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ArcticMyst Security by Avery

Tesla Coil Build Thread

Joined
Dec 11, 2011
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Congratulations on first light! That is a milestone to enjoy. Now that you know your coil works in theory it is time to do some bulletproofing!

Now then; I am thinking that due to the use of a counterpoise you are getting capacitive coupling between secondary and primary which is causing dielectric breakdown between high side drain and gate as RF feeds back in to the high side fet. Either that or there was some physical touching of the gate resistor and the drain lead.

Run that long ground wire to a RF ground and ditch the counterpoise. I think at this point you've blown up more components than the spool of wire would have cost.

Increase the spacing between the pins of your fets by angling the gate pins about 45degrees outward off the left side of the fets. Do the same with the source pins out towards the right side of the fets. Lay the freewheeling diode parallel over the plane of the fet and make 90degree bends in the diode leads to interface it with the Drains and Sources. Do not use such thick wire terminations right at the body of the fets. I know you were aiming to keep inductance and resistance low by maximizing connection surface area, but keep in mind there is 1 to 2mm of pin length inside the plastic case of the fet that you cannot correct for. Yes it is ideal to minimize any unecessary additions to the length of thin conductor present, but another 2mm isn't going to cause the bridge to fail at TC frequencies. 2mm more spacing yeilds 2kV more voltage isolation. Leave 1 to 2mm of pin length between the body of the fet and the heavy gauge wire, and a cm or so of length on the freewheeling diodes won't hurt anything as this does not handle much current, it is only there to protect the fets from flyback voltage.

If you can, get some back to back zeners across gate and source of those fets! The two zener's anodes are connected together and one cathode goes to gate and the other to source. You want a low impedance disposable path for any spikes and short currents outside of your GDT. Every time the drain shorts to the gate you risk complet;y destroying your GDT. I don't know about you but I F^%$#ng hate winding toroids by hand. Lead lengths on the zeners are not critical, as they ideally will not carry any current.

Add a very sharp breakout point to the toroid.

Place a snubber capacitor (2x DC Bus voltage, 0.1 to 1uF, film, foil, or ceramic type capacitor) across the high side fet Drain and low side fet Source.



Here are some photos of my latest half bridge. This thing is SOLID. If I had a higher voltage bridge rectifier I could run it off a doubler easily. Runs for 10min at 2.2kW before getting hot to the touch with no fan. Get as much info out of the construction as you can, the only thing better than this design would be laminated bus with actual bus bars with the caps closer to the fets (tight bend radii of stranded 10ga is hard to do).


photo1mjl.jpg


photo3gwn.jpg


photo4le.jpg
 





catman

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Wow, nice , good job.

Ever thought about building a rail gun? Or some kind of high powered laser (like couple of MW pulsed)? Or an EMP pulse generator?
 
Joined
Jun 22, 2011
Messages
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Congratulations on first light! That is a milestone to enjoy. Now that you know your coil works in theory it is time to do some bulletproofing!

Thanks! Yeah, it was so cool I didn't even get too pissed with the explosion :p

Now then; I am thinking that due to the use of a counterpoise you are getting capacitive coupling between secondary and primary which is causing dielectric breakdown between high side drain and gate as RF feeds back in to the high side fet. Either that or there was some physical touching of the gate resistor and the drain lead.

Run that long ground wire to a RF ground and ditch the counterpoise. I think at this point you've blown up more components than the spool of wire would have cost.

Will do, I found a nearby store that sells 2.5m rods and will try to place one. If I succeed I might buy a second rod.

How bad do you think it'd be if I used 2.5mm^2 wire (it' almost AWG 13) for the ground? I asked in several electric supply stores, none have AWG 6 and all of them say 2.5mm^2 is the standard for grounding purposes.

Increase the spacing between the pins of your fets by angling the gate pins about 45degrees outward off the left side of the fets. Do the same with the source pins out towards the right side of the fets. Lay the freewheeling diode parallel over the plane of the fet and make 90degree bends in the diode leads to interface it with the Drains and Sources. Do not use such thick wire terminations right at the body of the fets. I know you were aiming to keep inductance and resistance low by maximizing connection surface area, but keep in mind there is 1 to 2mm of pin length inside the plastic case of the fet that you cannot correct for. Yes it is ideal to minimize any unecessary additions to the length of thin conductor present, but another 2mm isn't going to cause the bridge to fail at TC frequencies. 2mm more spacing yeilds 2kV more voltage isolation. Leave 1 to 2mm of pin length between the body of the fet and the heavy gauge wire, and a cm or so of length on the freewheeling diodes won't hurt anything as this does not handle much current, it is only there to protect the fets from flyback voltage.

Will try this on my next bridge build but not on this one because I don't have enough stuff to rebuild from scratch and it'd take a long time to arrive... I'll have to try and repair this one :/

If you can, get some back to back zeners across gate and source of those fets! The two zener's anodes are connected together and one cathode goes to gate and the other to source. You want a low impedance disposable path for any spikes and short currents outside of your GDT. Every time the drain shorts to the gate you risk complet;y destroying your GDT. I don't know about you but I F^%$#ng hate winding toroids by hand. Lead lengths on the zeners are not critical, as they ideally will not carry any current.

Add a very sharp breakout point to the toroid.

Place a snubber capacitor (2x DC Bus voltage, 0.1 to 1uF, film, foil, or ceramic type capacitor) across the high side fet Drain and low side fet Source.

Already have a sharp breakout point (doesn't really show up in that picture) but I'll see if I can find a sharper one.

If I blow my GDT core I'm fucked, no more SSTC on the foreseeable future! I had to buy them on Digikey.ca and had a cousin bring them... No way to buy locally and ebay didn't have any at the time.

I'll see if I can get some zeners on a local store but my hopes aren't very high.. any specific models I should look for?

