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Default Transformer shot! (was scope SMPS/ capacitor venting)

On 20.02.2016 13:55, Cursitor Doom wrote:
On Sat, 20 Feb 2016 01:14:09 -0500, legg wrote:

Recheck pin function before jumping to conclusions.


Right; now re-checked. DC measurments proved (unsurprisingly) too
close together so I re-tested using 100khz instead. These are the
impedances WRT ground of the output taps of the long winding in the
order they actually come out of the transformer: GND, 0.17ohms,
0.17ohms, 0.26ohms, 0.28ohms, 3.7ohms, 3.8ohms. So this doesn't seem
to tally up with the schematic. Or does it? I need a pint of strong
coffee to kick-start my head on this one. :-/ Anyway, later...


Inductive reactance goes up with the square of the winding's nominal
voltage, so you take the square roots of your impedance values.

If you then take into account that the lower impedance values are
strongly dominated by the DC resistance (which stays linear and does not
square) and the upper one is mostly dominated by the AC reactance (which
does square), the ratios seem to look just fine (well, so far as I can
see, within a reasonable margin of error).

But the ratios don't tell the whole story. Even if there is a winding
short, all impedances will be very low (which they sort-of are, I would
have expected higher values everywhere, but then 100 kHz is maybe too
high, try testing at 10 kHz and see...), but the ratios between the
windings would be still be mostly correct.

Try to run it on higher voltage (like 15 V applied to 12.7 V secondary),
and see if it pulls excessive current and warms up. That would indicate
damage more clearly.

Dimitrij

 
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