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Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Hi guys. I know it's really embarrassing that after all this time I haven't even built a proper sgtc. It is absolutely nothing impressive for your point of view but it is a great step for me, and I still have big concerns about it. I started it last winter but only seen first light this summer.
I know my talk has no value here so I'll just proceed to posting some pics.
Primary forms
Bent and screwed on a stand. I did not photograph the painful process of stripping the insulation from the wire.
Closeup
Somewhere halfway through I figured out that I'm actually forcing the wire backwards, but couldn't be bothered to disassemble it and start all over..
Copper pipes cut and promptly polished for spark gap.
And soldered like this. The gaps were way too wide, I only ended using like 3 of them and this needs redesign.
PCB for a low pass filter and safety gaps, soldered so they don't accidentally move. Two 100pF capacitors made out of PVC pipes are going to be used.
Crappy 5W resistors died soon after first light. Strings of durable carbon resistors are far more durable but don't fit this filter, it needs redesign as well. Im going to use ceramic caps as well instead of crappy PVC pipes. The transformer on the pic is a 12kV 20mA OBIT.
Setup. I don't want to assemble anything together until I'm free of problems (is there such a thing as ''free of problems''???)
Caps. Not using bleeder resistors
Spark gap. I was limited by it pretty much due to very wide spacing, 3 gaps are too little and at 4 my safety fires. Need to rebuild it with much narrower spacings.
Lookup
Kind of how a first light looked, but I was frustrated by poor sensitivity of my camera. Sparks were already about 40cm long!
Only slight tweaking and got pretty continuous 43cm hits. Spark is very faint even for naked eye in daylight, it's hard to photograph with my low sensitivity camera.
For a single 240VA transformer my freau factor is currently about 1.1 if I count my apparent power into equation... how much W is actually going in is unknown but surely substantially less. But most efficient coil should have about 1.7 right?
Also my spark gap spacing can be increased by at least about 1mm so there's surely space for some more sparks.
Now, some questions:
- I have not ran the coil too much in great fear that my transformer may Die, if it does it will ruin the project. How worried should I be? I set my safety gaps narrowly and am using a low pass filter with 2x 100pF capacitance and 5k resistance.... but I own another equal transformer and plan to use it together with this one in parallel. I'll increase capacitance to 200 or 330pF in new version of the filter...
Should I invest into some TVS's or MOV's to protect the transformers? Or they may just die no matter what I do?
I'm asking that because I'm using a rather weird type of transformers and not NST's like almost everyone does.
- Second question: I happen to have no other caps but a large batch of 942C20P15K caps. I got into difficulty designing an MMC with them.
For single 12kV 20mA transformer resonant value was just 4.4nF, but I used 7.5 nF and bigger tank cap - number of seriesed caps was just an overkill, but it was required to get the capacitance low enough. I would used less caps and higher capacitances if my transformer allowed it...
20 caps is a lot but they work fully within ratings and were very happy with single transformer...
Problem is what should I do if I use two transformers.
Resonant value is 8.8nF and I should use some 12.5nF for proper LTR. Problem is, that this would equal only 12 caps in series which I'm very afraid would not make up for enough voltage rating.
I can use a bit lower value than that, maybe 14 or 15 caps, but more than that gets me dangerously close to resonant value... I don't know how close can I get to it. I can't decide what a compromise value should I use.
The only other option would be to use two strings of caps, which would need over 40 caps and basically drain my whole stash out! If that is the only option it may render the whole thing about an extra transformer implausible.
I'm going to keep this work up in a quest of reliable source of sparks (which I have none yet!).
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141
As a certified killer of NSTs and OBITs I'd say you should be worried (a little), however, if you set your 'safety' gap(s) to 'just' fire with just the transformer (no primary capacitor) then I think you'll be OK. The problem is that once you've tuned-up the coil, you will want more arc length here is where self-discipline comes in. RESIST THE TEMPTATION TO OPEN THE GAPS ANY WIDER !
Sadly NSTs die even in Neon Sign duty where once the tube has 'struck' the actual operating voltage is only a few kV,
Before the days of the 'Terry'Filter' I was already using a filter for my NSTs - they still died eventually.
So I'd say use the NST/OBIT gently,
P.S. I think you'd get more spectacular results without the breakout point. The topload doesn't get to full voltage otherwise. I found a single (tungsten) static spark gap with a good airflow to be better than the TCBOR-type multi-gap spark gap. If you try different spark gaps keep us updated.
P.P.S. How did you manage to wind the inner and outer primary layers in different directions ?
P.P.P.S. Regarding the capacitor ratings, for a small sgtc you can use them up to the rated dc voltage and still be within ratings due to the low duty-cycle (a couple of hundred us every 10 ms.) For this though I've always used 'balancing' resistors of 33MOhm (because that's what I had). I don't think ommiting the resistors is a good idea in terms of capacitor stresses AND being able to work on the coil quickly after powering up. The charge on an sgtc primary capacitor may not kill you, but it can certainly hurt!
