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Oil furance ignition transformer. Proper resistance for primary

Discussion in 'Electronic Repair' started by micky, Dec 21, 2013.

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  1. micky

    micky Guest

    Oil furance ignition transformer. Proper resistance for primary.

    Is it possible that a winding with only 4 ohms DC resistance would
    have 52 ohms impedance at 60 Hertz**?

    I have an ignition transformer for an oil furnace burner.

    It's designed to make a continuous spark to ignite the atomized fuel
    oil sprayed into the firebox. It runs on 120 VAC. And the
    secondary is meant to provide a voltage that will jump across a
    specified 1/8" gap, but probably will jump a 1/4" gap. I see that the
    transformer secondary is rated at 10,000 volts.

    Disconnected and measured with an ohmmeter it shows 20,000 ohms in the
    secondary, and 4 ohms in the primary!!

    Is that possible? Esp. the primary.

    I don't know how to measure impedance at 60 Hertz.

    The transformer primary uses 2.2A at 120 volts, according to its label

    (The current in the secondary is 23mA. The transformer is a lot like
    this one:

    E=IR, R = E/I = 120/2.2 = a little over 52 ohms, right?

    That would make the impedance at 60** Hertz about 52 ohms, right?

    Is it possible that a winding with only 4 ohms DC resistance would
    have 52 ohms impedance at 60cps**?

    Or is the transformer bad?

    Or what am I doing wrong?

    The transformer seems to fail when it's hot, but it wasn't hot when I
    measured the resistance of the windings.

    **Or maybe 15 to 30KHz. Some replacement transformers specify that,
    but I think it's only for quite modern furnaces. I don't think my old
    furnace would have that. It has a circuit board with about 30 parts,
    but I don't know what parts to look for that would be an oscillator.

  2. John-Del

    John-Del Guest

    There are two types of transformer controls: interrupted and intermittent. If you have an interrupted control, the transformer is actually shut off some time after the flame is established. This prolongs the life of the ignition transformer and points to almost a decade. Intermittent means the transformer runs the entire time the burner does, so the trans and points arebeing used even after they are no longer required.

    Someone already mentioned that coils can fail when hot, and burner transformers are no exception. And btw, when they're right, they will fire a gap well over half inch if the points are shaped properly.

    If you live in the States, home stores such as Home Despot carry "ignitors", which are drop in replacement for the old style transformer. They mount and are wired in the same manner, but use a high freq switch mode arrangement. They are less than $40, so it's a good idea to have one at home as these things are generally timed to fail nights, weekends, and holidays.
  3. SOunds ok to me.. The wire is of small gage so you would expect some DCR
    readings of significant value, especially on the secondary side.
  4. Phil Allison

    Phil Allison Guest


    ** The primary impedance with no load is actually much higher than your
    naive calculation.

    Ohms law has nothing to do with it.

    If you check similar size 120VAC transformers, you will find their primary
    resistances are all around 3 ohms.

    With 2 amps of current in the winding, the heat generated ( given by I
    squared R) = 12watts.

    Expect a similar amount of heat in the secondary and you can see why the
    things get quite warm.

    ..... Phil
  5. What you have sounds correct.

    I have here a new Dongan 250VA, 8000 volt 23ma ignition tranformer. It's
    oil filled and has lovely steatite insulators.

    The primary resistance is 2.7 ohms and seconday is 18.5k. For a plain
    shunted ignition transformer the numbers look safe, and it's close to what
    you have.
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