Coulter's Smithing Home.
Once you get those nice energy storage capacitors and some sort of switch or spark gap to control them, how do you get them charged up in the first place? Well, if you're not in a big hurry, here's one way, using a Cockroft–Walton multiplier.
This uses an inverter for a CCFL backlight, available from Digi-Key
and lots of other places. This one is rated to drive two lamps, and
puts out on the order of 6 watts. I put a series resistor in the input
line to control surge current when it is fired up across a discharged
capacitor. This version makes something a little over 30 kV, and is
used to charge a very nice .5 uF Maxwell capacitor I managed to get.
It's the kind we didn't want Saddam to have, the series inductance
of this puppy is under .02 uH! That's a difficult number to maintain
with wire leads, so we use a transmission line of copper flashing and
For lower voltages, one can just use fewer stages of multiplication.
This many stages rated laying out a little PCB board, made here.|
There's a couple little secrets to making this work well. The inverter puts out AC at about 50 kHz, so the diodes have to be fast enough for that. If you look closely, the capacitors on the board have builtin spark gaps, which flash over at about 2.5 kV. This is crucial to protect the diodes from excessive reverse voltages when the big cap discharges, and the little caps aren't all equal. These work fine with the 3kv diodes I managed to get, nothing fries that shouldn't. The diodes are Diotec DD300 types, rated at 3 kV and 20 mA, plenty for this. The capacitors are .01 uF safety caps, also obtained surplus. TV's used to have these in some of the horizontal circuits for safety, and across the power line. Don't think they are used for that any more, as this was actually a little dangerous in the presence of all that dust inside most old sets. The series resistor is 8.2 ohms, 3 watts, from Digi–Key. This is just to keep the little 1 amp fuse on the inverter from frying when the output is shorted.
|Here is another version that is regulated. The outputs of the little inverters are more or less proportional to the inputs. So, we took a nice big box (room for a 12 volt gel cell in there) and made a regulated ± 1200v supply. This has a 10 turn pot for adjustment, a nice blinking neon danger indicator, and most importantly, a quiet output (no ripple) so it is good for things like photo–tubes and Geiger counters. No voltage multiplication is used here, just a diode and a two stage RC filter for the output. Sorry, I do not have a schematic of this one, but will make one if someone asks. It's a pretty simple circuit, IIRC, just an LM358 op amp, a zener reference, and a good old IRL 3705 as a current booster for the op amp output. One of the output voltages is divided down to compare to the reference. The red stuff you can see on the board is Glyptal, a very good insulating paint. Without this, the high value resistors used in the voltage divider would have their conductance swamped by surface leakage. This is more heat resistant than paraffin.|
|Here's a picture of the box, with the battery we use. This makes the thing totally portable, and floating from ground, which can be nice. One portable use of this is something we are playing with over on the chemistry side of things, a detonator for high explosives that needs no primary explosive at all. That's nice for both safety and toxicity. For this we use a 30 uF 2500 volt capacitor, obtained surplus, and a spark gap to switch the output into what amounts to a miniature rail gun that is the detonator. Next time I make a spark gap for this energy level, I will leave room for pipe caps on the ends. This gap is noisy.|
These are a few of the large energy storage capacitors we have here.
Both the .5uF/30kV Maxwell and the 30uF/2.5kV Aerovox store about
225 joules. When fired through a spark gap they will make you
jump out of your skin (or pee yourself). Although this isn't all
that much energy, it comes out really fast, in the billions of watts
during the pulse. It is enough that the magnetic field will rip
speaker wire apart. We will have to do special things for the Maxwell
to get down near its intrinsic series inductance, as in make a big
flat transmission line. The spark gap you see hanging off the
Maxwell was made with two 1/4–20 bolts and a 4–40 screw as
the trigger electrode. Alignment was insured by drilling the main hole
all the way through in one pass, and tapping the same way. Next time
I make one of these, I will leave room for end caps. This thing sounds
about like a pistol shot. Not healthy indoors. |
The plexiglass box in the foreground is the primary capacitor for our Tesla coil, and other things. It is 50 .47uF 1000V caps in series, serpentine style. In use, the box gets filled with mineral oil for cooling, and the whole thing is wrapped in copper flashing to cut inductance. Even without these steps, the peak discharge current is such that it heats up the "little" silver plated 18 ga wire quickly when fired at a 120 hz rep rate by the 1 kw neon sign transformer we use. Haven't measured the peak current, but it must be pretty huge. We didn't really need a 50kV rating here, but did need to get a capacitor that the neon sign transformer could charge in one half cycle, and had these lying around.
1000 v, 6W. DigiKey number 289–1025 is a nice one. There are
other ones that are probably just as nice.
.47uf 1000v pulse rated capacitor is DigiKey BC1918. We bought a hundred of these guys. They are nice. We also used them for the primary capacitor for our Tesla coil, 50 in series to get .01 uf, the correct load for our 15 kV 60 mA neon sign transformer. Got that guy at Glantz. Those capacitors are very good pulse caps. #18 silver wire in series with them and a spark gap triggering at 120 hZ gets HOT FAST. So the peak currents must be really enormous. I will measure it someday.
Diodes are Diotec DD300, and I'm sorry, I have no idea where you would find these in single–user quantities. I also don't know where to find the nifty little spark gap capacitors any more. I got both of those items from a friend who had found them on e–bay.
The big energy storage caps are around if you look, but at prices that will leave you breathless, mostly, especially if the dealer knows what they have. The best deal I've gotten so far is on that 30 uF 2.5 kV unit, which was from an airport landing strobe. I got it for $30.
The ten turn pot I rescued from an analog computer that was being thrown out. I have almost 100 of these if anybody wants to buy one. They are 5k, have all the hardware, and seem to be in good shape.
Coulter's Smithing Home
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