Baffled by lamp failure

Discussion in 'General Electronics Chat' started by steveparrott, Nov 9, 2007.

  1. steveparrott

    Thread Starter Active Member

    Feb 14, 2006
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    Here's the situation:

    On a low voltage (12V, AC) lighting system:

    A contractor responds to a homeowner's complaint that a group of fixtures is not working.
    • The contractor finds that all 4 fixtures are spliced to a single field splice.
    • He test all lamps, they are all Ok.
    • He tests the voltage at each fixture (with the lamp in place) - all fixtures are receiving 12V.
    • He disconnects one fixture and brings it back to the transformer and connects it to the common and 12v tap and the lamp lights.
    • He concludes that the problem was at the field splice or the home run wire. The splice is Ok so he replaces the home run wire.
    • The system works - all lamps light.
    My question is this, How can you get a reading of 12V at the fixture and still the lamp doesn't light?
     
  2. Ron H

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    Apr 14, 2005
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    When the voltage at each fixture was tested, was the measurement across just the fixture, or from the fixture to the other end of the home run wire?
     
  3. steveparrott

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    Feb 14, 2006
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    The voltage was measured at the lamp - one lead at each socket contact.
     
  4. nomurphy

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    Aug 8, 2005
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    Base upon the way you have phrased it, the obvious answer is that the lamp is burned out. :)

    Only a guess, but:
    It's possible that there is a conduction path through earth/ground. The earth could be acting as a high(er) impedance path, relative to the transformer, that cannot supply sufficient current to light the lamp but the voltage can be measured with a high input impedance DMM.
     
  5. Ron H

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    But if the lamps are all good, you won't get 12V across them. I'm baffled.
     
  6. steveparrott

    Thread Starter Active Member

    Feb 14, 2006
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    Ok, the lamps were all good.

    What would happen if the wire going out to the field (about 50 ft. of #10/2 stranded) was partially cut - leaving only a few strands making contact? I assume without any lamp load you would still measure 12 volts at the fixture.

    The puzzling part is that the lamp load (about 80W) was connected to the circuit and the lamps didn't light even though a voltage of 12V was confirmed at the lamp.

    As NOMURPHY notes, it is possible that a cut could have resulted to a path to ground but I can't see how that could affect it since the secondary on the transformer is not grounded.
     
  7. nomurphy

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    Aug 8, 2005
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    Right. Plug in the lamps, no 12V, because the pseudo-circuit can't supply the current.

    It seems to me the lamps were tested separately, not in the non-operating fixtures. The lamps test good in other operating fixtures, the bad fixtures seem to have 12V, but no light.

    Leakage currents can be a very tricky thing, especially when related to AC and earth ground.

    As a test, it may have been interesting to place a 1K or 100 ohm resistor across the test leads (some kind of load) when measuring the 12V at the fixture.

    Have you ever used a DMM to measure a 1.5V battery, and think it was good only to find out it didn't work? Because without a load, the depleted battery would still read 1.5V.
     
  8. Ron H

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    But he said in his first post,
     
  9. steveparrott

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    Feb 14, 2006
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    The good lamps were tested in the non-working circuit. He used a pigtail designed for testing MR16's (one end has two pins that plug into the socket, the other end is spit into two MR16 sockets - one for the dmm leads, the other to plug in a working MR16) this allows for testing under load. The test for the non-working circuits at the fixture were under load.

    Are you saying that if the wire to the fixture was reduced to a couple of strands, then this is like a dead battery and can read 12V yet fail to light the lamp?
     
  10. nomurphy

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    Sorry, I think we're missing something.

    How did the person know the lamps were good? Since they did not work in the "bad" fixtures, they couldn't be tested in those fixtures.

    Are you saying that the lamps were tested in the "bad" fixtures using an extension of sorts and they would light? In that case, the only solution was that the lamp was not making a good connection in the socket. Forget everything else!
     
  11. steveparrott

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    Feb 14, 2006
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    The lamps were tested in other working fixtures on the system. He also tried brand new lamps that he testing in working fixtures.
     
  12. nomurphy

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    Aug 8, 2005
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    Okay, that goes back to what I presumed earlier, the lamps were tested in known good fixtures.

    1) And, when he put these known good lamps in the "bad" fixture, they didn't light, correct?

    2) And, even when he used the lamp on the pig-tail on a bad fixture, the lamp did not light, correct?

    If you answer "yes" to those two question, then I restate my original thesis -- leakage current. If you answer "no" to number 2, that it did light, then it was a bad lamp to fixture connection and nothing else.

    Now -- if you try to tell me that, with a "bad" fixture and his pig-tail, he measured 12V across a good tested lamp that would not light, then that too derives a solution. But I don't think you want to hear the answer (other than that his pig-tail was a piece of crap).
     
  13. techroomt

    Senior Member

    May 19, 2004
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    a poor connection can show the available potential (as with a meter) but the current carrying capability is not there. much the same as if too small a wire were used, say one small strand. the full potential can be read with no load. an inadequate connection shows the same symptoms.
     
  14. nomurphy

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    Yes, the key is that the potential is seen at NO load. If the potential is seen at load, then a good lamp MUST light.

    If all the lamps were removed from bad fixtures, and the focus placed on a single bad fixture, then I think that even one or two strands of wire could supply sufficient current to light one lamp to some extent.

    I really hope we don't get into areas where the lamp was half-on or half-off.


    Q: How many Ph.D. instructors does it take to change a light bulb?

    A: None, it's a problem for the student.
     
  15. steveparrott

    Thread Starter Active Member

    Feb 14, 2006
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    Thanks guys, I know I'm stretching credibility here.

    I just heard back from the contractor who insists that all is true as I stated. By the way, he pulled the suspect (buried) wire and found that it had been cut and poorly reconnected with an ordinary (non-waterproof) wire nut then re-buried.

    Without wasting more of your time, I'm left with the theory that he did measure the voltage correctly but that he was not measuring it under load.

    If he measured it under load, he probably would have got a reading of something under 4 volts (the min. voltage for the lamp to light).

    Unless anyone else wants to chime in, I'll consider the matter closed.

    Thanks for your help.
     
  16. techroomt

    Senior Member

    May 19, 2004
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    i am afraid that is not quite true. an available potential (voltage) does not mean a load will operate correctly unless the appropriate amount of current is available, and with a poor connection that is the case.
     
  17. Ron H

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    If the lamp is good, and has 12V across it, it must light.
     
  18. JoeJester

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    Apr 26, 2005
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    I don't think the potential was seen at the load with the "good" lamp properly installed.

    We need to hear the rest of the story with this one.
     
  19. Ron H

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    Agreed. There has to be at least one error or omission in this story.
     
  20. nomurphy

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    Aug 8, 2005
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    Yes, techroomt is correct. Which is why I mentioned using only one lamp above. I thought I covered this earlier, but still had my mind wound up on leakage current.

    I once had a car that, at one point, had trouble cranking over. The engine ran fine when started, the battery was was fine and all its cable connections were checked and looked good. It was quite a puzzler at the time. Must be a bad/weak starter, right (cheesy starters are so notorious)?

    Nope. It turned out that the negative/black cable going to the motor/chassis ground was about 5-6 feet long (somewhat unusual), and had become corroded internally. With too much cable resistance, the battery could not supply the needed current to the starter.
     
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