Remember the hockey puck of doom that powers the Tesla Gun? It’s a ZVS driver (also called a Royer oscillator) that drives a flyback transformer using an 18V drill battery. I used to make them as a dead bug circuit potted in silicone. Fast forward a couple of years, and now you can find them all over eBay for less than $20.
It occurred to me that it should be possible to charge the quarter shrinker using a battery and a couple of transformers in series. Let’s try it and find out!
Looks legit.
After digging through my old box of transformers, I found a couple that didn’t arc too badly when powered up. I connected the outputs in series, using two independent ZVS circuits to drive them. The final output is approximately 20kV at 30mA with a charged battery.
Ready to fire. And for fire. And hopefully no explosions.
I also added a little fan to help cool the IGBTs on the driver boards.
It began charging as expected, quickly climbing to a couple of kilovolts. At around 7kV, the rate slowed a bit. The fan gradually slowed down, and various components (the battery, main switch, and IGBTs) all started to heat up.
By 8kV (about two minutes charge time) it was clear that the battery was about to give up the ghost. I switched it off at 8280 volts and pulled the trigger.
We have battery powered shrinkage!
The results were excellent! The coin in the middle was shrunk on battery power. The one on the right was from an earlier run with the usual NST supply, fired at 10kV. The standard quarter on the left is for scale.
I think it was a very successful experiment. If I swap the cheap IGBTs on the driver board for beefier switches, I think I can put together a reasonable power supply that lets us shrink on battery power all the way up to 10kV.
That will be handy next time there’s a tiny coin emergency during a power outage.
Back in 2009, I was one of several folks that built the coin shrinker at Hackerbot Labs. After several hundred firings and sitting in a closet for months, it badly needed some love. We moved it to my shop and I got to work.
As with most projects on this site, the hazards here are many and subtle and include high voltage, extreme UV production, supersonic shrapnel, a several Tesla EMP, charging and discharging hazards, toxic smoke… In short, DON’T TRY THIS AT HOME. Come to my shop instead and we’ll shrink some coins. ^_^
Here’s a breakdown of how the Quarter Shrinker does its magic.
This machine eats kilojoules, copper, and vice-grips to make tiny coins.
Energy is stored in three 10kV/100uF Aerovox capacitors wired in parallel with thick copper bus bar. The capacitor bank is quite heavy, weighing in at about 200 kg (450 pounds). When fully charged, the capacitor holds 15,000 joules. Is this dangerous? To quote Wikipedia,
Any capacitor containing over 10 joules of energy is generally considered hazardous, while 50 joules or higher is potentially lethal.
Caution is clearly advised.
(Aside: we do in fact see dielectric absorption hysteresis after every firing. This toy can be lethal even after you think you’ve turned it off!)
Copper bus bars. For when the current absolutely, positively has to be there on time. The jumper cable safety short keeps the capacitor “turned off” when not in use.
The capacitor is connected to a high voltage DC power supply for charging. It includes a neon sign transformer, variac, and high voltage rectifier. The state of charge of the capacitor is monitored using a cheap semi-disposable volt meter and a 1000:1 voltage divider. Every volt on the meter represents 1000V on the bank.
There’s a gigohm resistor between the red lead and the positive terminal of the capacitor. The internal resistance of the meter is 1 megohm, making a convenient voltage divider of 1000:1.
The red and black twisted leads are connected to a hinged plate that is raised during use, but can be brought down at any time by pulling a long rope. Doing that immediately puts a large resistor across the capacitor, quickly and safely discharging it.
Resistance is not futile. In fact, it’s absolutely necessary for safely discharging the cap bank.
The main switch is a mechanical trigger with tungsten carbide contacts and an HDPE and delrin housing. It too can be gravity-closed by pulling a long rope. This is assisted by a rubber band made of surgical tubing, closing the switch as quickly as possible.
The trigger is shaped so that the contacts come very close together, but don’t actually touch. This keeps them from welding to each other or getting damaged from physical impact.
The trigger. Aim away from face when firing.
Current starts to flow well before the contacts come together, creating a fantastically bright flash of light (including ridiculous amounts of UV). Do not look at trigger with remaining eye.
The light on the left is the trigger closing. The light on top is the coil exploding.
The big plastic box on top is the blast chamber. It’s designed to absorb the force of the exploding coil without breaking. A baffle system allows the rapidly expanding air and copper vapor to escape while trapping the copper shrapnel inside.
There’s a quarter inside that coil, about to be shrunk.
The copper coil consists of twelve windings of 12 gauge solid copper wire. Vice grips clamp the wire to the copper bus bar. As with nearly everything on this machine, the vice grips, sacrificial chunks of HDPE, and bus bar ends are all semi-disposable.
Physics Girl has a fantastic description of the physical forces at play, including how the quarter shrinks and why the coil explodes.
After pulling the trigger, the capacitor discharges in to the coil, shrinking the coin in less than 40 microseconds. The explosion is extremely loud. Ear protection, distance from the machine, and yelling “fire in the hole” are all mandatory.
The resulting shrunken coin glows white-hot during the process, and is quite hot for several minutes afterwards. Some of the copper coil is vaporized by the electric arc, creating a green copper plasma. The coin is typically covered in an atomically thin layer of copper, discoloring it. (Note to self: try to get an SEM photo of the copper plating on a coin when Millie is back online.)
Today’s money just doesn’t go as far as it used to. Also, copper plating a couple of atoms thick.
My shop-mate Sirus made a nice 4k video of the setup process and first post-rebuild firing. (The charge time is probably greater than your attention span. To skip directly to the trigger pull, CLICK HERE.)
Popular Science has published the online version of the print article on the Tesla Gun! It includes a quick (and very geek-tastic) interview with yours truly.
The year was 1889. The War of the Currents was well underway. At stake: the future of electrical power distribution on planet Earth. With the financial backing of George Westinghouse, Tesla’s AC polyphase system competed for market dominance with Edison’s established (but less efficient) DC system, in one of the ugliest and most epic tales of technological competition of the modern age.
More than a hundred years after the dust settled, Matt Fraction and Steven Sanders published The Five Fists of Science: a rollicking graphical retelling of what really happened at the turn of the last century. (Get yourself a copy and read it immediately, unless you’re allergic to AWESOME). On the right is the cover to this fantastic tale of electrical fury.
See that dapper fellow in front? That’s a young Mr. Tesla. See what he’s packin’?
As I read this fantastic story, gentle reader, certain irrevocable processes were set in motion. The result is my answer to The Problem of Increasing Human Energy: The Tesla Gun. For reals.
I gave another talk at Ada’s Technical Books: this time about the (nearly completed) Tesla gun. The audio isn’t perfect, and it very tragically ends just before the spectacular spark demonstration, but here it is:
After tragically melting my previous HV switch, and given that a 15kV multi-ampere switch isn’t exactly something you find at Radio Shack, I’ve had to resort to more drastic measures. It was time to make one from scratch.
And when I say scratch, I mean mud. And by mud, I mean porcelain.
The fine folks at Metrix CreateSpace had just to tools I needed: a 3D powder printer to make a slip cast mold, and a kiln to fire the clay into porcelain. After a couple of revisions, I settled on something vaguely resembling a cooling tower. This will let me install a tiny turbine in the bottom to pull out hot ions, while using a minimum amount of material for the switch itself.
Install some tungsten welding electrodes (held in place with high temperature silicone) and voila: a handy switch capable of handling a couple of kilowatts at 15+kV!
Did I mention that, given the incompressibility of porcelain and the shape of the cooling base, it is perhaps a little loud?
