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Thread: How to check transformer operational

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  1. #1

    Default How to check transformer operational

    okay i admit it: i'm a total knob.

    i'm trying to figure out if it's the transformer or the component that is not operational. I disconnected the transformer. how do i check to see if it's the culprit before spending money on a new transformer?

    thanks

  2. #2
    Retired Defense Industry Engineer jadnashua's Avatar
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    Well, if it is entirely out of the circuit, the easiest thing you can check is the input windings and the output windings. A transformer is two coils of wire, insulated from each other, wrapped around a (typically) iron core. The exact shape of the core and how the wires are wrapped will vary, but all are the same. The input windings current causes voltage to be induced in the output windings as the magnetic fields change with the a/c signal. The ratio of input to output depends on the amount of loops relative to the input vs output. So, you can make the output voltage higher or lower depending on which side has more windings. The amount of current it can handle is dependent on the size of the wire, the amount of windings, the core, and heat disipation capacity.

    With an ohmmeter, the input windings should show continuity from one lead to the other. The output windings should show continuity between them. There should not be any continuity from the input windings to the output windings. Now, exactly what those readings will be will depend on the current capacity and whether it is a step-up or step-down transformer. If it is more than say 20-30 ohms, (usually much less but depends on the design) there may be a problem.

    Typically, a transformer is pretty stable item. If it doesn't smell from having been overheated, and it has continuity in the windings, it is probably okay.

    Note, this only applies to a real transformer...some, for low-voltage supplies use an electronic equivalent which has active circuitry in it...this simple test won't work.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  3. #3

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    thanks for the reply. i'll see if i can figure it out from there.

    cheers
    j

  4. #4
    Plumber jimbo's Avatar
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    Before you charge off....how about more description of what you are doing? What equipment is this transformer in? In its most basic application, a transformer has a certain AC voltage input on one winding(primary), and a smaller(step down) or larger ( step up) AC voltage appears at the secondary winding.

    It can get complicated from there, so how about more info?
    Last edited by jimbo; 06-16-2007 at 02:48 PM.

  5. #5

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    jimbo-

    (you sound like my dad. lol.)

    have you heard the saying about reaching the level of one's incompetency?... i got the friendly folks at ********* (yep, HD) to check it for me. i know a cop out.

    thanks

  6. #6
    Electrical Contractor/Instructor jwelectric's Avatar
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    None the less this is a good topic to discuss.

    Both Jim D. and Jimbo have made some good points.

  7. #7
    DIY Junior Member bykviking's Avatar
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    Quote Originally Posted by jadnashua View Post
    Well, if it is entirely out of the circuit, the easiest thing you can check is the input windings and the output windings. A transformer is two coils of wire, insulated from each other, wrapped around a (typically) iron core. The exact shape of the core and how the wires are wrapped will vary, but all are the same. The input windings current causes voltage to be induced in the output windings as the magnetic fields change with the a/c signal. The ratio of input to output depends on the amount of loops relative to the input vs output. So, you can make the output voltage higher or lower depending on which side has more windings. The amount of current it can handle is dependent on the size of the wire, the amount of windings, the core, and heat disipation capacity.

    With an ohmmeter, the input windings should show continuity from one lead to the other. The output windings should show continuity between them. There should not be any continuity from the input windings to the output windings. Now, exactly what those readings will be will depend on the current capacity and whether it is a step-up or step-down transformer. If it is more than say 20-30 ohms, (usually much less but depends on the design) there may be a problem.

    Typically, a transformer is pretty stable item. If it doesn't smell from having been overheated, and it has continuity in the windings, it is probably okay.

    Note, this only applies to a real transformer...some, for low-voltage supplies use an electronic equivalent which has active circuitry in it...this simple test won't work.
    Please help, I'm trying to discern if a transformer from my oven is faulty and therefore the reason for my woes. Your test parameters are one of the most informative and specific to my question that I have found, I'm just hoping you can clarify something for me.

    Checking the Ohms on the transformer in question (PRI: 120V 60Hz 3/16" tab / SEC: 23V 11.5VA 1/4" tab) gives me some readings I don't understand based on your comments. My multimeter reads 0.4-.6 Ohms just crossing the leads. Sitting on my desk not hooked up to a power source, the transformer's SEC gave me a nice low 2.4ish reading, but the PRI was closer to 57.4ish Ohms which is higher than your 20-30 range. There is no reading (or maybe it was Overlimit) across PRI and SEC leads in any combo so I know that a crossing of the windings shouldn't be the culprit.

    Does this help or is it too ambiguous, would more info help? My next step unless you can help is to strip a 120V power cable, hook it up and measure the SEC output to see if it jives. I just don't want to cut up a power cord if I do not have to. Thanks in advance!

  8. #8
    Retired Defense Industry Engineer jadnashua's Avatar
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    The transformer is probably okay. There should not be any continuity between the primary and the secondary on a normal transformer. Raw resistance only gives a rough idea of the viability of a transformer...the real measure is inductance, but that takes other tools to measure. I think you need to look for something else to figure out what's going on. Even if you input the 120vac, the output would unlikely be the 11.5 unless it was under load - it would likely be somewhere near 15-18vac with no load, but that's dependent on the core design. Transformers are often used as inputs to a power supply. Without a schematic, it's hard to know what's going on.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

  9. #9

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    What I would typically do if something is not working is measure the voltage at the primary for 120 VAC with a multimeter.

    Then measure the voltage at the secondary.

    Then go from there.

    Very rarely have I ever needed to replace a transformer for a non-working electronic power supply. Usually no main power, fuse blown on the electronic circuit board, capacitor, or voltage regulation components (main power regulating transistor).

    What is the problem you are having?

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