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Registered Member #1134
Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351
ben123324 wrote ...
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.
The only people I found who do bare coated discs is calramic here:
The best way I think, would be to have them machine a rod of barium titanate the same length as the coil with silvered ends, but as far as I know, they only make small "pucks" hence the idea of stacking the discs.
Killa-X wrote ...
Hmm..Rather interested in possibly trying one out for myself. Is the capacitor string literally in parallel with the secondary?
Yes, electrically the MMC is in parallel with the secondary. As Steve Ward pointed out, having the capacitor external to the coil, would mean taking care to ensure the voltage gradient across it is even (ie equipotential rigs etc etc). The advantage to having the caps inside the coil is that we already have the smooth voltage gradient created by the coil itself.
Steve Conner wrote ...
Well done Les, that wee coil looks great! I'll have to try this capacitive loading trick myself.
You can get small (150pF?) Russian doorknob caps on Ebay, and it may be possible to screw a bunch of them end to end.
Thanks!
For such a small coil, it really does pump out the volts!
I had considered doorknobs in the beginning, and thinking about it, I might have been better off. Those Russian caps really can take a beating, and there would be a little more capacitor volume to take care of heat.
Another possibility, especially if I try to build an even smaller coil, is to de-pot existing capacitors, and build a tiny stack myself. I de-potted one last night. They are epoxy coated, so I just heated it with a small blowtorch until the epoxy softened, and peeled it off.
These were tiny caps, so it was a fiddly job, however if I use slightly physically larger caps, I don't suppose it would take too long to do 20 or so.
Above, one of the original caps, next to its ceramic disc. A large volume of my current MMC is taken up with epoxy and lead in wires. There is potential here to manufacture a very slender MMC for much smaller coils.
Note that the silvered face of the ceramic has a blob of solder on it. This would make the job of seriesing them up pretty simple, just tin each of the faces, stack them in a metal tube to hold them in place, and bake in the oven for a while...
Registered Member #3806
Joined: Sat Apr 02 2011, 09:20PM
Location: France
Posts: 259
Steve Conner wrote ...
You can get small (150pF?) Russian doorknob caps on Ebay, and it may be possible to screw a bunch of them end to end.
I was thinking to use these K15Y-1 kvar rated Russian capacitors, but the small ones (Ø ~25mm) are rated only 6kV. That would be only 60kV for a string of 10, isn't it too low ?
There are higher voltage ones (20kV) but they are Ø 52mm and 40mm thick, not suitable for tiny coils
plazmatron wrote ...
The only people I found who do bare coated discs is calramic here:
These bare disc capacitors also seem a good solution... but how to ensure a good contact between them ? (With a spring ?) And I wonder if CalRamic sells small quantities...
plazmatron wrote ...
Note that the silvered face of the ceramic has a blob of solder on it. This would make the job of seriesing them up pretty simple, just tin each of the faces, stack them in a metal tube to hold them in place, and bake in the oven for a while...
Isn't there a risk of damaging the caps if you heat them up at 300+°C for too long ?
Registered Member #2099
Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716
plazmatron wrote ... The only people I found who do bare coated discs is calramic here: work, Les. And thanks for the link to the Calramic datasheet (which lists density in units of grams per cubic inch !)
plazmatron wrote ... Note that the silvered face of the ceramic has a blob of solder on it. This would make the job of seriesing them up pretty simple, just tin each of the faces, stack them in a metal tube to hold them in place, and bake in the oven for a while...
Yes indeed. Normal solder reflow cycles are measured in seconds; with too much cooking the molten solder might eat through the metal capacitor plates.
If the requisite number of disks doesn't fill the desired physical length, add metal spacers.
You could dispense with soldering. In well fitted insulating tube, alternate capacitors and conductive springs! (whose inductance may well be negligible). Springs don't need much compliance. Could be zigzag strips of brass ribbon from model shop, designed to hold the disks flat and/or to contact their centers.
[edit] I see that Phil beat me to the Send button on a couple of points.
Registered Member #1134
Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351
PhilGood wrote ...
These bare disc capacitors also seem a good solution... but how to ensure a good contact between them ? (With a spring ?) And I wonder if CalRamic sells small quantities...
Isn't there a risk of damaging the caps if you heat them up at 300+°C for too long ?
I don't see why a spring of some description couldn't be used. There are small very thin washers available that are sprung, the ones that are bent into sort of a saddle shape.