Nice work so far.
P.P.P.P.S. I am building an eht supply using multiple TV flyback transformers, one of it's uses will be to power an sgtc, so I'll be joining you soon!
P.P.P.P.P.S. Isn't it amazing how six components can have so many interesting interactions ? (Transformer, spark gap, primary capacitor, primary inductor, secondary inductor, secondary capacitor.)
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Sulaiman wrote ...
As a certified killer of NSTs and OBITs I'd say you should be worried (a little), however, if you set your 'safety' gap(s) to 'just' fire with just the transformer (no primary capacitor) then I think you'll be OK. The problem is that once you've tuned-up the coil, you will want more arc length here is where self-discipline comes in. RESIST THE TEMPTATION TO OPEN THE GAPS ANY WIDER !
Sadly NSTs die even in Neon Sign duty where once the tube has 'struck' the actual operating voltage is only a few kV,
Before the days of the 'Terry'Filter' I was already using a filter for my NSTs - they still died eventually.
So I'd say use the NST/OBIT gently,
So should I abandon this project and save the transformers for some more fruitful use?
I'm happy with my current spark length from this transformer, if the second adds a sqrt2 more it will be great.
My safety gap is set so it just doesn't fire, so maybe I should bring it a bit back.
Sulaiman wrote ...
P.S. I think you'd get more spectacular results without the breakout point. The topload doesn't get to full voltage otherwise. I found a single (tungsten) static spark gap with a good airflow to be better than the TCBOR-type multi-gap spark gap. If you try different spark gaps keep us updated.
P.P.S. How did you manage to wind the inner and outer primary layers in different directions ?
I used breakout point in try to prevent racing sparks, which I had even that way. I put two more thick ugly coats of resin onto the secondary now, will see how it holds up on next run.
Is it really anything magical about tungsten apart from magical price? It oxidizes well in the air as far as I know. I can't think of anything better to use than copper or brass. I thought a multi gap would ease quenching in a small coil where it's impractical to have large fans to provide enough air flow. From other side Richie claims that multi gaps are less efficient... but I'd rather avoid having to use a vacuum cleaner for quenching in this case.
The primary coil is entirely wound in wrong direction, not just one half :P
Sulaiman wrote ...
P.P.P.S. Regarding the capacitor ratings, for a small sgtc you can use them up to the rated dc voltage and still be within ratings due to the low duty-cycle (a couple of hundred us every 10 ms.) For this though I've always used 'balancing' resistors of 33MOhm (because that's what I had). I don't think ommiting the resistors is a good idea in terms of capacitor stresses AND being able to work on the coil quickly after powering up. The charge on an sgtc primary capacitor may not kill you, but it can certainly hurt!
Meh, how did the bleeder resistors become 'balancing' and 'reducing capacitor stresses' after all? I don't buy it, current they pass is way smaller than any other current in the TC. Remember that their time constant with the caps is several seconds, I thing they'll have hard time balancing anything that way.
A bleeder resistor across each cap is probably a proper safety procedure but in this case I think it's just overkill, and difficult to mount. A single bleeder across entire bank is enough for a tesla coil in my opinion, but even without it the mmc normally simply discharges into transformer's output resistance.
P.P.P.P.S. I am building an eht supply using multiple TV flyback transformers, one of it's uses will be to power an sgtc, so I'll be joining you soon!
Yeah, it may be interesting if you could get a lot of identical flybacks... even a single flyback can probably put out as much power as my obit for some time!
Only problem I see is their high output voltage (20..30kV) and low available current, this would require very high characteristic impedance of primary circuit. I wouldn't be surprised if the design ends having like 40 or so primary turns!
Hazmatt_(The Underdog) wrote ...
Copying my gap design I see. It is nice to be immitated. ;P
Oh, really? Honestly I didn't know you solder your spark gaps this way, did you post about it on the forum? I thought to glue the copper pipes at first but figured out it's just easier and firmer to solder them onto a pcb. Very precise alignment between pipes can easily be attained this way.
