It was a lot of fun and I've wanted to do something like it again for a while. I have a large number of ~400V rated pulse capacitors:
that I've used before to blow up wire, small railguns, etc with good success. However, I also have an 80 kV death capacitor that I've always wanted to use:
The death part is pretty literal:
"WARNING: THE ENERGY STORED IN THIS CAPACITOR IS LETHAL". Not "may be"...*IS*. And they are probably right: don't touch it while its live.
But the capacitor is pretty boring without a way to power it. 4 years or so ago I used a 5kV capillary electrophoresis power supply to give it a token charge:
This made a reasonably large snap but, because capacitor energy is 0.5 C V^2 was only 0.4 % of the max energy. After that it was put into storage and didn't see any use for some time due to lack of a large power supply. I bought some capacitors and diodes to make a CW multiplier for my neon sign transformer but never built it.
Roll forward to present day. This capacitor came out of an x-ray (diffraction or crystallography IIRC) system at RPI. Somewhat related, I've been doing some x-ray work and had a unit fail (more on that in another post). I opened it up and found I was able to salvage the high voltage (100kV) transformer:
For the following I'm currently only using the left half which seems to give about 60kV DC max. Worth mentioning I could have grabbed the transformer but it was very heavy/large and left behind.
Started with some spark testing to verify the transformer worked. The transformer must be submerged in oil to work properly:
About a gallon of oil in there and the box was tipped to one side that it fully covered. Then I made some arcs:
The gap in the above picture is about 2"
But this is AC and to fully charge the capacitor I need DC. I still had the original full wave rectifier that powered the capacitor but, for reasons I don't recall, I only have the bare diodes and not a rectifier assembly. So next step was to assemble them into a rectifier module. Originally I did a classic rectifier square design:
But then realized this required crossing an output high voltage line over an input source, potentially leading to a breakdown. So redesigned int in an x pattern so that the polycarbonate sheet provided insulation between the two halves:
Per the labels, AC comes in on the left and DC exits on the right.
Next, we need a trigger. In the simplest tests its fine to set a spark gap and let it automatically trigger but longer term In the past I used a pneumatic switch to trigger a spark gap which worked pretty well. I had some 2" stroke solenoids which aren't ideal (remember I said I could jump 2" above?) but was good for a first test. Setup:
Note on the right you can see 40kV probe for "low voltage" tests. The blue silicone tube goes to a compressor on the right. I simply open up the regulator to close the switch. I also built a polycarbonate platform that you can see in the above picture.
This system needs a way to safely discharge. Long term I want an emergency stop, but short term I'm fine with simply bleeding off charge. My goal is to have the system safely discharged (50V) within about 10 seconds. I think this didn't work out in the end, but in theory this should be V = e^(-t /(RC)) where:
- V = 50V
- t = 10 s
- R = ?
- C = 0.25 uF
Putting it all together, this is what the prototype schematic looked like:
Some artistic license on the schematic: I loosely interchanged electrical paths with pneumatic paths.
Time for results. The first victim was a galaxy 2 phone donated by a coworker. Couple minute video of the test process here: https://www.youtube.com/watch?v=Q-sTKhyaM3Y
The phone wasn't blown in half but was a good start: the screen was cracked and burned. Ultimately the phone succumbed though so I call it a success.
But the goal was to blow up some fruit. First I tried placing a tomato directly inside the gap: https://www.youtube.com/watch?v=CgCAwcUGMeg&list=UUq3z1paLNFugoH3yTYokMIg
But this didn't work so well: a tomato has a lot of water that serves to nicely absorb the energy. Next idea: put the electrodes inside the tomato and separate by 0.25". This focuses the energy into a small space and forms a proper explosion.
This worked great: https://www.youtube.com/watch?v=E4cW6JSh3lc
Mission success! There is one less tomato in the world. I also decreased the camera ISO from auto to fixed 400 ISO to get better explosion coverage. I also upped the frame rate from 30 fps to 60 fps to catch more gore. I have a 1000 fps Casio that I'll try to use in the near future.
Finally, how to blow the phone in half? I have a tenative agreement to upgrade the system from 800J to 12-13 kJ. More to come
(note the ruler for scale)