Other possible alternatives include conductive epoxy, or that soft woven metal mesh you find in laptops.
Re heating, probably not. The caps are mode from microfine barium titanate that is compressed in moulds at thousands of pounds per square inch, then sintered at likely well in excess of 600C. However the less heating the better. Odd things can happen with ceramics and liquid metals like solder.
The only thing I found is that the leads must be removed when the solder is good and wet, else it takes the silver coating with it.
====== Edit: I tried multiple ways today of relieving these things of their epoxy shell, including soaking in hot petrol, brake fluid, various paint strippers, acetone, methanol, formaldehyde, and even fuming nitric acid! This stuff will not shift, unless it is heated. Since the heated, softened epoxy will often peel off the silver coating with it, the best option that I see, is to dissolve off the remaining silver, and re-coat the dielectric. I mentioned before I was considering joining them with conductive epoxy anyway, so this might not be a big deal. ======
Registered Member #3806
Joined: Sat Apr 02 2011, 09:20PM
Location: France
Posts: 259
plazmatron wrote ...
Edit: I tried multiple ways today of relieving these things of their epoxy shell, including soaking in hot petrol, brake fluid, various paint strippers, acetone, methanol, formaldehyde, and even fuming nitric acid! This stuff will not shift, unless it is heated. Since the heated, softened epoxy will often peel off the silver coating with it, the best option that I see, is to dissolve off the remaining silver, and re-coat the dielectric. I mentioned before I was considering joining them with conductive epoxy anyway, so this might not be a big deal.
Thanks for all these tests that might be useful to many of us !
If we completely remove silver coating, what about double-sided adhesive copper foil tape with conductive glue, that would act both as plates and sealant ?
Registered Member #1134
Joined: Tue Nov 20 2007, 04:39PM
Location: Bonnie Scotland
Posts: 351
PhilGood wrote ...
Thanks for all these tests that might be useful to many of us !
If we completely remove silver coating, what about double-sided adhesive copper foil tape with conductive glue, that would act both as plates and sealant ?
No problem. In the end it turns out epoxy is hellish to remove!
By the way, after roasting the epoxy off with a gas pencil torch, the capacitance was exactly the same after cooling, so the ceramic elements survive moderate heating. Just be careful not to thermally shock them in any way.
Sure, why not. There are plenty of options here for re-coating. You could paint them with silver paint too.
The only issue I could see is if the adhesive on your copper tape is nonconducting. For this I see two possible failure scenarios. Either, the adhesive will just break down on its own under the high electrical stress, or any trapped air in the adhesive will produce corona, and thus hot spots within the adhesive.
Personally I'm going to go with either conductive epoxy or silver painted electrodes and Aluminium discs.
From a manufacturing perspective sputtering silver is cheap, and fast, but as hobbyists, we get to do as we like. I see no real problems in a low average power application like this.
I will throw a bare element into some nitric acid tomorrow, just to determine whether or not it will etch the ceramic........
Registered Member #4800
Joined: Wed May 09 2012, 04:23AM
Location:
Posts: 39
Thats really good :) is it possible to use this method of using the caps to lower the reasonant frequencie to about 50-70khz cause i want to use my flyback transforker to make a sstc. And im guessing i would have to make the secondary caps a higher value to get the KHz lower right :)
Registered Member #3900
Joined: Thu May 19 2011, 08:28PM
Location:
Posts: 600
Ive had no problem with the epoxy peeling off the plates. I fired up the old propane torch and softened up the epoxy. Then just cut and slid off the coating with a small flathead. Then again, I didn't have silver in mine
What's ironic is that after torching off the epoxy on some 2kv ceramic caps I get the elements all fine. But the instant I touch my soldering iron to them to tin them, 2/3 split their dialectics. Maybe bringing the entire device up to temp uniformly is the key.
Registered Member #1643
Joined: Mon Aug 18 2008, 06:10PM
Location:
Posts: 1039
So how are the caps connected exactly? Your adding capacitance to it, correct? I'm guessing in series with the topload and top wire output?
Recently purchased some fine magnet wire (all I had was 24awg). Going to construct a mini DRSSTC myself and give this secondary MMC a shot! So, from my understandings, Your removing the epoxy/resin off the caps so you can solder them together closer and reduce corona issues? I mean..is there anything wrong with leaving them as-is and just putting them in wax / mineral oil? Got 4 gallons of the crap, might as well use it... I just want to verify how things are being connected here...
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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.
