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Transformer Teaser

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Rufus:
This had me scratching my head for a while but I think I understand now - what do you make of it?

The circuit is a mains transformer secondary one side ground, a resistive load one side ground, a fast switch between the other end of both.

In the attached traces yellow is the transformer side of the switch, pink the load side, blue an irrelevant control signal.

The switch turns off at zero current and optionally on again shortly afterwards. 

Why does the transformer secondary voltage jump when a load carrying no current is disconnected and jump back when reconnected (with very little drive voltage available)?

IanB:
I can think of two possible candidates for a cause, but I don't know the answer.

One possibility is that connecting the load changes the inductance of the coil by partially shorting it out. Since at the instant the inductance changes the stored magnetic energy in the core is the same, the voltage across the coil will change (much like a spinning ice skater speeding up when folding her arms).

A second possibility is that even though the current is zero when the switch is operated, the system is not at steady state. There are non-zero dynamics present at that instant, and those dynamics react against the change in circuit topology to produce the observed effect.

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