Instructables

Microwave Oven Tesla Coil

The NST is the preferred line transformer in a Tesla Coil. With 15kV and about 20mA it is perfect. The only probem for me is finding a cheap one (it is possible, but not with my luck)

  I though since people throw out microwaves far more than Neon signs an easy solution is to use a MOT. A large number of people say it is not possible to build one with only one MOT, or at least some form of voltage multiplier circuit.
 a MOT puts out only 2k and over 500mA (A cap on the low voltage side will limit current)

   Well it is possible to build a tesla using a single magnetron microwave (It is disappointingly weak though, an ignition transformer from a car puts out more juice, and is my preferred tesla coil transformer) I got about a 1cm spark, but it throws RF like crazy. It is a semi decent beginner coil being made mostly from scrap, and needing no power transistor drive circuit.
 
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Step 1: The Circuit

The only difference between this and a normal coil is the use of the diode.
A MOT will jump the spark gap, but it will not jump it in a coil circuit. To let it build the voltage I used a diode. The diode does not let the cap discharge. So the voltage may build and jump the gap. (Still appears to fire each AC cycle) It is not a voltage multiplier just a rectifier. A multiplier would improve the output considerably but cannot be made with one Microwave.

WARNING USING THE DIODE ALOWS THE CAP TO KEEP ITS CHARGE FOR SEVERAL MINUTES AFTER USE
There is a built in resistor in most microwave caps but please short the terminals anyway before servicing
(should go without saying) the same with the ballast cap.
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my wookie1 year ago
man i gave the perfect low voltage capacitor for this, if it weren't for the fact that its polar :( 250V and 680uF i guess that isn't technically low voltage but still, you understand what i mean.
oops got not gave and i thought id take a look at it again to double check i had the right voltage, it actually says "200 WV" here's a pic.

and just for fun, there's a pic. of the one i built based on this. only have one MOT and was only able to get around a 1/4" spark, course i also didn't include a capacitor on the low voltage side.
100_1347.JPG100_1348.JPG100_1350.JPG
jazzzzzz (author)  my wookie1 year ago
Pretty nice looking, the giant knife switch adds that extra bit of danger.
sciencetor22 years ago
did you ever get your hands on an Neon Sign Transformer? if not, i found a great ebay seller that sells really old but still working non-GFI NSTs for cheap. i used one of his for my coil and it worked great, they dont look pretty but mine worked
jazzzzzz (author)  sciencetor22 years ago
Thanks I have not yet got one, if I can get one cheap I will buy it.

I'm still leaning toward ignition transformers.(already have one)
Of course I blew through 10+ 555's and 4 CD4047BE's driving one. (PC123 might help)
I've got plenty of power transistors though.

What was the kV of the NST you got? I would settle for 7.5 but would rather have 10-15kV (then I would need better caps too)
if you need a circuit that wont burn out your 555's, then check out the circuit on my website at http://k-labs.site11.com/teslacoil/tesla-coil.html it really works, i demonstrated it's use at school
what the heck? that comes up as a blocked web page for me... no clue why, its catagorised as "Web Hosting Suspicious"
What i have heard is that NSTs are the best, and about as safe as you get with high voltage as they are current limited, mind you they can still kill, but only if the charge crosses your heart, so still not something to touch. but anyway, i personally went with a 9kv because i was a complete newcomer to coiling at the time, and thats what the instructable called for, i have since learned quite a bit more thanks to a lot of help from Xellers and google. But anyway this guy just sells what he gets, so he could have anything, its also worth asking if hes got one unlisted, thats how i got mine. Anyway i gotta go find his name, but remember, always explicitly check to make sure it is NON-GFI/GCFI, as that determines whether it is useful for tesla coils.
jazzzzzz (author)  sciencetor22 years ago
If I could get a center tapped 15k it would be the best of both. Ignition coils hurt
bad enough I do not touch anything near them while it is on. I'm ok with high voltage (still get shocked pretty often), but better safe than sorry.


If I could find a cheap GCFI I wonder how hard it would be to bypass. If the case opens it really should not be a problem. It probably is not the best idea in the world, but neither is an un-ballast MOT.(of course I sort of current limited mine)
ok here is the seller i bought from http://myworld.ebay.com/staushmanjd he doesn't have any listed right now, but its always worth asking, he gives great deals on old ones with no GFI or GFI removed, and he might have a few unlisted.
jazzzzzz (author)  sciencetor22 years ago
I will definantly watch that seller thank you.

