[Argus25]

Kalee20,

Yes in essence the high voltage transient is caused by a rapid rising edge 24V potential (the 12V supply + 12V stepped up by the other half of the primary) shock exciting a resonant circuit composed of the primary leakage inductance and the associated capacitances. The Q is high as the resistances are relatively low. As noted this happens to a large extent only in a force or osc driven circuit, not self oscillating(see below). I do agree that the reduction in load current in the leakage inductance plays a part, however on testing I found that the voltage transient magnitude was relatively independent of that, and a nearly identical transient is generated with negligible load. It is primarily because a new large step voltage is applied to the capacitances, rather than the energy stored in the leakage inductance's field generating the spike. As noted from the recordings the transient is only about 10uS wide at its base or half cycle time.

In a modern SMPS, normally, the primary winding is bifilar wound, minimizing the leakage inductance which helps. But these old vibrator transformers were not wound like that, its easy to tell, just by measuring the DC resistances, larger on one half of the primary winding than the other.

In the case where the circuit is configured to self oscillate, switching timing of the two devices is such that it takes longer for the device coming out of conduction to do that.This controls the transient and the changeover between one mosfet (or transistor) going into conduction and one coming out of conduction is more gradual, but probably less efficient. But it doesn't matter because the switching events per unit time are only in the 100 to 200Hz vicinity so the energy dissipation on account of this is low. So I preferred this method to others and the circuitry more robust than an IC driver. But I do agree an SMPS IC , like an SG3524, can be used to replicate the vibrator contact timing. Self oscillating types are intrinsically short circuit protected too.