12 volt transformer

Discussion in 'Electrical Forum discussion & Blog' started by Snafu34, Jun 28, 2007.

  1. Snafu34

    Snafu34 New Member

    Messages:
    5
    Good Morning- I have a question regarding a 12 volt transformer I'm hoping some one can help with. I'm trying to repair a low voltage lamp that is no the blink. I found the 110 to 12 volt transformer in the base of the lamp
    and proceeded to test it. I found 110 going into the transformer but nothing coming out the 12 side. I assumed the the transformer was bad so I ordered the exact same transformer from an electrical supply house. When the new one arrived I bench tested it only to find the same behavior. 110 going in but no 12 volts going out ? Am I missing something here or did I get a bad unit ?

    Best Regards
    Snafu34
  2. jwelectric

    jwelectric Electrical Contractor/Instructor

    Messages:
    2,540
    Location:
    North Carolina
    It is possible that you got a bad transformer form the supply house.

    If a voltage is applied to the primary then there should be a voltage present on the secondary
  3. frenchie

    frenchie Jack of all trades

    Not sure this is relevant, but most transformers like that also go from 120 AC to 12v DC. Your tester should have different settings for AC and DC, make sure you're on the right setting... ?
  4. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,128
    Location:
    New England
    Whoa, there, a simple transformer doesn't work on DC...it will only work on a/c in and produce a/c out. Now, there could be a rectifier circuit somewhere on the output generating DC voltage, but that is totally independent of the transformer. If the black box has a transformer in it, it could be a hybrid circuit with a rectifier.

    When you measured, where did you do it? At the light socket? at the output of the transformer? Sometimes, there is an in-line fuse or fuseable link, all it may take is a new one of them. It could be soldered in and may not be obvious.

    With the thing unplugged, do some continuity checks of the primary, secondary, and from the secondary to the lamp socket. You may just find an open. If you didn't actually measure on the input wires to the transformer, it could be the switch as well.
  5. frenchie

    frenchie Jack of all trades

    Well, I did say "most"...

    Snafu34, check the label on the transformer, make sure it matches up with your tester setting.

    All I was saying...
  6. Snafu34

    Snafu34 New Member

    Messages:
    5
    Thanks for looking gentlemen-

    The transformer is a WH-1501E6A
    AC 120V -60Hz 1.2A
    AC 12 V 150W output
    Auto reset - soft start
    overload protected.
    For 12 Volt Halogen lights only.
    hxxp://www.atlantalightbulbs.com/ecart/nw012104/WANGWH-1501E6A.htm
    (change hxxp to http)

    The transformer has two lines in for the 120 side
    and two out for the AC 12 out

    I connect the transformer to 110/120 and confirm it is getting power
    but yet there is NO reading on my voltmeter on the output 12 volt AC side.
    My meter is a cheapie that only has AC setting of 200 and 600 volts
    but I think it should show the 12 Volts anyway------- correct ?

    THX
    Snafu34
  7. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

    Messages:
    22,128
    Location:
    New England
    It sounds like the transformer has a circuit breaker in it (the overload protection). It could be sensing an overload and shutting down the actual input to the primary; you could have a shorted wire or something in the secondary.

    The physics of a transformer are that it only passes a/c. If you put d/c in, all you'll do is heat it up if you don't short out the dc supply. Transformers work by the generation and collaping of the magnetic field. It is the change that induces the field that allows the primary to transfer energy to the secondary. No changing of current, no output, thus, you can't get DC power out of a transformer. That doesn't mean there may not be some there as the result of other components, as you can have a/c riding on a dc bias.
  8. Snafu34

    Snafu34 New Member

    Messages:
    5
    Jim- excuse me for being dense - but- are you saying it may be defective ?

    "you could have a shorted wire or something in the secondary"

    If it is putting out anything at all I should be able to read it on the 12 v out side
    when it is not connected to the lamps bulbs (bench test) Correct ?

    thx
    Snafu34
  9. Rancher

    Rancher Guest

    I'm assuming you tried the new transformer and it didn't make the light work?

    The transformer is probably an electronic halogen soft start transformer, you don't have the resistance in the meter to make it do it's soft start... so you will not be able to measure it's voltage that way.

    Did you try a new bulb first?

    Is the switch after the transformer or before?

    There may also be a wiring problem after the xfmer, or in the socket it's self.

    Try ohming out the wire from the xfmer to the socket, and ohm out the bulb also.

    Rancher
  10. Snafu34

    Snafu34 New Member

    Messages:
    5
    Roger- Many thanks for the reply-

    "The transformer is probably an electronic halogen soft start transformer, you don't have the resistance in the meter to make it do it's soft start... so you will not be able to measure it's voltage that way."

    You may have the missing link here- First I'll say this is not my lamp and I would have thrown the ugly thing out before fixing it.

    So- the xfmer with soft start will not provide any current at all until it is
    asked for by the bulbs and therefor the meter will not pick it up - do
    I understand this correctly ?

    The floor lamp has 12 small 10w bulbs danging from the top of the lamps
    stem. A bunch of wires treaded thru a small dia. stem and then into
    even small tubes that feed the bulbs. There is NO switch so I assume it was
    controlled from a wall switch. Sounds no fun to try and track down
    all these wires but it probably the only way to find out what is going on.

    Regards
    Steve
  11. Snafu34

    Snafu34 New Member

    Messages:
    5
    Back again- what a job pulling all those little wires.
    But I found the problem. A broken wire running up the stem.
    I wish I had checked them before ordering the xfmer. ;-(

    THX for the assistance
    snafu34
  12. hj

    hj Moderator & Master Plumber Staff Member

    Messages:
    26,811
    Location:
    Cave Creek, Arizona
    Frenchie

    Yes, you said "most" but you should have said "very, very few, if any".
  13. frenchie

    frenchie Jack of all trades

    You are absolutely correct.

    I thought it was typical - but it's really only LED's, isn't it? - everything else is 12v AC.

    I learn something new every day...
  14. snafflekid

    snafflekid Electrical Engineer

    Messages:
    45
    oops, oh well, you know what happens when you assume?

    these transformers won't strike if the bulb is broken, it's a safety feature.
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