I always install a bleed resistor(or use caps with them built
In like MOT caps) and cover the Cap bank. I think a manual discharge
Through an inductor and low value high watt resistor
may be a good idea.

the GCFI probably is epoxyed into the case
(for safety) and I will avoid them.
try just contacting the seller, if he has one he will probably just list it for you. the bleed resistors i used are sold by a tesla project supplier and are 10megaohms. they are sold in packs of 200 dirt cheap
here are the cheap bleed resistors if you want http://www.ebay.com/itm/10-Meg-Ohm-Bleed-Resistors-for-Tesla-Coil-Capacitors-/360313237049?pt=LH_DefaultDomain_0&hash=item53e457ae39#ht_506wt_1166
jazzzzzz (author)  sciencetor22 years ago
I found a good price on a 7.5kv NST on eBay and bought it
I also got some 15kv caps ( that will require bleed resistors)
So now I've got some HV fun stuff
That's a fairly low voltage... Is that to ground or across S1 to S2? Also, have you calculated the resonant capacitance and configured the capacitors accordingly?
jazzzzzz (author)  sciencetor22 years ago
S1 to S2 it is somewhat low but so is a MOT
My caps are in the mail but i shall configure for
max power at some point
What's the capacitance of your capacitors, and how many do you have to work with?
jazzzzzz (author)  sciencetor22 years ago
I bought 5 .0022uF caps.(all 5 for .0106uF)

I did a fair share of research before construction of my coil, with the thought of making a program that would allow me to optimize a coil from almost anything.

The URL’s you gave are a nice resource and one can never do too much research.
ok, well assuming your output is also at the US standard 60Hz and 30mA(correct me if that is not true, as different values require an adjustment of the equation) your perfectly resonant capacitance is 0.0106uF(or 10.6nF) a very slightly higher capacitance will extend the life of your NST(this slightly damages resonant rise,which keeps your final peak slightly lower. Perfect resonance puts extra strain on your transformer, but can provide more power, so it all depends how much you like your transformer. risk to your transformer can be mostly mitigated by using the configuration shown here in regards to the location of the capacitor/spark gap, ignoring the polar symbol in the cap, it's usually bipolar. The use of a filter such as the one known as the "Terry Filter" created by Terry Fritz can also greatly extend your transformer life.) you can use this page to calculate max safe voltage and combined capacitance in certain configurations on this site http://deepfriedneon.com/tesla_frame6.html
keep in mind that you want your maximum safe voltage equal to or greater than twice your transformer's output voltage, while keeping your capacitance as close to resonance as possible. always settle on slightly greater rather than slightly lower. i apologize if you already know all this, i'm just relaying all the stuff I had to learn building my coil. its entirely possible you know all this already ;P.
Its also probably worth noting that touching the capacitor or capacitor bank of an NST driven coil is pretty much instant death, and can be for several minutes after the coil is shut down, or days if you dont install bleed resistors(ALWAYS INSTALL THE 10M BLEED RESISTORS IN PARALLEL WITH EVERY CAPACITOR!)
I have seen it bypassed in certain transformers, however it is a risk as to whether you will be able to or not, you see the majority of the internals of an NST are sunk into a solid block of tar, while some tranformers obviously have it accessible, there is probably a possibility that it wont be. I dont know whether the GFI circuit is ever sunk into the tar or not, i havent researched it, but you would probably be better off buying one with no GFI or pre-bypassed and tested for arc than taking the risk.
Nice job just built one!
legowave2 years ago
Can you tell me what type of cap you use on the low voltage side please?
jazzzzzz (author)  legowave2 years ago
It is a 250V (AC) 45uF cap for the motor in an air conditioner (I am sure the value is not to critical)
Since I'm in the US would i want to use one with a lower voltage?
jazzzzzz (author)  legowave2 years ago
You could use a 125V, but using a higher voltage will not hurt anything, and is a good idea. I try to use 120% the voltage I am working with.
also, dont forget the AC/DC rating difference, a DC rating should be double your working voltage at least if you are working with AC, also, polypropylene film/foil or metalized polypropylene radial capacitors are best in a tesla coil, as they blow less frequently at high voltages
also, as for no arcs, did you add a breakout point?
legowave2 years ago
EDIT: Could it just be too bright outside?
jazzzzzz (author)  legowave2 years ago
It might be too bright. What are you using as a capacitive top load, primary coil, and secondary coil?

If the gap fires regularly then it is most likely a problem with the air core transformer.
The top load is a big cookie tin, the secondary is about 510 wrappings of magnet wire, and the primary coil is some insulated wire wrapped around the secondary.
jazzzzzz (author)  legowave2 years ago
That coil configuration sounds good. When the gap fires, is it a loud spark every 1/2 to 1/3 seconds or relatively continues?
It continues.
jazzzzzz (author)  legowave2 years ago
Well that is a problem.

The HV cap is not charging. (or does not work)

Make sure it is connected in parallel with the MOT out (through the diode), and if it is take it out and test it. (short it first just in case) If you do not have a cap meter charge it with a low voltage (12V or less DC). Then test for voltage a minute or so after charging.
Okay... it was taken from a working microwave so that's why I'm puzzled, but I'll try that then test it again.
jazzzzzz (author)  legowave2 years ago
Well in that case it should work. It would almost have to be somthing in the wiring. Is there anyway you could post a photo of the full assembly?

(working microwave, working magnatron, sounds fun)
I got it to work, I just wired it incorrectly. My cfl bulb glowed, but no arcs sadly.
jazzzzzz (author)  legowave2 years ago
I am glad you got it to work. Although the arcs are almost the whole point.
Yeah... Im going tinker with it a little more and see if I can get any.
jazzzzzz (author)  legowave2 years ago
Did you ground the bottom of you secondary to earth? I just clipped a wire to it, then clipped that on a screwdriver and drew arcs that way.

It defiantly needs more power for corona.
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