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So I have a dual MOT coil that I have been building with the help of KillaX (I think that's his name here..) videos can be found here: (uploading an RSG video now) And my doubler has died and I really cannot figure out why. I cannot test the MOT diodes as they simply wont read on my meter and I never tested the MOT caps pre installing them, however now they are both around 58nf with a posted capacitance of .9uf. Here is the diagram I used: Since the caps still have capacitance, although they got stupid hot when the doubler worked, I presume they are not the issue. When I put the doubler in though, nothing happens, no voltage out, nothing. Currently I am running 4kv into the coil and getting about a foot out max which I figure is fairly respectable for an AC SGTC. I recently built a RSG, uploading the video of it soon and the carnage it did to my drill bit (Don't drill grinding discs with drill bits you like..). So because of the failure I am just re-building the doubler from scratch using 1N4007's and some caps I have laying around. Not sure what ones yet, but I was told to stay away from ceramic.
When working with the doublers, what should the components be rated for? I want to double a dual MOT tesla coil and I killed the last doubler (using 2 MOT caps and diodes) so I am making it all from parts now, diodes from 1N4007's I just dont know what voltage I need everything to be rated for. Output from the doubler after RMS is 11.8kv or so at around 250-500mA. Any idea?
Any help is greatly appreciated. I am happy to make specific videos if anyone has questions regarding something about the coil. I really want to get better output on it so I can begin to use it for testing of HV resistance of sensitive electronics.. and to kill a couple of old cell phones as per the android forum's request. This is my first coil for anyone that is asking so please bear with my lack of the complex workings of these coils. Yes it has bit me before, hurts like hell, please, if you cannot help me, I dont need your criticism unless it is constructive :) This coil will hopefully be donated to the local science museum (MOSI in Tampa, FL) as their largest one is a small dual MOT with no voltage doubler and a stationary gap.. it has also not been cleaned in what looks like years, primary is all corroded and it apparently is burning through its MMC capacitor. Anyways, thank you in advance for the help :)
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
If you're using this to power a Tesla coil, you need a DC bus inductor and a de-Qing diode.
You can't just connect the doubler's DC output across the spark gap, because the doubler's capacitors will dump into the spark gap every time it fires, hence why they get "stupid hot" and die.
If you're using this to power a Tesla coil, you need a DC bus inductor and a de-Qing diode.
You can't just connect the doubler's DC output across the spark gap, because the doubler's capacitors will dump into the spark gap every time it fires, hence why they get "stupid hot" and die.
That makes sense.. It did work whilst it was alive though, which made me figure that one of the components simply could not handle the power from the coil itself. And yea, its a Tesla Coil power supply. I never noticed it on my bank capacitor because it is rated so high, but those puny little MOT caps do not like that kind of abuse. Would you be able to help me design/ steer me in the right direction to building the necessary components (the inductor and de-Qing diode)? Also, any thoughts on the ratings of the diode stack and caps for the new doubler? My bank capacitors are .1uf @ 20kvDC and I have 5 available
Registered Member #195
Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111
you need to dump the DC cap and diode. and go with a AC system find some caps with a low ESR like polypropylene or mica caps and make a old school spark gap tela.
you need to dump the DC cap and diode. and go with a AC system find some caps with a low ESR like polypropylene or mica caps and make a old school spark gap tela.
This is an old school Spark Gap coil. A Dual MOT setup. My MMC is made of nice poly caps (well really it is currently only one capacitor) from Custom Electronics Inc. in NY. By biggest limiter here is my input voltage, hence the doubler and DC rectification.
you could phase the transformers 180 deg out of phase and get 4200vac and then you might be able to use a cap like this
you can try the cap you have as well. how much experience do you have building teslas
Wouldnt out of phase cause them to not function at all? I am an audio guy and that's more or less how 180 out of phase speakers work, quite odd actually if they are placed right.. Either way, I am getting about 4kv out as it is. My first post brings you to my youtube page where I detail the coil a few times including the set up. The caps I have I believe are very good, run about $1000 a piece (yes a thousand dollars) but I asked nicely and got them for free lol. The coil was designed to have 6 turns and 500nf but that was soon found to be WAY wrong, not sure why, both SolidWorks and JavaTC says 6 turns at 500nf is good, but it is sitting at 100nf at 1.3 turns or so.
This is my first coil and my first real experience in the TC world. I play with high amperage (210,000 watts [144v dc @1400amps continuous, its an electric vehicle]) but high voltage is new to me. I am sure the coil is slightly out of tune as I do not have anything to measure the precise tuning of the coil itself, but I did play the whole try this spot, and this spot and this spot and see what is best. I have a decent knowledge of electronics but up until a few months ago I have never had the ability to practice it (lived at college where they wouldn't let me mess with high voltages.. I tried once.. it didnt work out well..) TC's have always interested me if not for their scientific properties than at least LIGHTNING.. I also had a motive as well.. I have a couple of old cell phones that need to die and the droid forum guys agreed that a Tesla coil is the best way to kill it, they even told me to not part the phone out, simply kill it as it stands and see what happens.. Anyways, yea I am a noob at this, I know the dual mot coils are dangerous, but it is the best i can do with my budget. I simply cannot afford to spend 100 on an NST (they are expensive near me or generally broken) and I have been cleared to pull any parts from old university microwaves that I want, so I essentially have an endless supply of MOT's if I need them. SGTC made the most sense, it is simple and doesn't have a high chance of blowing up. I am taking what safety precautions I can to remain safe. As for the caps, I do not have an ESR meter to test them, but I know they are damn good caps, 5 all custom made polypropylene I believe as well. You can see them in my TC vids on youtube, but I am happy to post better video if I have it or take video for you of a specific part of the coil. I just finished a rudimentary RSG for it, and it works meh at best. I am somewhat limited on my materials so I used steel carriage bolts (round heads) and because not everything is perfect when you dont have CNC stuff there are some issues with alignment. I have never had to circular interpolate but I was within a 1/16 of an inch from round and within 1/128th inch in height (I am good at woodworking) but it does all add up in the end.. The RSG is fun though ha ha! I should probably get a transformer to de-tune the angle grinder as past 40% on my variac it seems to be spinning too fast to catch an arc, mind you, this is all without any voltage doubling, straight MOT output.
