The Big Bang Z-Pinch Machine

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Do you like doing science experiments? I don’t mean vinegar and baking soda volcanoes; I mean building a real scientific apparatus that uses a Z-pinch effect to squeeze the daylights out of a soda can in the blink of an eye. KA-BANG!!!

All you need are a few parts from an old microwave oven, a special capacitor from eBay, and a mousetrap. (You know, the old-fashioned kind of wooden mousetrap that goes SNAP!)

Actually, you’ll also need a few other minor components like a knife switch, a voltmeter, and some power resistors. A Variac would be handy too. Figures 1 and 2 show the apparatus and the amazing, pinched results.

FIGURE 1. The Z-pinch machine uses a high-power pulse of stored energy to magnetically squeeze the can in 10 microseconds.

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FIGURE 2. An eight-turn coil induces an intense current flow and opposing magnetic field in the can. The totally collapsed piece on the far right consists of three wraps of aluminum foil.

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Figure 3 is a schematic of the machine. No microcontrollers or software are required.

FIGURE 3. A mousetrap is used to rapidly discharge two high-voltage capacitors into the coil by slamming the contacts together.

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A short video of a typical Big Bang event can be found at https://youtu.be/CpqZouJPIf0.

THE BACKSTORY

When I was a teenager, I visited a physics lab at General Atomics in San Diego, CA where a scientist demonstrated the pinching effect on a piece of aluminum tubing; 3,2,1 … BAM! It squeezed the center of the one inch diameter tube completely shut. Wow!

Now, years later, I wondered if I could do something similar in my shop. I checked the Internet and was very surprised to find that lots of hobbyists were squeezing the heck out of poor little defenseless soda cans. Many of them used great big capacitors and transformers that generated tens of kilovolts, and they used spark gaps as switches. Very impressive, but a little too much for me. I really wanted to build something that other experimenters could also make, with parts that were readily available at a reasonable cost. That became my goal and I think I achieved it.

HOW IS THE CAN PINCHED?

Nothing physical squeezes the can, it collapses because of the intense magnetic field generated by the coil and the induced opposing field generated by the can itself. Here’s how it works:

1. The energy in the charged capacitor is dumped into the coil by the mousetrap, causing a large current to flow around the coil of copper wire.
2. This current flow creates a rapidly changing magnetic field both inside and outside the coil.
3. The inside field cuts through the can, inducing a large circulating current flow in it, in the opposite direction from the coil.
4. In turn, the current flow in the can also creates a magnetic field that “opposes” the coil’s field.
5. The two opposing magnetic fields do the following: The coil’s field tries to push away from the can’s field, and the can’s field tries to push away from the coil’s field.
6. Result: The coil which is relatively strong, only expands a little, while the poor can which is made of thin aluminum, collapses (is pinched) inward. With a BANG, of course!
7. In scientific language: Because of Lenz’s Law, the magnetic fields created within the can and coil strongly repel each other.

Wikipedia has several informative articles that explain the principle behind the pinching. Please look at “Pinch (plasma physics),” “Z-pinch,” and “Electromagnetic Forming.”

This effect is normally used in scientific research to control the shape of conducting plasmas and by industry to shape metal pieces into desired forms.

FAST DISCHARGE CAPACITORS

The most critical component of this project is the fast discharge capacitor(s). Figure 4 shows two types of capacitors that I purchased on eBay.

FIGURE 4. Special pulse discharge capacitors are essential for Z-pinching and can be found on eBay. The pair on the right were removed from commercial defibrillators.

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They weren’t especially cheap ($65-$100 each), but they were the heart of the project so I bit the bullet. The rest of the components were relatively inexpensive.

Regular capacitors like electrolytics and motor caps won’t work mainly because their ESR (Equivalent Series Resistance) is too high. Fast discharge pulse capacitors are precision wound, use special dielectrics, have controlled capacitance, and low ESR. So, of course, they cost more. (Thank goodness for places like eBay!)

The flat ones on the left of Figure 4 were made by Maxwell and the round ones by Aerovox Corp. The Aerovox units (which I used in the final machine) were salvaged from commercial defibrillators. Can you believe it, defibrillators! I did a search for several months on eBay to find them and they actually popped up quite often. Just search with: capacitor, discharge, pulse, high voltage, Maxwell, Aerovox, 196, 50.

As you might have guessed, the bigger the capacitor, the bigger the bang. However, in storing energy, the voltage rating has more effect than the capacitance. If you double the capacitance, you double the energy, but if you double the voltage, you square the energy. The formula is J = 1/2 x C x V2. A Joule (J) is a measure of energy named after the physicist James Joule from the 1800s.

Bottom line: When designing a Z-Pinch system, if you want more bang, you need to trade off increasing the capacitance versus charging to a higher voltage. This machine is somewhat limited by the easy-to-find microwave transformer, but there are hobbyist pinching machines out there that use much higher voltages and bigger capacitors that sit on the floor and can store kiloJoules of energy. Scary!

Do not discharge this type of capacitor with a piece of wire! Use a series resistor to limit the current. Always store unused capacitors with a shorting wire or resistor across the terminals.

BUILDING THE MACHINE

Here are some thoughts about the design of this machine and its components. Please refer back to the schematic in Figure 3.

HOUSING: I decided to put all the high voltage circuitry inside a sturdy plastic enclosure, mainly to protect myself from doing something stupid in the heat of demonstrating it to my friends. Only the controls and coil are external.

VARIAC: A Variac™ is handy to control the maximum charging voltage, so you don’t accidentally exceed the voltage rating of the pulse capacitors. Be really careful not to overvoltage the caps. They can explode.

