Transformer repair

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Thanks, Phil.

Well yes, it is a quite rapid action, but does not occur till the current in the primary is sufficiently high. Saturation does not occur over the whole input wave, and drops and reverses as the sine wave input reverses. So during the non saturated period, the input and output voltages are proportional to turns ratio.

peter

"Peter" "Phil Allison"

** Strangely enough, the peak saturation currents occurs near to each AC supply voltage zero crossing.

With a small transformer and double the rated AC input voltage, the (off load) current wave is very peaky in shape and rises to about 15 times the usual RMS value.

Shorted turns in the primary results in a few seconds, the resistance drops suddenly and blows a fuse even 10 times the correct size.

The PSU in question contained only electros, resistors and zeners - all operating well within their ratings in normal circumstances, so well able to take a brief increase in voltage.

... Phil

Yes phil, that is interesting, and why I referred to current, as the current amp-turns causes the magnetising force to result in core saturation. When the secondary load is small, the primary current is nearly at 90 degrees to supply voltage as the transformer is essentially an inductor, so the high current part of the wave will be near 90 degrees. Of course, when the core saturates, the inductance drops to very low value, and the main limiting factor is the primary circuit resistance, which in the case you quoted gave 15 times rated input current. In the case you quoted, the failure in a matter of seconds means you have little time to turn things off before a catastrophic transformer failure.

peter