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Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
I think the main problem is that they are *used* overheads. This means they've been handled, which means hygroscopic salts from sweat are contaminating the surface. Surface tracking over time is such a difficult problem above 10kV that you need to start with the deck stacked in your favour, like using squeaky-clean, never-been-touched dielectrics. You could try washing and drying them, but ...
I've often wondered about using parafin wax (candles) as an encapsulant. A big down side is you'd have to built the stack at 70C or higher to keep the wax fluid, but once it cools it doesn't rush around like oil, and it's easier to dismantle than hot-melt or resin. Just a (wild) thought.
Registered Member #325
Joined: Fri Mar 17 2006, 12:42AM
Location: Turku, Finland
Posts: 55
Ok, thank you for all the useful input. I believe I will just use the two capacitors in series and build a few more to get the required capacitance. They seem to work fine in this mode at about 12 kV per cap.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
I don't want to discourage you, but even at 12.5 kV I think that they will not be reliable, search eBay for suitable Doorknob Capacitors e.g. Four 10 kV in series should be ok for 25 kV.
This forum is used mainly by experimenters who push components to their limits and are (theoretically) prepared for the unexpected. (frequently un-explainable!) Since your main interrest is the photography I suggest you don't over-stress high-voltage parts too much - you've already had a taste of possible results.
If you need really short duration arcs have a look at web pages for Nitrogen Laser, sub-microsecond discharges are normal.
Registered Member #325
Joined: Fri Mar 17 2006, 12:42AM
Location: Turku, Finland
Posts: 55
Sulaiman wrote ...
I don't want to discourage you, but even at 12.5 kV I think that they will not be reliable, search eBay for suitable Doorknob Capacitors e.g. Four 10 kV in series should be ok for 25 kV.
This forum is used mainly by experimenters who push components to their limits and are (theoretically) prepared for the unexpected. (frequently un-explainable!) Since your main interrest is the photography I suggest you don't over-stress high-voltage parts too much - you've already had a taste of possible results.
If you need really short duration arcs have a look at web pages for Nitrogen Laser, sub-microsecond discharges are normal.
Ok, I will try to use the capacitors at half the voltage for a while to see how well it works out in the long run. If they prove unreliable also at this voltage I shall look into doorknob or other ready-made capacitors.
I am well aware of the uncertainty factor in DIY high voltage experimentation and am not afraid of overstressing the parts--the only thing I've invested in these things is time (I got the material for free). I'm doing it mainly for fun and don't mind a few failures here and there, since they make a good opportunity to learn and improve designs.
BTW, thanks for the suggestion about checking out laser experiment pages, I will have a look at these.
Registered Member #56
Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445
I kind of doubt that a N2 laser would work for you because the the small amount of energy in each 'bang' The peak power way be in the MW, but if you only have 1j you can't get enough of an exposure.
As you your caps, if the transparencies are free could you putting like a 3"x5.5" strip of foil (5" tracking distance) at the ends of the pater, and assemble it with some jellied oil (like vaseline) to keep the air out? I can't see any way that would ark over... I think that your problem now is that you have air where the foil isn't, so you are getting arking there. With a layer of something nonconductive there I think like will be a lot better. You might be able to get away with using a piece of plastic about the same thickness as the foil where the foil isn't (but that would require a lot of cutting).
Registered Member #325
Joined: Fri Mar 17 2006, 12:42AM
Location: Turku, Finland
Posts: 55
I have now tested the capacitors at half the target voltage (~12.5kV) for about 50 firings with my triggered spark gap and they seem to hold up nicely. I haven't had a single arcover after lowering the voltage. There are some sizzling and crackling noises from inside the capacitors, but it seems to be harmless. Of course, being home-made I understand it might still just be a matter of time before it fails.
The disappointing thing is that the light levels are too low. I assume that most of the light emitted by the spark gap is in the UV range, which is useless for photos (I actually use UV filters on my lenses). So I need to build two more capacitors, which will take about one whole day.
About the printer/copier toner--even if it might be sligthly conductive, the surface tracks I've had do not seem to follow any kind of pattern. I've had some tracks over empty (non-printed) areas of a transparency. I am aware of the possibility of salt and fat deposits on the slides from handling, so I did try to clean them of a bit (using alcohol) before assembly. Buying unused transparencies is not an option--they are prohibitevily (sp?) expensive!
I would like to use doorknob capacitors or other "proper" capacitors if I can find them for a reasonable price. However, I'm a bit tired of browsing eBay all day, only to be overbid at the end of an auction. Buying new is just too expensive and high voltage components are difficult to get around here.
Anyway, thank you all for your valuable suggestions.
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Homemade plate stack capacitor, surface tracking problem
