• Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

Mot Tesla coil not working

billybob

Active Member
I need y’all’s help here. The Tesla coil is finally complete. I had to completely reduce the spark gap design and even still it’s a little off balance and makes the primary connection disconnect (hence the tape)
My problem is sometimes I see the spark gap arcing like it should and other times it just stops. I had to replace the 20amp fuse for my variac 4 times now and I’m wondering if its just not enough for the two MOTs is there anyway I can fix this? I haven’t seen any streamers yet.
Thank you,
Ben
 

Attachments

KeepItSimpleStupid

Well-Known Member
Most Helpful Member
Orientation, may prove to be important too. See this http://zeiss-campus.magnet.fsu.edu/articles/lightsources/xenonarc.html

Arc lamps are DC and I did maintain a 1000 W Xenon lamp. Fun times just to replace a light bulb. Suited up wuth thick clothing, thin gloves and a face shield. Door got shut. Do not enter posted. No fingerprints were allowed/
The lamp was pressurized with 15 atmospheres, so 15*14.7 PSI.
 

DerStrom8

Super Moderator
I disagree with KISS. For a properly-tuned SGTC you do NOT need a breakout point, and in fact that may cause its own problems. It could reduce your overall arc length because it causes corona leakage and does not allow enough charge to build up on the topload to form a proper streamer.

As for your popping fuses, what are you using as a ballast for your MOTs?

Regarding the spark gap not firing, there are a few things that could cause this. The first is that as the electrodes heat up, their properties change and they may become less conductive, and they may start to shift. Steel is generally not preferable for this type of application as it is a poor conductor at high frequencies and the melting point is relatively low. Tungsten is much better for use in spark gaps, though I understand it is harder to come by and more expensive. The other problem is that two MOTs will only give you about 4-6kV, which is very much on the low end for a Tesla coil. While it is definitely still doable, the spark gap must be finely tuned and cannot shift at all during use. You mention vibration, which could very well be the main cause of this issue. Additionally, I suspect your spark gap is asynchronous (not timed to match the charge/discharge cycle of the tank capacitor) and that will affect performance. If timing isn't right, the capacitor charge/discharge cycle will be disrupted and your spark gap may not fire reliably.
 
Last edited:

rjenkinsgb

Well-Known Member
Most Helpful Member
You appear to have some connections screwed directly to wood?

Dry wood can be a reasonable insulator up to a few hundred volts, but when you have possibly thousands of volts it may be causing leakage.
 

DerStrom8

Super Moderator
You appear to have some connections screwed directly to wood?

Dry wood can be a reasonable insulator up to a few hundred volts, but when you have possibly thousands of volts it may be causing leakage.
This is a great observation. Especially if the air is particularly humid, the wood can absorb some of it and may cause some issues.
 

DerStrom8

Super Moderator

billybob

Active Member
I disagree with KISS. For a properly-tuned SGTC you do NOT need a breakout point, and in fact that may cause its own problems. It could reduce your overall arc length because it causes corona leakage and does not allow enough voltage to build up to form a proper streamer.

As for your popping fuses, what are you using as a ballast for your MOTs?

Regarding the spark gap not firing, there are a few things that could cause this. The first is that as the electrodes heat up, their properties change and they may become less conductive, and they may start to shift. Steel is generally not preferable for this type of application as it is a poor conductor at high frequencies and the melting point is relatively low. Tungsten is much better for use in spark gaps, though I understand it is harder to come by and more expensive. The other problem is that two MOTs will only give you about 4-6kV, which is very much on the low end for a Tesla coil. While it is definitely still doable, the spark gap must be finely tuned and cannot shift at all during use. You mention vibration, which could very well be the main cause of this issue. Additionally, I suspect your spark gap is asynchronous (not timed to match the charge/discharge cycle of the tank capacitor) and that will affect performance. If timing isn't right, the capacitor charge/discharge cycle will be disrupted and your spark gap may not fire reliably.
Thanks Matt for the input, I realize I underestimated the difficulty of MOTs. I’m using .86uf microwave oven capacitors as the ballasts. Maybe add some more? I’m not using any coils as a current limiter because I was afraid the MOTs would eat them up. I have had some problems with the variac before on even low current applications so I don’t know if that could be a problem.
You mentioned vibrations being an issue, I just can’t seem to get this spark gap right. The first one was too big and the contacts were not evenly spaced as well as the metal rod sometimes shot out. This one is at least adjustable, but yes it does rattle the whole thing.
 

DerStrom8

Super Moderator
One of the simplest ballast options for MOTs is to take an additional MOT, short the secondary winding to the core, and connect the primary in series with the HOT wire feeding your high voltage MOTs.

Rotary spark gaps are also not as simple as many people think, and it in fact takes a lot of thought and design in order to create a reliable one. I would definitely reconsider your angle grinder one. Maybe stick with a static gap for now? Either that or read up on RSG design and start fresh.

Best,
Matt
 

billybob

Active Member
One of the simplest ballast options for MOTs is to take an additional MOT, short the secondary winding to the core, and connect the primary in series with the HOT wire feeding your high voltage MOTs.

Rotary spark gaps are also not as simple as many people think, and it in fact takes a lot of thought and design in order to create a reliable one. I would definitely reconsider your angle grinder one. Maybe stick with a static gap for now? Either that or read up on RSG design and start fresh.

Best,
Matt
Sounds good. Thanks a lot, you guys are keeping this thing alive XD I’ll try those suggestions.
 

Latest threads

EE World Online Articles

Loading
Top