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Blown primary, hopw to determine unknown secondary voltages of a mains transformer

Discussion in 'Electronic Repair' started by N Cook, Apr 26, 2007.

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  1. N Cook

    N Cook Guest

    Folowing on from thread lower down. I just tried this to see what sort of
    results you get simulating such a multi-secondary transformer using a known
    good one but not using the primary.
    I used a variac supply near the bottom of its range at 18volts and a 25 ohm,
    20W dropper to feed 50Hz (UK)ac into a secondary. Assuming you have a
    reasonable idea of the voltage of one 'unknown' secondary.
    The transformer I used was 240V (UK) with marked 2 separate secondaries of
    6.3V, 0.6A and a 150-0-150 at 25mA.
    With 3.43V ac on one '6.3V' secondary there was open circuit 3.40 on the
    other isolated '6.3' and 161.4V end-to-end on the '150-0-150' and incidently
    116.4 on the primary.
    Then loading with different resistors
    100K, 161.4 drops to 159.1

    5.8K on 161.4 drops to 55.8, 3.43 input drops to 1.64
    swapping to 5.8K on 3.4 , no change

    1K on 161.4 to 12.1 and 3.43 to 0.771
    swap to 1K on 3.4 , drops to 3.39

    270 ohm , 161.4 to 3.34V
    270 on 3.40, drops to 3.37

    56 ohm on 161.4 to .704 and 3.43 to .54V
    56 on 3.4 , drops to 3.28 and 3.43 to 3.42

    8.2 ohm on 3.4 , drops to 2.55 and 3.4 drops to 2.99V


    The size of the transformer gives an idea of the overal sum of Volt x amps
    as well as repeating the above with load resistors on each secondary should
    give an idea of voltages and currents per secondary, without powering the
    primary.
    Anyone care to make an empirical formula from the above loading data and
    give an idea of its applicability to the general case ?
     
  2. N Cook

    N Cook Guest

    corrections
    Blown primary, how to determine unknown secondary voltages of a mains
    transformer
    for
    8.2 ohm on 3.4 , drops to 2.55 and 3.4 drops to 2.99V
    read
    8.2 ohm on 3.4 , drops to 2.55 and 3.43 drops to 2.99V
     
  3. N Cook

    N Cook Guest

    A bit more generalised.
    Noting that for one secondary for this test transformer was rating 300V,
    25mA then V/I of 12K and the 6.3V, 0.6 secondary of 10.5 ohm.
    Doing as before powering a 6.3V secondary to 3.43V and '300V' was 161.4V
    then loading it until the voltage ratio was 80 per cent that is 161.4V down
    to 101.5V and 3.43 falling to 2.69V so 101.5/2.69 = .8 then that R is 12K.
    So for similar transformer construction and high V, low I then find that
    value of R for 80% then if V is known then current rating is V/R.
    Doing the same for the low V,high I one then for R=10.5 ohm then
    corresponding ratio drops from 1:1 ie ==3.43:3.4 down to 3.03/3.43 is 88%
    for high current , low voltage.
    So for similar transformer construction and high I, low V then find that
    value of R for 88% then if V is known then current rating is V/R.
    Other clues would be the gauge of the wires if they can be seen and the
    overall size and weight giving an idea of the overall power rating.
    Resistance checks would show which are more likely high V or high I.
    Usually you would get some idea of one rectified V from max or min, by
    capactitor ratings or a regulator voltage etc.
     
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