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Old 01-18-2011, 01:11 AM   #1
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Halogen light fixture voltage reading


Hi all - Recently my halogen light fixture started acting up. It won't turn on when I flip the switch, but if I touch the bulb with a pen, pencil, etc. it sparks to life. I figured maybe something in the fixture or junction box was loose, so I took it apart and tried to get a reading on the multimeter. It shows 0.2V out of the transformer which is supposed to be a 120VAC -> 12VAC transformer. (Found here: http://www.lightingdirect.com/access...former/p739720)

Does that seem normal? I thought using the 200 AC V setting on the multimeter would show 12, not 0.2. So i replaced the transformer with a new one, and it shows the same reading: 0.2 VAC. So I'm assuming that is right but it isn't intuitive to me.

What's really weird is the lamp working when I tap it. I'm wondering if it has something to do with grounding (as in, I'm creating a ground when I use a pen to tap the bulb). The wiring in the jbox doesn't appear grounded (I get a 0.4 VAC reading when touching hot to the junction box) so that may be the problem, but why would it start now? I've had this working for over a year.

Any help or suggestions would be great.

Thanks in advance,
-gc

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Old 01-18-2011, 07:30 AM   #2
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Halogen light fixture voltage reading


Bad bulb, bad socket contacts, wiring.

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Old 01-18-2011, 07:38 AM   #3
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Halogen light fixture voltage reading


I, too, would expect 12V across socket contacts. This makes me suspect supply or wiring problems.

Do you have a way to confirm lamp is not failed? Another fixture, somewhere? Do you have a replacement lamp to try out? While I don't normally like throwing parts at a problem, this is so easy that it is worth trying.

BTW, what types of lamps are you talking about here? MR-16?
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Old 01-18-2011, 08:22 AM   #4
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Halogen light fixture voltage reading


Touching a dirty or oxidized contact with your voltmeter can yield a voltage less than what you expect and unpredictable. Also the voltmeter reading can jump all over the place as your probe moves slightly and makes a better or worse contact.

Because halogen lamps get so hot, it is very common for the socket contacts to become lightly oxidized. Wires held on to contact terminals using rivets can develop irrepairable oxidized connections too.

Turn the light off and let it cool down. Then (not touching the bulb with your bare fingers), unsnap and reinstall the lamp four or five times. This will clean the contacts somewhat.
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Last edited by AllanJ; 01-18-2011 at 08:26 AM.
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Old 01-18-2011, 11:03 AM   #5
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Halogen light fixture voltage reading


Thanks for the replies all. So the voltage reading was taken directly after the transformer. I checked to be sure 120V was being delivered to it first when flipping the switch. Then I hooked the transformer up and tested the output, which is where I read 0.2 VAC. But like I said, the lamp started when I tapped the bulb with something, so I think the bulb is ok. It might be the socket as you suggest, which I'm looking into next.

The type of lamp I'm using is this: http://www.seagulllighting.com/pc27-...Assemblies.htm

I did wire up another lamp in the junction box and it works, so I'm pretty sure the wiring to it is good. I'm just not sure what's wrong downstream of that.
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Old 01-18-2011, 11:46 AM   #6
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Halogen light fixture voltage reading


How are you measuring the voltage? You need to measure across the two contacts, not to ground.
A burnt bulb could be your problem. The filament is dangling and tapping it makes the two ends short together. The socket is the next likely problem. Pull the lamp and inspect the pins. It will probably be obvious if the socket is bad. The pins will be all burnt.
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Old 01-18-2011, 12:15 PM   #7
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Halogen light fixture voltage reading


Yeah, the bulb may in fact be bad. The first thing I did was buy another one to replace it, but then it didn't light up at all. So that made me start thinking something inside the fixture was bad. There's not a whole lot to this pendant fixture since it's really just a low voltage wire that's threaded down to a socket connection. That's why I started with the transformer. I did just pull the socket apart and the pins are not burnt. But I'll reinstall it again when I get home and see if anything changed.

Sorry, I was a little confusing. I did measure the voltage across the two contacts for both the jbox wiring and the transformer. But I also tried to see if the fixture was maybe grounding out. So I checked the ground connection between the hot and the Jbox and it is apparently not properly grounded since I'm only getting a very small voltage reading.
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Old 01-18-2011, 06:16 PM   #8
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Halogen light fixture voltage reading


A common cause of trouble with halogen light fixtures is
Loose connections at the lamp base,
and/or dirty contacts in lamp base,
sometimes even loose contacts in the lamp itself.
The fact that tapping it can make it work,
suggests a loose/ bad connection somewhere.
You can some times fix them, but I prefer to replace
the lamp base with a new one.
Even a small amount of resistance in a low voltage/
high current circuit like this, will lead to problems.

You cannot always measure the output voltage of an
electronic power supply (SMPS) with a voltmeter,
As it is not pure ac,
As it could be non sinusuidal,
it could be high frequency pulses.
Most cheap ones are.
So unless you have a really good quality volt meter.
You should not rely on a volt meter.

Quote:
Originally Posted by goingcommando View Post
Hi all - Recently my halogen light fixture started acting up. It won't turn on when I flip the switch, but if I touch the bulb with a pen, pencil, etc. it sparks to life. I figured maybe something in the fixture or junction box was loose, so I took it apart and tried to get a reading on the multimeter. It shows 0.2V out of the transformer which is supposed to be a 120VAC -> 12VAC transformer. (Found here: http://www.lightingdirect.com/access...former/p739720)

Does that seem normal? I thought using the 200 AC V setting on the multimeter would show 12, not 0.2. So i replaced the transformer with a new one, and it shows the same reading: 0.2 VAC. So I'm assuming that is right but it isn't intuitive to me.

What's really weird is the lamp working when I tap it. I'm wondering if it has something to do with grounding (as in, I'm creating a ground when I use a pen to tap the bulb). The wiring in the jbox doesn't appear grounded (I get a 0.4 VAC reading when touching hot to the junction box) so that may be the problem, but why would it start now? I've had this working for over a year.

Any help or suggestions would be great.

Thanks in advance,
-gc

Last edited by dmxtothemax; 01-18-2011 at 06:25 PM.
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Old 01-18-2011, 07:00 PM   #9
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Halogen light fixture voltage reading


Many of those transformers have diagnostic circuitry and will not show a secondary voltage without a load.
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Old 01-18-2011, 10:32 PM   #10
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Halogen light fixture voltage reading


Thanks all, I think the transformer problem is explained by what brric and dmxtothemax wrote. I didn't think of that, but it makes sense (oh well, $29 for a spare transformer.)

I took the pendant socket connection apart and it looks like it may be blackened on the wire right at the connection. So maybe that's where the problem originated. I quickly took a wire brush to it, but it's hard to get it clean so I'll try harder and see if that works.

What may have caused this is that I discovered that the light switch was wired for a switched neutral. So a current was being delivered to the lamp at all times and the switch just cut the neutral wire to keep the lamp off. I'm not sure if that causes things to shorten life expectancy, but I changed it to a switched hot in hopes that it will not occur again. I have no idea why you would wire it to switch on the neutral, but I guess that's what you get in an old house.

Thanks for the help!
-gc
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Old 01-19-2011, 10:09 AM   #11
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Halogen light fixture voltage reading


Switched neutral while a bit dangerous is not the cause of your problem.

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