Registered Member #195
Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111
the caps that you have are low ESR caps I can tell by the terminals what is the voltage rating and the listed capacitance? I was at work where you tube is blocked. a SGTC has a schematic like this
I have seen pictures of mot tesla's with 4 mots and it made 4 to 5 ft sparks. since you have 3 so phase and parallel and use the caps in a series or parallel to get the capacitance that you want. I found this
by using the criteria " tesla coil construction guide " on google and found a plethora if guides. On a side note the high voltage diodes are bunch of single diodes connected in series in side the package. each segment has a forward voltage drop of .7v so a diode with 40 segments will have a forward voltage of 40 X .7 that's why your meter won't measure them. The way you test them Is to use a power supply that will overcome the forward voltage so for my example you will need a supply that is grater than 28 vdc you should add some series resistance to limit the current say 1000 ohms. Once you have the setup you can add a series led to see the current flow one way the led will be on and the other it will be off. If the LED is off or on in both directions it is bad
Do you have a picture of the front of your capacitor? If you get it going check the temperature make sure they don't get warm, hopfull they are AC rated. If you can find one more mot you can do a series parallel to get 4 kv ac. mots are terribly inefficient and the will draw 2 to 3 amps idle
the caps that you have are low ESR caps I can tell by the terminals what is the voltage rating and the listed capacitance? I was at work where you tube is blocked. a SGTC has a schematic like this
I have seen pictures of mot tesla's with 4 mots and it made 4 to 5 ft sparks. since you have 3 so phase and parallel and use the caps in a series or parallel to get the capacitance that you want. I found this
by using the criteria " tesla coil construction guide " on google and found a plethora if guides. On a side note the high voltage diodes are bunch of single diodes connected in series in side the package. each segment has a forward voltage drop of .7v so a diode with 40 segments will have a forward voltage of 40 X .7 that's why your meter won't measure them. The way you test them Is to use a power supply that will overcome the forward voltage so for my example you will need a supply that is grater than 28 vdc you should add some series resistance to limit the current say 1000 ohms. Once you have the setup you can add a series led to see the current flow one way the led will be on and the other it will be off. If the LED is off or on in both directions it is bad
Do you have a picture of the front of your capacitor? If you get it going check the temperature make sure they don't get warm, hopfull they are AC rated. If you can find one more mot you can do a series parallel to get 4 kv ac. mots are terribly inefficient and the will draw 2 to 3 amps idle
My 3rd MOT has the secondary shorted for current limiting, the issue with the 4MOT set up is that the MOTS on the end float really high, and having this to show to kids I dont want little johnny getting a nasty shock. I would prefer to use the dual MOT set up and just voltage double the circuit, and frankly I do not want to build another circuit, this one is pretty heavy as it is..
The caps used in my MMC are roughly the size of my hand and weigh around a pound or so a piece. The tabs are 3/4 inch wide by about 4 long (width of the cap) and are some form of tinned copper (I only found this out when I drilled for a bus bar setup to get the caps connected as solder would not stick) Ratings are 20,000 WVDC at .1uf +-5%. No rating for temperature. The caps were never guaranteed to work from the company, they had them laying around and said if it works I get good video, if they explode I get good video. His logic was sound and they work great. They stay ice cold, no heating at all, I am only feeding them 4kv (well at this point it is just one) and it seems to just laugh in my face. At 500 NF here is what it does to a stationary gap using Cap Nuts: That was 15 seconds of run time.. 500nf is stupid loud and made a really hot arc. Even 100NF will break down steel over time, hence my issues with the RSG currently.
The mots will happily draw a 4kv arc all day long, I an actually melt a 5/32 inch tungsten welding rod with them. Before the coil was even built I made the power supply and has fun melting the rods.. Drawing a constant arc the ballast MOT will get to 300 degrees (where it begins to smoke) in roughly 45 seconds. When on TC duty it takes much longer and now with the RSG heat is significantly reduced.
Maybe I have not made it completely clear, but I have a working coil lol. I want to make it better with a voltage doubler. With only 4kv in I can religiously get 8-12 inches out of it which seems par for the course in this low voltage. With the doubler I can expect almost 12kv out and DC as well which is where the fun can begin lol.
Registered Member #195
Joined: Fri Feb 17 2006, 08:27PM
Location: Berkeley, ca.
Posts: 1111
this is what I meen by phasing. when you connect them the way you have it the case of one transformers will be at 2100vac which can flash to the mains if it arcs. keep the 3rd mot for a ballest but remove it if you want more power.
those caps should work fine and even better if it was a ac setup. I relise that you are trying to use a dublier to increase your voltage for your spark gap. When using dc a rotery spark gap might be necisary. your caps are a pulse caps. if you are willing try a ac setup reduce the amount of gaps you have till you get to start arcing. if you can't get it to start make some gaps with smaller spacing. Ither way it will take some tinkering
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Dual MOT doubler woes
