We recently had a chance to work with the fine folks at Intellectual Ventures to quantify some of what goes on with our coin shrinker. Using a bunch of neat gear (including an extremely high speed camera), we were able to learn all sorts of things about our amazing chamber of shrink. Here is some of what we learned:

• The coin shrinks in just 30 to 40 microseconds. Even when filming at 100,000 frames per second, that’s just 3 or 4 frames of video!
• By the time the coil begins to explode, the coin has already been shrunk for several microseconds!
• The collapsing coin gets extremely hot: the thinnest part of the center of a quarter glows bright white-red for a few microseconds just after shrinking.
• At one meter away from the spark gap, the shrinker generates a big bang: about 135 dB, or roughly as loud as a jet engine taking off from 100 meters away.
• The magnetic field created by the coil is extremely strong: at 10 cm from the coil (just on top of the lid of the blast chamber) we measured a peak field strength of 0.22 tesla, or about as strong as a small neodymium magnet.
• During the explosion, some of the bits of shrapnel fly past the camera’s view at about twice the speed of sound.
• The edge of the coin moves about 1/4″ in 36 microseconds, or roughly 400 miles per hour!

There’s a lot more info over on Intellectual Ventures’ blog. More photos of the camera rig and actual shrinkage are available here.

Coin Shrinking from Jeremy Ruhland on Vimeo.

Turn half dollars into quarters! Turn quarters into dimes! Turn dimes into little semi-molten balls of metal! All this is possible with the proper application of insanely high electrical current at very high voltage.

“God made the quarters. It’s up to us to make them shrink.”  –anon.

The coins don’t just shrink; they keep their identity and fine details. But do they keep their value?

Hackerbot Labs is proud to contribute to America’s economic downsizing.

How does it work?

A high voltage DC supply charges the big 300 µF capacitor to 10 kV, or about 15,000 J of stored energy.

This energy is released as quickly as possible into a small copper coil wrapped around a coin.

The fast changing current creates a very powerful magnetic field in the coil, which creates a very powerful magnetic field in the coin itself. The strong opposition between the two fields causes the material of the coin to contract, while at the same time the copper coil expands. A few dozens of microseconds later, the coil has violently exploded inside a specially built blast chamber. And the coin itself is much smaller!

Where does the coin go?

No material is lost from the original coin: the weight and volume are the same as the original. But the coin now has a smaller diameter and is thicker, while retaining much of its surface detail. It’s also extremely hot just after firing!

Isn’t this dangerous?

Oh yes. An average automated defibrillator (AED) provides a jolt of about 150 joules. The energy stored in the big cap is about 100 times that much.

There’s also the tremendously loud noise, a very bright flash of UV when the spark gap switch closes,  the possibility of fire and toxic gas when stuff in the chamber burns or pyrolyzes, and bits of copper shrapnel moving with enough energy to blast clean through six-ply blocks of plywood. And somebody always burns themselves when they try to pick up the newly shrunken quarter.

To contain and prevent much of that danger, we have designed a custom-built blast chamber out of very thick high density plastic. It contains the shrapnel and covers most of the electrodes, and provides a solid platform to hold the mechanical spark gap trigger. The trigger is engaged by yanking a very long rope: simple but effective.

Who invented this thing?

It wasn’t us. We just built one. It’s an actual industrial process called electromagnetic forming. See some of these links for more coin shrinking madness:

• Bert Hickman’s “Makin’ Small Change” and Shrinker Gallery
• Ross Overstreet’s Pulsed Power Page
• Tesla Downunder’s Can Crusher
• More HBL coin shrinking photos on flickr

It uses a TL494 to switch a MOSFET very rapidly, dumping current into the primary coil of a TV flyback. The air rapidly heats up when the spark is on, and it cools quickly when switched off. This causes the air to vibrate, making sound just like a speaker.

I used a Hitachi flyback (about $10 on eBay) but any flyback with an open primary will work. You can use jumper clips for electrodes (as above), but they will melt eventually. Tungsten electrodes are much better. You can also use anything else that is conductive and will tolerate heat, such as chunks of pyrolytic carbon.

Here it is running in a vase full of argon. This gives you much bigger sparks at the same voltage!

Here is Plasmana’s original schematic. Replacing the IRF540 with a bigger MOSFET (such as the FQA16N50) will let you dump more current at higher voltage across the flyback, making even bigger sparks. Adding a gate driver between the TL494 and the MOSFET would also likely help.

A good heat sink is critical to keep the MOSFET from melting. I used a large copper CPU cooler with an integrated fan.

Datasheets:

• tl494 PWM controller
• irf540a MOSFET
• fqa16n50 MOSFET