I could try a series of two of those voltage divider caps I'm using as snubbers but I'm not sure it'd make any difference since the voltage dividers should also serve as snubbers.


Here are some photos of my latest half bridge. This thing is SOLID. If I had a higher voltage bridge rectifier I could run it off a doubler easily. Runs for 10min at 2.2kW before getting hot to the touch with no fan. Get as much info out of the construction as you can, the only thing better than this design would be laminated bus with actual bus bars with the caps closer to the fets (tight bend radii of stranded 10ga is hard to do).

Great work! Hope someday mines will be that good :)
 
Joined
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I've used 14ga for a long ground wire with no problem, so you should be fine. You can always parallel up multiple wires.

You've got to unsolder all those components from the blown mosfets and put new ones in anyway, right? When you do that just leave a bit more lead length on the mosfets so you can spread the pins a bit.

PM me where you live (just the region/country) I'll see if I can find a better source of cores for you. Hopefully yours is fine still (normally the windings just short to the core and then they blow up, but it can break the core from thermal stress).

Any 15V zener will do, I use 5W ones but that's probably overkill.

The snubber caps have to be a lower value than the divider/dc-blocking/series/storage caps and have to be exceptionally low ESR. I've run bridges without them, but they're worth adding if you're able to.

It is a pain in the ass but you can try hot glue to insulate between the pins of the mosfets. The stuff I get locally has rather high dielectric strength, but I am sure makes will vary.
 
Joined
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Cool, I can do multiple wires. Any way to test the ground before I run the coil?

OK, I'll add the extra space :)

Hot glue is actually a very good idea. It did save my flyback from arcing once, no reason why it shouldn't work here as well. In fact I'll make sure I pot all solder joints to be sure none of them touch the damn resistor!

The only low ESR caps I have in hand are the voltage divider ones and if I buy more it'd take at least a month to arrive.
 
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There's no real good way to test the ground if you don't have a reference to compare it to. You might be able to see if you can power a low wattage lamp between your hot mains line and the new ground. Dangerous but a barebones test.

Just give it a shot, had to be better than the counterpoise, lol.
 
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Managed to drive the rod and it was much easier than I thought..
Just used a garden hose to soften the earth, pressed down the rod a bit, removed for the water to penetrate, repeated again and again. Didn't even have to use a hammer!
 
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Excellent!! I did similar for mine; I plumbed a garden hose on to a 1.5" copper pipe about 8ft long and used the water pressure to drill the hole in front of it, then just used my weight to sink it. The extra soil moisture usually makes it a better ground too.
 
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Yeah, that's a really cool technique!

Managed to find an old 6mm^2 solid copper wire (between 9 and 10AWG). I don't have enough for multiple wires but I think it should be good enough for this ground.

Pity I can only work on the coil on weekends :(
 
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I was going to fix that bridge but after checking with my DMM I found out the low side MOSFET was shorted as well. Ended up rebuilding the whole thing from scratch. Had to reuse the 2 MURs that were on the low side on the blown bridges, because I only have one more (they tested fine on the DMM). Also reused the sil pads.

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low.jpg


I tried to follow most of your advice, even added the same zeners. Used hot glue on all uncovered conductors that could even remotely touch something.

I haven't tested it yet and will wait to do so on the weekend.

Let me know if you think anything needs to be changed or is looking prone to failure. If this one blows up it'll be my last, at least for a couple months - I'm almost out of components.
 
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Hehe, my my that looks an awful lot like my bridge XD! Very nice work! If it blows up there has to be an issue elsewhere, looks spotless to me (as long as the zeners and MURs are facing correctly - can't tell from the pics).

Remember to keep the distance between the bridge rectifier (and any filtering caps) and the inverter bridge as short as possible.
 
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Thanks a lot, I have a really good feeling about this one.. like they say, third time is the charm :)

As a side note I've also ditched the SMPS for a transformer like they suggested on 4HV.

Also made some improvements to the toroid (wasn't very well centered) and found a *very* sharp needle to use as breakout point.
 
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IT WOOOOOOORKS!!!!!!

About 3" sword-like streamers @ about 30Hz on Steve's interrupter. Increasing frequency diminishes the arc until it gets down to 1cm - seems like my 420uF cap isn't near enough.

Got zapped by touching the heatsink while it was on.. my finger's still hurting :/ Won't touch it again!

The antenna has some corona on the top, should I be worried?

Will upload a video later tonight :)
 
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Excellent! Glad my help helped.

Yes, move the damn antenna before you blow your control electronics, haha. I had the same issue so don't worry, just don't run it again until you reroute the antenna farther away. Also place a solder blob on top of the antenna if you can for corona suppression - has to be nice and shiny/round.

It is normal for the spark length to decrease with frequency and is not just caused by too low bus capacitance. Though once you have enough capacitance it normally doesn't diminish too bad until you're over 600Hz or so. I'm running mine with about 1100uF on the bus, and while I would always like more, the surge current for charging those caps is insane. If I use an interrupter instead of CW and run the interrupt frequency low enough to fully charge the caps each time it loads down the mains voltage of my house enough to dim lights and make certain devices fail due to brown outs.

Don't touch any metal while the coil is running that you were not touching before turning the coil on. If you were not securely grounded before starting don't touch a ground. Everything within the e-field becomes capacitively coupled to the coil and will carry the RF current.

Very glad you're tasting success! Now, fix the antenna and get some pics!
 
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You sure did help!

Will move the antenna first thing tomorrow and place a nice blob on it. If that doesn't help I might try CT feedback.

I noticed the dimming of lights as well... I do have 9 more of those caps here, might try with 2 tomorrow.

Here are some videos I took earlier - sorry for the shakyness:

Let me know if you have any suggestions!
 




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