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’?
Yep. Tesla Guns. Akimbo.
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.
The Tesla Gun is a hand-held, battery powered lightning machine. It is a spark gap Tesla coil powered by an 18V drill battery. You pull the trigger, and lightning comes out the front.
It is functionally inferior to that of Tesla’s design in the Five Fists in a few important respects. Notably, it is a bit longer and heavier than Tesla’s own. It also cannot (yet) create an ion wind strong enough to cushion the user when leaping from a four story building.
On the other hand, my design is an improvement in two important respects: 1) It is battery powered, and 2) It actually exists.
I’ve given a few talks about how this project came to be, and it’s a bit of a long story. I could not possibly have built it without the help and expertise of Seattle’s many hackerspaces. Take a look at the basic components, and you’ll see what I mean.
The housing is made from a nerf gun cast in aluminum. I had never made a metal casting before, so I went to the expert: Rusty from Hazard Factory. With his expert metal working skills and my limited ability to gather scrap aluminum, follow directions, and stay the hell out of the way, we had a pretty good aluminum housing in a couple of evenings.
Sand casts inevitably have a few rough edges. Since I needed both halves of the housing to fit together perfectly, the next stop was Hackerbot Labs to put in some time on the Fadal 3-axis mill.
The milling process took a couple of days, but in the end I was able to remove a lot of the bulk of the interior aluminum, and the two halves lined up perfectly. With the housing finished, I set off on the next engineering challenge.
The HV switch
The heart of any spark gap Tesla coil is the high voltage switch. It needs to be able to withstand repeated switching events of many thousands of volts at an instantaneous current of a couple of thousand ampere, generating more than a little bit of heat along the way. This meant finding a material that was a good electrical insulator that was tough enough to withstand high temperatures. With the help of the fine folks at Metrix Create:Space, I decided to make my switch housing out of porcelain.
The first step required the use of a 3d powder printer. This kind of printer is perfect for printing molds for slip casting.
Once the mold was printed, I made a couple of castings using porcelain slip. After air drying for a couple of days, I fired them in the kiln at Metrix, let them cool for another day, and… Ta da! A custom sized HV switch housing, complete with little lightning bolts.
Then it was just a matter of inserting a couple of tungsten welding electrodes, and I had a fully functional high power switch. The shape was chosen to fit inside the aluminum housing while still providing room for a cooling turbine fan: a CPU cooler reclaimed from a discarded 1U server. This draws hot ions out of the switch, making for bigger and more rapid lightning.
The power supply
Power is provided by an 18V lithium ion drill battery. That powers a ZVS driver circuit which drives a flyback transformer, stepping up that 18V to around 20,000V. This stage is affectionately known as the HOCKEY PUCK OF DOOM.
The circuit is small enough that it fits neatly in a 2.5″ PVC plumbing end cap. It is potted with household-grade silicone (yes, Home Depot was an important supplier for this component). The output goes to a center tapped coil wrapped around the ferrite core of a flyback transformer salvaged from a TV.
That leads us to…
The capacitor bank
No, I didn’t roll my own capacitors for this project. But I did make a nifty laser cut housing for them. Also, bleeder resistors are important for preventing unexpected surprises. Like waking up dead after touching this crazy toy.
The caps are 942C20P15K-F by Cornell Dubilier (the cap of choice when your current absolutely, positively needs to get there ON TIME). Since the housing is made of highly conductive aluminum, electrical connections are made with 40kV high voltage wire.
All of that circuitry strobes the primary coil, protected by a couple of chunks of black HDPE (also milled on the Fadal).
The HDPE sandwich makes a great electrical insulator, helping to prevent arcs between the primary and secondary coils. The bottom of the secondary is also wound with PTFE tape (another great insulator, commonly found at Home Depot). The coil form is a piece of 2.5″ ABS pipe (Home Depot again FTW!) wrapped in 30 gauge enameled wire, then sprayed with polyurethane finish (can you tell that the Home Depot is just a few minutes drive from my lair?)
The top load is an aluminum toroid purchased from Information Unlimited (sadly I don’t have access to a lathe big enough to turn one myself.) Put it all together and there you have it: instant lightning at your trigger-happy fingertips.
Of course, the devil is in the details. How do you tune this beast? What about eddy currents in the housing? What do you use for an earth ground? Why is it so LOUD? How do you not die while operating it?
I’m afraid that this post has already gone on far too long. I’ll explain a bit about those topics in future posts. Until then, stay safe and make AWESOME.