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Registered Member #1134
Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351
I set out to build an even smaller DRSSTC after my last effort. It is based on my simple DRSSTC driver that I mentioned back here: so nothing special there.
Because I wanted it small, the high frequency it would be projected to run at would become something of a problem. The finest wire I was prepared to wind was 0.125mm, and it was wound on a 110mm x 50mm form, which gives an fres of around 830kHz, a large enough toroid could drop this to around 500kHz, but this is still uncomfortably high.
The solution was to incorporate a SECONDARY MMC into design as mentioned by Steve Ward in his epic new DRSSTC coil here:
I designed a MMC using a string of 14 10kv 220pF caps, to give me a total of 16pF @ 140kV. This string was placed inside a Pyrex test tube, that I cut the bottom off, and filled with paraffin wax. This assembly fits right down the middle of the secondary coil.
The fres of the secondary now sits at around 295kHz, and with a small toroid, courtesy of John Freau, the fres drops to around 260kHz.
The primary cap is 0.188uF, with a 5.1 turn primary wound on a 65mm plastic form.
After solving a couple of issues getting the feedback to work properly with the secondary, and a little tuning, this thing pumps out 7-8inch arcs!
There are a couple of little bugs need sorting out, for example it absolutely does need a strike rail! And perhaps another few coats of varnish on the secondary wouldn't go amiss either.
There is also an interesting quirk I noted. When the coil is powered, it takes several bursts from the interrupter before there is any output at all, if you start with a very short pulse width. However, if you start at 100µs or so, the thing bursts into life right away, however with each subsequent burst, the arcs get bigger and bigger! So then you have to back off the pulse width a little to keep the thing under control.
Perhaps Steve could explain that one. I'm sure it has a lot to do with the huge secondary capacitance. My best guess is, that the secondary LC combo is still oscillating for a long time between bursts, so there is still energy in the resonator, when the next burst arrives?
It seems to be a very all or nothing coil, and for its size it is very impressive!
I am totally convinced now that secondary MMC's are the way to go with mini coils like this, and I am sure one could be made much smaller!
I will try and get some spark pics up this evening.
Registered Member #1134
Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351
Ok, so the obligatory spark pics!
I apologise for the poor quality, I always have problems trying to catch sparks well on my cheap camera, so I simply videoed it, and stripped out all the frames as jpegs with ffmpeg.
The target is an ungrounded coke can (sat on top of a candle, as an insulator).
Interestingly striking the can doesn't seem to detune the coil much, and the first time I set it up it was too close the coils case, and arced to the can, then from the can to a bolt in the plastic case, that fortunately wasn't connected to anything important!
The gap between the topload and the can is 6.5" in the photos, I had it at 8", but the arcs were hitting the base of the coil more often than the target, and I have not built a strike rail yet.
This thing is going to get a new case too, I have a pair of small PC power supply cases from the "book PC's" that were popular in the 90's, that should make a suitable (and much smaller) shielded case for the electronics.
Registered Member #3900
Joined: Thu May 19 2011, 08:28PM
Location:
Posts: 600
what type of caps do you use inside the secondary? steves qcw uses mkp film caps, do you use the same? i have a cool secondary that i half assed a while ago, didnt do any resonant guesstimations boforehand knowing the ~fres would be high. i had no idea that it would be so high as to be out of 4046 range! i'd like to run it somehow, whether its via dr mode or class e- depending on the final resonant frequency.
Registered Member #1134
Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351
ben123324 wrote ...
what type of caps do you use inside the secondary? steves qcw uses mkp film caps, do you use the same?
I was stuck for room in this small coil, so I went for wire ended disc ceramic caps.
If I had real money, I would have bought a bunch of bare ceramic capacitor elements, and stacked them (yep apparently you can buy them without leads or epoxy coating!), but this worked out well. The value I chose for the caps, meant they were physically small enough to slide easily into the pyrex tubing.
Although Steve didn't pot his, on small coils like this, I think it is a good idea, since there is not a lot of clearance between the walls of the cap, and the inside of the secondary.
Registered Member #3900
Joined: Thu May 19 2011, 08:28PM
Location:
Posts: 600
plazmatron wrote ...
If I had real money, I would have bought a bunch of bare ceramic capacitor elements, and stacked them (yep apparently you can buy them without leads or epoxy coating!)
Although Steve didn't pot his, on small coils like this, I think it is a good idea, since there is not a lot of clearance between the walls of the cap, and the inside of the secondary.
Do tell! Where can I find these, or what can I search for? If I could stack a thin pile of ceramic and pot them in epoxy, then it would fit right in without me having to worry about the stray secondary wire.
And although Steve didn't literally pot his, for all intents and purposes he did. A piece of PVC inside his secondary held them. Probably wouldn't suppress much corona, but it kills the issue of that stray wire.
Registered Member #1643
Joined: Mon Aug 18 2008, 06:10PM
Location:
Posts: 1039
Hmm..Rather interested in possibly trying one out for myself. Is the capacitor string literally in parallel with the secondary? I have a mini coil going, but given i have the right wire laying around, as well as the caps not costing too much...rather interested in trying this build and see what I can get out of it myself :)
Registered Member #3806
Joined: Sat Apr 02 2011, 09:20PM
Location: France
Posts: 259
Amazing results plazmatron !
plazmatron wrote ...
There is also an interesting quirk I noted. When the coil is powered, it takes several bursts from the interrupter before there is any output at all, if you start with a very short pulse width. However, if you start at 100µs or so, the thing bursts into life right away, however with each subsequent burst, the arcs get bigger and bigger! So then you have to back off the pulse width a little to keep the thing under control.
Hehe sounds good, something to explore further ^^
I imagined something interesting with this secondary MMC...
I'm gonna try that soon too, but the only coil I built so far has a secondary ID of 18mm, I found some small PP caps that could fit, but they will be very close to the winding. As Steve explained, I hope that the voltage gradient along the cap string being the same as the gradient along the coil will help in preventing internal arcing.
Registered Member #152
Joined: Sun Feb 12 2006, 03:36PM
Location: Czech Rep.
Posts: 3384
Yes, the russian RF caps would be ideal for this, no power loss ( ), easy stacking with screw threaded terminals and you can even get them in near-toroidal shapes.
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A small DRSSTC with a Coke can sized secondary and an Fres of only ~260kHz !
I'll have to try this capacitive loading trick myself.
), easy stacking with screw threaded terminals and you can even get them in near-toroidal shapes.