I have now measured the high voltage pulses during can crushing (lower trace). The lower trace shows 2000 V peak with around 75% voltage reversal. Current in the upper trace is around 12,000 A.
“Continue reading” for more details and photos.
Voltage measurement is important to look at voltage reversal which is a bad thing for caps. Voltage reversal occurs when the energy dumped into the coil begins to return to the capacitor which is how an inductance and a capacitance oscillate. The initial 5000 V drops to zero and then reverses polarity and back again in a decaying manner.
I initially ran my CRO and Rogowski from a 12 V 150 W inverter to isolate it from the MOT supply which is at a potential to earth. Later I made a set of dual contacts with a long insulated handle as a DPDT switch to isolate them. Voltage divider was 27 M ohm (330 K x 82) and 270 k ohm for 100:1 division.
Surprisingly there was no voltage reversal with that setup (see left photo below). The voltage decayed in a damped sinusoidal waveform to the baseline. This was not expected and I wondered about stray capacitance with my leads and the relatively small currents through my divider creating excessive damping.
So to see if there was voltage reversal I tried some expendable diodes 5 x 2.5 kV 60 ns to catch the reverse EMF. If they explode then there was voltage reversal. Well, they exploded all right. The only piece I ever found of the diodes was the small black fragment embedded in my right wrist (every other part of me was behind a blast shield).
Accordingly, I rebuilt my divider with 10 x 1 M resistors to give 10:1 then used my 100 MHz 1.5 kV probe and minimized wire length.
This is the inaccurate voltage curve showing a more gently decaying voltage (lower trace) without voltage reversal. contrast this with the more accurate trace at the top of the page which shows voltage reversal of over 75 % for a 2 kV (=200 J) shot with a can in situ. Scale is 10 kA/div for current (upper trace) and 1 kV/div for voltage (lower trace) with time base 20 us/div.
Related pages
Measurement of high current pulses
Try something else
External links
See the discussion about my can crushing and current measurements on the 4HV forum.
Altair ‘s 4 kJ setup 100 kV but only run to 400 J.
Bert Hickman is renowned for his small change (coinshrinking) and has a few pics of can crushing.
Sam Barros used electrolytic capacitors and SCR switching. Electrolytics are slow and his 3000 J gave similar results to a 300 J shot with my setup.
Tristan Stewart has nice pictures of different energies and the can being torn apart with 2100 J. It was the inspiration for my efforts.
Mike Harrison (Mike’s Electric Stuff) has some high speed videos of can crushing.
Photo Date: 2005