MOT PSU FAQ
Q: How is this thing better than an NST?
A: It isn't. For hobby-type coiling, NSTs are superior in every respect. I built this because I can't afford new NSTs, and I have no convenient source for used NSTs. If you can get ahold of a few NSTs, use them. They are a lot less trouble.
Q: I don't know anything about electronics. Can I still build one of these?
A: Yes. If you have the tools and skills to build a Tesla coil, then you can build this MOT power supply too. Mastery of the theory of operation is not a requirement. If you don't understand how it works, then just carefully copy mine. If you know enough to hook the wires up in the right places, it will work fine.
Q: About those wires--how do I phase the MOTs?
A: If you are using a 120vac power cord, you must wire the primaries of the two MOTs in parallel, but in opposite phase. This is not as complicated as it sounds. Just wire them in parallel, then test the power supply. If it won't fire a narrow spark gap, then reverse the primary connections on one of the MOTs, and try again. If you get the phasing backwards the first time, it won't damage anything--it just won't work. If you plan to use a 240vac power cord, then you must wire the MOT primaries in series. Again, test the power supply and see if it can fire a narrow spark gap. If not, then reverse the leads on one of the MOTs and try again.
Q: I don't live in North America. I have 240vac/50Hz wall outlets. Can I still use this design?
A: Sure you can. Assuming your MOTs are pulled from 240vac/50Hz ovens, you must wire the primaries in parallel, reverse-phased. A 10 Amp outlet will suffice for 1080VA or 1600VA. For 3200VA (maximum ouput), you'll need at least a 13A outlet. Also, the 50Hz current will affect the ballasting a bit. If you use the same value MOT caps I did, the power output will actually be a bit lower--in the range of 900VA, 1300VA, and 2700VA respectively.
Q: How much did it cost you to build this thing?
A: I don't know. I'm a packrat, and I've been saving up MOTs and assorted electrical junk for years. I used a mix of new parts and junk box parts. I probably brought in both power supplies for under $50 out-of-pocket. I propose a simple test. If you can get a couple of good NSTs for no more than twice the cost of the MOT PSU, you are better off buying the NSTs.
Q: Where can I get MOTs?
A: You can scrounge at a dump or landfill. You can drive around apartments, hotels, mobile home courts, and similar rental properties. The landlord of such places often discard broken ovens every week. You can ask for broken ovens at a Goodwill or Salvation Army thrift store. People often "donate" broken ovens, and these charities usually discard them. You can beg at a repair shop. Cheap ovens are not worth fixing, and they might have a dumpster full of them. You can ask friends and family to hold broken ovens for you. You can put an ad in the classifieds offering free pickup plus a small payment for broken ovens.
Q: Where can I get microwave oven capacitors?
A: From the ovens, of course. Or you can buy them. I needed six oven caps all exactly alike, so I bought them from an outfit called USA Manufacturing. Their prices are very good. Service is good too. You do have to establish an account with them before you can buy anything, but it's not that much trouble.
Q: Must all six microwave oven caps be identical?
A: They need not be physically identical, but they should all have the same capacitance. If you use different values of oven cap, the voltage will not divide evenly among them, and the arithmetic to figure out the power output becomes more complex as well. USA Mfg sells many different values of oven cap. As long as you buy six all alike, you'll be OK. Of course, if you use anything other than .97uF caps, you'll have to calculate your own VA rating.
Q: Your power supply seems complicated. Is there an easier way to do this?
A: Yes. The adjustable ballast thing is not mandatory. You can build it using only two doubler caps (one on each leg), and live with fixed power output. Simpler still, if you can get four or six MOTs all just alike, you can wire them in series for a very beefy 8kvac or 12kvac power supply, with no need for any electronics at all.
Q: Where can I get the other electronic parts?
A: You can get the 1000pF/10kv ceramic disk capacitors at All Electronics for $1 each last time I checked. They are catalog #HVD-102. They carry the diodes too (1N4007). I found the diodes cheaper at Mouser Electronics.
Q: I already have a nice, big variac (or slide choke, or other reactive ballast). Can I use it as a ballast on the 120/240vac side, use only two doubler caps, and still have a variable power PSU?
A: Yes!
Q: I don't have any 28AWG magnet wire. Can I wind my chokes with something else?
A: Yes. I used 28AWG because it happened to be on hand. Whatever you wind your Tesla coil secondary with--use that for the chokes too. It will be fine as long as it can stand up to 750ma or so.
Q: What size tank capacitor should I use with this power supply?
A: Good question. The usual capacitor calculations that work so well with NSTs won't work for this power supply, but I do have some general suggestions. You can forget about that 6 pack of beer bottle caps you used with your NST. The twin MOT PSU has lower voltage and higher current than any NST, so it requires a bigger tank cap. 50nF seems like a good compromise size that works well at all three power levels. Along these same lines, I think a Tesla coil based on a 4" x 24" secondary winding is a good size for this power class. If you want to have a reasonable number of turns on the primary with a 50nF tank cap, you need to wind that secondary with finer than usual wire--I suggest 24AWG magnet wire or smaller, and even 28AWG isn't out of line.
Q: What kind of spark gap should I use?
A: Due to the modest voltage and rather stiff current, RQ gaps and similar multi-gaps are not a good idea. For reliable firing, I suggest a single static gap with a fairly narrow electrode spacing. For any hope of good quenching with such a gap, you'll need plenty of mechanical assistance, such as a blast of compressed air or else a powerful vacuum cleaner motor to blow out the arc. My single static sucker gaps have both worked well. I made one using 3/4" copper pipe and fittings, and a bigger one using 1" copper pipe and fittings. They both perform very well with this power supply.
Q: Can I use microwave oven diodes for the voltage doublers instead of building that huge diode string?
A: Yes, but be cautious. Most oven diodes are only rated at 500ma. This power supply is capable of over 700ma at maximum power. If you are satisfied to run only at medium or low power, you can probably use the oven diodes. You'll still need two of them in series, plus my bypass (filter) capacitors in parallel.
Q: Why did you immerse your MOTs in an oil tank on the compact power supply, but not on the breadboard power supply? Do I need to dunk my MOTs in oil?
A: I put my MOTs under oil back when I built the original MOT supply because at 3000VA, they got pretty hot after a few seconds. Also, I was having trouble with the MOTs arcing over to the inside of the steel ammo can. The oil bath cured both problems. Unlike the tight confines of the ammo can, the open-air breadboard layout lets plenty of air get to the MOTs for cooling, and there's nothing nearby for them to arc too, so they don't need an oil bath. If you plan to pack your MOTs into a tight little enclosure, you'll need to fill it with oil. If you use the breadboard layout, you don't need the oil.
Q: You mounted both your power supplies on high-density fiberboard. Can I use plywood as the foundation for mine?
A: I don't like plywood for high voltage. Plywood has long, unbroken strands of wood that can break down and form a carbon track, instantly ruining your power supply. Ditto for regular wood. Fiberboard and particleboard are made of millions of chopped up bits of wood held together with glue. This means there are countless broken wood strands and glue barriers to prevent high voltage creep. Better yet, use hard plastic or phenolic. A plastic cutting board from a cheap discount store would be a "natural" for the breadboard design.