Re: Testing a transformer for shorted turns
Any inductor will try to oppose a change in current flowing through it. If you suddenly force the current to zero, for example by opening the switch, the stored energy in the magnetic field will induce a voltage that will attempt to make the curent flow again. This voltage can be very high, limited largely by stray capacitance. This is why you get a spark at the contacts when you switch off an inductive load. Also why you need a diode across a relay coil driven by a transistor. The high voltage will easily light a neon.
This is sometimes called "flyback" because it's seen in in TV sets during flyback when the current in the scan coils has to change rapidly.
A transformer winding is simply an inductor. Unless there are shorted turns or a load on another winding which will severely reduce the inductance.
In mathematical terms, for an inductor:
V = L * dI/dt
In simple terms: The voltage across an inductor is equal to the inductance multiplied by the rate of change of current.
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