lol, i am quite confident that an irf510 breaks down at about 100-110 volts and that the supply rails are not affected at all, except by the supply side inductance destroying the 555
here's an example of what the supply side voltage would look like At The 555.
View attachment 65113
the lower trace is dc coupled and it was 10 volts per div. the mosfet was a irfz48.
the reason it is clamped is because the mosfet and the flyback diode conduct in the reverse direction, the mosfet breaks down and conducts in the other, but only for the first microsecond or however long it takes. The remaining decaying oscillation is the supply side inductance resonating with the local capacitance on the board.
Now i should make clear that the second portion of the decaying sine wave is what the voltage looks like across the 555, when there is a local capacitor connected across it.
the entirely of the waveform is actually the source to drain voltage.
top waveform is the gate source voltage.
since he originally had no cap on the 555, it would have been destroyed instantly upon turn off. the voltage on the dc bus at the 555 would rise to whatever the mosfet avalanches at
plus the dc voltage drop through the transformer coils. (assuming the capacitance of the mosfet is negligible) but after the energy was dumped into the mosfet the supply would ring down just as it does in the oscope shot.
had a low inductance 1uF cap and a few hundred uF electrolytic to back it up, only the mosfet would be affected.
View attachment 65114
here is what his actual schematic looks like.
with two modifications, the transformer replaces R
and a virtual 555 would be connected across the capacitor. (or more accurately, lack of a capacitor)