem breaker, put the transformer onto its own connection and the lights onto the other.

Now the lights work fine, well I get 120V on both the fixtures, I just have to put in the fixtures. Also compounding this was the fact that the dimmer switch was not working. Putting it on full would do nothing but give me 18.7V.
 
  #11  
Old 09-27-12, 05:01 PM
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Glad you figured it out, and thank you for the feedback.
 
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