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Finding a transformer's CT

Discussion in 'Electronic Basics' started by Rikard Bosnjakovic, Jan 5, 2006.

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  1. From a smashed VCR I plucked out its transformer that's got two primary
    taps and three secondary. Using the consistency checker on my DMM it looks
    like all three secondary taps are connected together. Something tells me
    that this means one of the three is center tapped, and to avoid shorting
    the mains when I use the transformer, I need to find out which one is the
    center one.

    Is this possible to do with a DMM? I learned how to find the primary and
    secondary windings using the resistance meter on my DMM, but I do now know
    how to find a center tap in a winding.
  2. You can use a resistance measurement to find the center tap, also, but
    you need to be able to measure lower values. The end to end
    resistance is about twice the resistance of either end to the center
    tap, but the total resistance may be below an ohm. Do you have access
    to a current regulated lab supply? If so, you can connect the winding
    in question across the supply, with the current limit set to a
    reasonable value (an ampere or a tenth of an ampere, or a guess of
    what the winding can stand without fusing, and measure the voltage
    across the winding with a volt meter. You can measure milliohms this
    way with reasonable accuracy. Just be sure to connect the current to
    the winding with one pair of connection points and the meter with a
    second pair of points, so the current path contact resistance voltage
    drop doesn't get involved in the resistance measurement. If you don't
    have a current regulated supply, you can just use a voltage supply and
    a series resistor at least 10 times the resistance of the winding,
    that limits the current to a safe current. The resistor will produce
    considerable heat, so use a large one and make the measurements quickly.
  3. The resistance you measure between the center tap and either end
    should be half the resistance between the end terminals.
  4. Bob Monsen

    Bob Monsen Guest

    No, there won't be a problem. The center tap is isolated from the
    mains on a working transformer. The worst that could happen is that you
    get 1/2 the voltage you think you'll get.
    Just hook up a 1M resistor between all of the taps, and plug the other
    side in. Now, measure the voltages. If the terminals are A, B, and C, and
    the C terminal is the center tap, then the voltages will probably be

    A-B = V*2
    A-C = V
    B-C = V

    for some V, defined by the winding ratio of the transformer. That
    implies that the the two terminals which are not the center tap will have
    twice the voltage across them.

    Bob Monsen

    "In the struggle for survival, the fittest win out at the expense of
    their rivals because they succeed in adapting themselves best to their
    -- Charles Darwin

  5. Actually, the worst that could happen is that the VCR has a switch-mode
    power supply, and when he tries to connect 60Hz mains to it, it saturates,
    overheats, and flames.
  6. Jasen Betts

    Jasen Betts Guest

    a centre tap will show less resistance to either end of the winding than is
    measured from either end, also you may be able to observe two wires from
    inside the transformer going to the centre terminal, or observe that it
    enters the winding half way through its layyering, furthermore the centre
    tap is usually located in the centre.

    don't connect any of the secondary terminals, connect the transformer to
    the mains and measure the AC voltage between pairs of secondary terminals.

    it should become obvious which is the centre tap.

    eg A-B 12V
    A-C 12V
    B-C 24V

    A is the centre tap.

  7. Assuming its a 50/60Hz transformer, a handy way is to use a small
    transformer, say 6 volts AC, to energise one winding, and then measure the
    voltages that arise in the other windings.

    All the best
    Ian Macmillan
  8. Use a lightbulb in series with the primary to limit the current.
    That way you won't damage the transformer if you hook it up the wrong
    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
  9. Rich Grise

    Rich Grise Guest

    Get a known good 6VAC transformer, and a .1A fuse for its primary. Pick
    any two terminals that have continuity, and put the 6VAC on them - that
    will be a winding or part of one.

    Measure and record the voltages. All of them! :)

    Scale them, and Voila! You know what all of the windings' relationships
    to the others are, center taps and all. :)

    Have Fun!
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