TRANSFORMER: I salvaged the transformer and HV diode from a kaput microwave oven that was about to be recycled; see Figure 5.

FIGURE 5. A 2100 volt microwave oven transformer can usually be salvaged from old ovens, along with the 12 kV high voltage diode.

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Please check the Internet for typical oven schematics. The salvaged transformer looked fine with no signs of arcing or overheating. It put out 2100 VAC on the red/yellow wire, with respect to the frame. The high-voltage winding measured 127 ohms. The other two wires were for the low-voltage magnetron filament and were snipped off and dressed out of the way. New transformers are available on eBay for $20-$30.

RECTIFIERS: Half wave or full wave bridge rectification can be used. A bridge will charge faster, but keep in mind that microwave transformers typically have one output wire tied to the laminations as shown in the schematic. In that case, the transformer laminations should be isolated from EVERYTHING, including your fingers. Additional 12 kV diodes are available.

CHARGE/DISCHARGE SWITCH: I would recommend using a suitably insulated switch and resistors to make sure the capacitors are discharged after a test and during future storage. The knife switch seen in Figure 6 is not actually a high-voltage switch, but I disassembled it and decided that it was sufficiently insulated inside for this application.

FIGURE 6. The handle of the large knife switch blocks the mousetrap from an accidental trigger during charging. For safety, it should be set on discharge during storage.

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RESISTORS: The charge and discharge resistors should be rated for the appropriate divided voltage, although I think I stretched the voltage rating a bit with the Dale resistors I chose. No problems so far.

DC VOLTMETER: You could use a Heathkit Handitester on its 5000V range as I did, a real high voltage probe, or build your own divider.

MOUSETRAP: The mousetrap shown in Figure 7 is isolated from the high voltage by using a nylon push rod to actuate the contacts.

FIGURE 7. The old-fashioned Victor mousetrap (invented in 1894) is held back by a 3D printed assembly until firing time.

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Just watch your fingers when you cock it.

CONTACTS: When I first started this project, I used a simple cantilevered piece of brass and a screw head to carry the hundreds of amps. They worked fine for discharges of a couple of hundred Joules, but more than that, the brass contacts welded together with almost every shot. I solved this issue by tearing apart a large power relay and using the plated contacts from it (Figure 8).

FIGURE 8. Repurposed high-current contacts from a big power relay are used to rapidly connect the capacitors, without welding together.

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This scheme works pretty well.

CONTACT BOUNCE: Figure 9 shows the scope trace from a typical discharge.

FIGURE 9. The scope trace shows the entire discharge event only takes about 10 microseconds.

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Notice that the sweep speed was 50 microseconds. The whole event was over in 10 microseconds, which included the negative collapsing field. It appears that there was no contact bounce as the mousetrap slammed the contacts together.

SPARK GAPS: Sometime in the future, I’m going to explore a triggered spark gap, like the big guys use. Figure 10 shows my first design, which I have yet to try.

FIGURE 10. Triggered spark gaps are normally used for Z-pinching instead of relay contacts. This design may replace the mousetrap scheme in the future.

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There’s only so much time.

COIL: It’s currently made of eight turns of regular #12 insulated solid copper house wire laced tightly together. It has survived dozens and dozens of discharges. I tried two turns a while back and the intense current actually blew a hole through the insulation in one area. Why? Who knows. Plus, the banana plugs on the ends of the coil wires melted inside the mating jacks. A total disaster as you can see in Figure 11. 

FIGURE 11. A two-turn coil was a disaster. The intense current blew

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WIRING: I used test probe wire for the high-voltage runs, except the discharge paths. On the right side of the schematic, please note that the lines are bolder. These “discharge” lines should be as short as possible (2” max) and at least #12 in size. Try to group the capacitor wires, coil, and contacts close to each other.

SAFETY INTERLOCK CIRCUITRY: The schematic in Figure 12 illustrates the interlock circuitry that was added to improve safety.

FIGURE 12. The safety interlock circuit turns off the transformer just prior to discharge and the LEDs indicate system status.

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The lights are not essential, but the two switches (SW1 and SW2) are important to make sure the power is shut off to the transformer and that the capacitors are fully discharged after a test or during storage.

3D PRINTED PARTS: If you haven’t already invested in a 3D printer, your shop is missing a really handy tool. Figure 13 is a collage of some of the parts I printed for this little project.

FIGURE 13. Custom 3D printed parts are used throughout the machine. They can be easily tailored to exact requirements and dimensions.

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In fact, I made a total of 12: one to hold the Variac; some nifty clamps to hold the round capacitors; and others that are just simple angle brackets. Nothing fancy.

SAFETY FIRST

Not to scare you off, but obviously this project is not entirely safe! However, if you are careful to build and operate it with safety in mind, it can be a real blast to show your friends the raw power of two opposing magnetic fields. The transformer and capacitors generate lethal voltages that can stop your heart in a second. So, if you haven’t had any experience with high voltages, please look at some videos on the Internet to learn about how to protect yourself and others.

DISCLAIMER

I made a number of judgement calls about the suitability and safety of components for this machine, but if you decide to do Z-pinching, please do your own research to make certain that each component is qualified for its function. I (or the magazine) cannot accept responsibility for any problems you might have with building or operating this apparatus or any derivative thereof. You do so at your own risk. Thanks.

ENDING WITH A BANG

Don’t forget to watch the video of the Z-Pinch machine in action at https://youtu.be/CpqZouJPIf0. Feel free to contact me at [email protected] with any questions, corrections, or suggestions. See you next time. 

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PARTS LIST