PS.. oh, dug up your thread and seen it... pipes are soldered indeed?? /glad to be rediscovering something from a cool coiler
Registered Member #160
Joined: Mon Feb 13 2006, 02:07AM
Location: Melbourne, Australia
Posts: 938
Marko, Nice work. Concerning your cap bank, why not go with a single string of 15? This will give you 10nF which will be enough for the two Obits. For safety's sake, the bleeder resistors should go over each capacitor. One resistor over the whole bank won't discharge each one. I'd like to ask you why you used this primary design? I find it fascinating as I have not seen it before but had a similar idea a long time back when I read Tesla's patents. What reasons do you have for winding it this way, and what makes you say it is backwards? Concerning spark gap, the sucker gap is by far the best performer in my opinion, and still easy to build. (I just hate the big noisy vacuum cleaner attached to the end). Perhaps you could unsolder some gaps and make them smaller so that you could use 4 or 5. I made mine adjustable by raising or lowering the tubes to set the spacing. I believe that if you take the time to set the safety gap correctly, you will not kill a transformer. I have not got any other filter or anything on my circuit and still use the same NST after several years and several different coils. Ideally I would have some power resistors and a doorknob cap on there, but I have found it unnecessary on my 900VA NST. I know there are differing schools of thought on this. Nice work as always with the plexy, would have been easier if I had bent my supports like you did. How hard is it feeding that tubing through the supports!! I called it 'wrestling copper', took me hours of muscle work but yours has twice the number of turns. Nice going. Michael
Registered Member #1526
Joined: Mon Jun 09 2008, 12:56AM
Location: UK
Posts: 216
I can attest that your caps will not discharge after use if you use one resistor across the whole mmc, there will almost always be voltages floating around somewhere. I always discharge the capacitors one by one now after being zapped a few times when working on the mmc with the ends shorted.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Coronafix wrote ...
Marko, Nice work. Concerning your cap bank, why not go with a single string of 15? This will give you 10nF which will be enough for the two Obits. For safety's sake, the bleeder resistors should go over each capacitor. One resistor over the whole bank won't discharge each one. I'd like to ask you why you used this primary design? I find it fascinating as I have not seen it before but had a similar idea a long time back when I read Tesla's patents. What reasons do you have for winding it this way, and what makes you say it is backwards? Concerning spark gap, the sucker gap is by far the best performer in my opinion, and still easy to build. (I just hate the big noisy vacuum cleaner attached to the end). Perhaps you could unsolder some gaps and make them smaller so that you could use 4 or 5. I made mine adjustable by raising or lowering the tubes to set the spacing. I believe that if you take the time to set the safety gap correctly, you will not kill a transformer. I have not got any other filter or anything on my circuit and still use the same NST after several years and several different coils. Ideally I would have some power resistors and a doorknob cap on there, but I have found it unnecessary on my 900VA NST. I know there are differing schools of thought on this. Nice work as always with the plexy, would have been easier if I had bent my supports like you did. How hard is it feeding that tubing through the supports!! I called it 'wrestling copper', took me hours of muscle work but yours has twice the number of turns. Nice going. Michael
Hi,
I feared 10nF might be too close to my resonant cap value (8.8nF), and not store enough energy. Are you sure that just 1.13 resonant value would be OK? :O
I never had any cap in mmc retain any significant charge without bleeders, one resistor at the end discharges all well unless a cap fails open and holds off significant voltage. Or, if caps were in various uses for longer time they could build up irregularities... still I don't find that very plausible. Still I have enough resistors around so could probably just throw some bleeders on in the end if any residual charge problems show up. But I have far more important things than that for now...
My primary is simply the result of need to cram a larger number of turns into small volume. I wanted a high characteristic impedance (over 100ohms) thus a lower frequency secondary and lots of primary turns were used. I only have about 150A of peak current now which is easy for caps and spark gap to quench.
Now I think how it might be even more space efficient to use several stacks to make some sort of helical/conical primary hybrid.
If you look at the first pic, you can see that I drilled holes in supports with decrement that was supposed to follow the coil spiral -
but instead of following the decrement I actually went against it, and didn't know it until I practically finished the winding of first half - I noticed it is excessively hard to pull the wire between sixth and first support!
But that's hardly noticeable in appearance now, I shouldn't have created the decrement at all I guess.
Registered Member #160
Joined: Mon Feb 13 2006, 02:07AM
Location: Melbourne, Australia
Posts: 938
Marko wrote ...
I feared 10nF might be too close to my resonant cap value (8.8nF), and not store enough energy. Are you sure that just 1.13 resonant value would be OK? :O
Well you could then go 12 in series @ 12.5nF This still gives you 2X the voltage rating on the cap bank. I only use 1.6X, which depending on who you talk to may not be enough, but I have had no failures. And I'm using the 940 series of CDE caps which are supposedly not as good as the 942.
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
Coronafix wrote ...
Well you could then go 12 in series @ 12.5nF This still gives you 2X the voltage rating on the cap bank. I only use 1.6X, which depending on who you talk to may not be enough, but I have had no failures. And I'm using the 940 series of CDE caps which are supposedly not as good as the 942.
Oh, really? I thought this looks like some serious overrating... DC rating is usually recomended to be 2.5x peak voltage (45kV in my case, really an overkill) but like 12 series caps it's only like 1.3...
Any more opinions on this, before I set an expensive amount of cde caps on fire?
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Marko's spark gap coil
