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I’m looking at a light fixture on Craigslist, but I’d like to see if it works first.

I have a multimeter that I needed for another project. Can I use the multimeter to test the light fixture before buying it? How would I use the multimeter to test the light fixture?

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Does it have a power supply that you want to test, or is it just a fixture with regular bulbs in it? – JPhi1618 Jan 26 at 21:49
    
@JPhi1618 It isn't hooked up to anything. The owner found it in their attic, and wants to sell. It's a chandelier that uses regular bulbs. – itsjustluck Jan 26 at 21:50
up vote 9 down vote accepted

You don't need a bulb at all to test the fixture. There should be one main "neutral", and one main "hot" wire coming out of the fixture. If there's more than one of each, you can tie each set all together, or test each one individually. The following procedure assumes a single "hot", and single "neutral".

First, set the multimeter to test continuity , or the lowest resistance setting.

Test the "neutral" wire

  1. Put one probe on the "neutral" wire.
  2. Touch the other probe to the threads inside each socket.

Next, test the "hot" wire.

  1. Put one probe to the "hot" wire.
  2. Touch the other probe to the contact at the bottom of each socket.

The threaded portion of each socket should show continuity (or low resistance), with the "neutral" wire. While the contact at the bottom of the socket, should show continuity (or low resistance) with the "hot" wire.

If one or more of the sockets doesn't test out, you may be able to repair the fixture. It's up to you if you're willing to put in the work.

If you know what size socket it is, you might want to fit a bulb in the sockets to make sure they're not physically damaged (deformed, etc.).


As @MichaelKaras points out, you'll also want to check to make sure there's not continuity between "hot" and "neutral", or either of those and the body of the fixture.

  1. With the multimeter set to test continuity (or low resistance). Place one probe on the "hot" wire, and the other on the "neutral" wire.
  2. With one probe still on the "hot" wire, touch the other probe to various metal parts on the fixture.
  3. Place one probe on the "neutral", then touch the other probe to various metal parts on the fixture.

If any of these tests show continuity, then the internal wiring is likely damaged.

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For a very old fixture where the wire insulation may have degraded it would also be a good idea to test for no continuity between the "neutral" wire and the "hot" wire going in when all the bulb sockets are empty. A connection in this case would indicate a direct short in the fixture and as such the unit would need wires to be replaced. – Michael Karas Jan 27 at 2:51
    
@MichaelKaras We're looking at a 1960's fixture. – itsjustluck Jan 27 at 3:35
    
A 60's fixture stored away in a baking hot attic for even a few years could very well have plastic insulation on the wires that has turned hard and cracking or waiting to crack at the moment of wire flexing. – Michael Karas Jan 27 at 4:34
    
Thanks, @MichaelKaras. I've edited the answer. – Tester101 Jan 27 at 10:55

If it is for incandescent lamps there is little that can go wrong with it but if you screw a good lamp into each socket and use your multimeter in continuity or ohms you should be able to test each socket individually.

The resistance of a 40 watt 120 volt lamp is about 360 ohms. Without any lamps screwed in you should get ~infinite resistance or no continuity. With a good lamp plugged in you should get continuity.

Good luck!

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Multimeter probably isn't going to tell you much

For a light fixture that takes incandescent bulbs, about the only thing that you can do is put known-good bulbs in the fixture and measure the resistance between the two wires. If there is infinite resistance, the wiring connections are bad somewhere, and if the resistance is lower than the resistance of a single bulb, the wires are shorted together.

However, in something like a chandelier that has multiple bulbs, the wires will have a junction point somewhere in the fixture where they fan out to each bulb. Each of those connections would have to be individually tested. Even then it will only tell you if a connection is completely broken. A bad connection might not be noticed until you actually try to plug it in.

Instead...

Take a cheap extension cord with the female end cut off, and wire it up to the fixture in question. Put good bulbs in it, hook up the two wires from the extension cord, set it on the ground and plug it in (without touching it). Either it all lights up and or it doesn't. You'll want to have a wire cutter/stripper and a few wire nuts with you to make a clean, safe connection.

That said...

There's not a lot that can go wrong with a chandelier that can't be fixed with a trip to the hardware store and redoing a few connections.

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Not sure why this got down voted. – Tester101 Jan 27 at 1:22
    
I'm sure someone thought it was too much work compared to your solution. I guess mine is ok if you don't have a multimeter but you proved it can be done with only a multimeter. – JPhi1618 Jan 27 at 2:12
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If I'm testing a lamp, it's with a cord and some bulbs. Shake it, slap it a few times, call it good. – Mazura Jan 27 at 10:30

Yes and no, some testing is possible but it won't reveal all possible faults.

What you can do with a multimeter

  1. You can measure the resistance of insulation but the test voltage will be low so certain types of fault will be missed.
  2. You can measure the resistance of conductive pathways (both those intended to carry current and those intended to be protective earths) but the test current will be low and the accuracy at low resistances will be essentially nonexistent. So you will have no way of knowing if a connection is just "hanging on by a thread".
  3. You can test the resistance with a bulb in place to ensure that the lampholder actually makes contact with the bulb.

What you can't do (or can't easilly do) with a typical multimeter.

  1. Test things are reliable, probing with a multimeter is unreliable enough that you are likely to blame intermittent breaks on poor probing.
  2. Test conductive pathways at a high current that will burn away any connectoins that are hanging on by a thread and with an accurate measurement of resistance.
  3. Test insulation at a high voltage.

Overall testing with a multimeter is better than nothing but not really a substitute for proper test gear.

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