I’ve been playing with capacitors for a long time, but I just recently began getting ones large enough to make REALLY cool sparks, not just nice snaps. After the “toy” caps I played with for a month (see farther down the page), I bought 16 400v 1500 uF caps on eBay. Those made some nice sparks, and were a huge step for me. It turned out however that I had bid on two auctions and won both of them, so 5 days later 8 150v 15000 uF caps arrived. [pics missing]
Since the voltage directly affects the temperature of the spark and the over all energy stored (1/2 Cv^2), the more the better. Thus, the array is connected as 2 sets of 4 150v caps in parallel, in series. Thus the whole bank can go to 300v. (Click on any image for full size)
The 400v caps are connected by thick copper wire between the terminals.
You can see the balancing resistors on the 150v bank. These primarily balance the voltage on the two parallel so as not to overvoltage one set (overvoltage==boom==bad) and leave the other at a smaller charge. The resistors basically create a classic voltage divider.
With all current connections, the bank holds 2430 Joules at peak. I’m planning to get 4 more 150v caps, which will let us bring the bank to its peak 400v, this increasing the max energy to 3520 Joules. That’s easily enough to cook your average garden squirrel.
What do we do with these caps, you ask? Melt things. Among the favorites are aluminium foil (which instantly sublimates and creates a huge shock wave due to the instantaneous heating of the air) and softdrink cans. The latter instantly ruptures and boils the drink, which then comes out the holes at high speed…
Here is a great video I put together of several soda cans being punctured (that’s Ken doing the puncturing): capfun-small.wmv (6.2MB), capfun-big.avi (153MB)
Frames from the video:
Before I had any “real” caps, I collected first 16 then 64 camera flash capacitors and wired them together to create some nice ize sparks. Nothing like what I have now, but nice nonetheless.
The old array is 64 120 uF, 330 volt capacitors wired in paralell. This stores 424.7 joules of energy. If you grabbed the terminals with two hands, you would be pretty done. We enforce the “pocket rule” vigerously, wherein you keep one hand (usually the left) in your back pocket anytime you’re near the caps.The videos are here: Video 1, Video 2. You can see all of these files in a directory structure here. Props to Nick and his camera for the photos and video!
My old cap collection with two other large but low voltage caps I scrounged from the trash at MIT.
The lab and charging circuitry. You can see Ken turning on one of the power supplies to charge the caps. We used 5 such supplies at 2 times 30v each, all linked together. The cap array is at the left of the picture, in front of me.
The first test: two pennies. Note the scorch mark on the table.
Whacking a sprite can with a penny produces interesting holes.
We melted a peice of copper to a penny. Note the edges of the penny… same one we had used on the can.
A pattern left by a ball of molten copper bouncing across the table.
One of the first few sparks, a frame taken from the video.
A few frames from the second video: