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

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 ?

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

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N Cook
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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

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

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N Cook

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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.

-- Diverse Devices, Southampton, England electronic hints and repair briefs , schematics/manuals list on

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N Cook

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