Re: MMC for 24kV: Thoughts on best construction?
..., Sun Jul 23 2006, 05:27PM

I don't think you really need to worry about corona, just make sure that you don't have both ends of the sting close to eachother...

Also, I think you have about 10x the needed caps in your bank (with the 942's you don't really need to overate the caps, with a saftey gap you should be able to run with strings of 15 caps without overvolting), so the easier method might be to use a single string of 15, and see if they get to hot...

Re: MMC for 24kV: Thoughts on best construction?
Kolas, Sun Jul 23 2006, 05:49PM

Hi, why not use ceramic door knob capacitors?
They can be bought easily rated for 50kV

I've run across the same problem for my coil, and what's the benifit of using MMC type configuration versus using doorknobs in ||?

Kolas

Re: MMC for 24kV: Thoughts on best construction?
J. Aaron Holmes, Sun Jul 23 2006, 06:06PM


JavaMMC doesn't like 15x2kV caps for 24kV. Even with three strings of 30 each, the voltage rating is only "Fair". I suppose it is applying the 2xVpeak rule of thumb, though. I've heard that people "get away" with using fewer of these CDE942s, but D.C. Cox recommends *3x* the DC rating regardless, so even with 40 caps, I'm not there. I suppose if I'm going to go to all the trouble to solder all these little caps together, I'd like to "never" have to do it again


That still qualifies as an MMC, I think They may not be "mini" as voltage goes, but they're "mini" as capacitance goes!

I was thinking of doorknobs too, but it seems relatively little information is generally available about them, and folks say they're often not meant for pulse discharge applications at radio frequencies, but rather are meant for filtering. So they don't have the "voltage reversal" spec. There's a guy selling brand new 1100pF 50kV doorknobs on eBay right now for $18 each. Not bad. The guys is in China, though, and I have relatively little faith that I'd get any adequate sort of datasheet out of him if I asked. I could be wrong, though. Anybody else have any experience with "doorknob MMCs"? Construction would definitely be a breeze!

Aaron

Re: MMC for 24kV: Thoughts on best construction?
Reaching, Sun Jul 23 2006, 06:10PM

doorknobs are the worst choice for a mmc in a spark gap coil. they will consume much power ,will get hot and crack off after a few seconds of runtime. doorknobs are the best for pulsed applications, such as marx generators, but in continious mode they will fail after a short time. there are many people out there who can tell you that.

and for the mmc, why do you want to overrate the mmc in this dimension?, ok i never used the CDE Caps but from what i heard they must be the best caps for this application,and really 350$ for a 15nF cap?for me its like a joke, i build sgtcs with 2kVa transformers and build up a mmc with wima caps (old stock) for 15€ per 10nF (15kV rating) and they never failed

Re: MMC for 24kV: Thoughts on best construction?
Sulaiman, Sun Jul 23 2006, 07:47PM

I have been using Arcotronics (Italian) polypropylene film/foil capacitors
at their rated peak voltage in sg coils - no problem.
(If CW etc then major de-rating required)

Manufacturers data sheets usually specify 2.5x over-voltage for testing!

D.C. Cox makes TC's 'professionaly' and needs reliability/safety,
home use TC's don't need such safety margins.
Just do a short run and check for overheating.
(A fan is a good idea for high power/VAR polypropylene capacitors)
as they do not handle high temperatures.
So I'd start with one string of 10 capacitors (with balancing resistors)
If they get too warm then go for more caps in the mmc.

Re: MMC for 24kV: Thoughts on best construction?
J. Aaron Holmes, Sun Jul 23 2006, 09:04PM


Perhaps then we ought to be asking why it is that programs like JavaMMC and MMCCalc don't like short strings of caps for MMCs? MMCCalc seems a little less adverse to shorter strings, however a "Good" rating from JavaMMC requires 2xVpeak, and an "Excellent" rating requires 2.5xVpeak, which would be even more than 40 series caps in my case! I suppose I'd assumed that these MMC calculators accounted for the general consensus on MMC construction, but again, perhaps the CDE 942s go by "different rules", as they're called "extremely high dV/dt" caps?


Cost versus cap size misses the point, though. Bang energy varies as the square of voltage, and as I have a stockpile of PTs giving me ample opportunity to experiement with voltages of 24-28kV, I'd rather have a cap that can take the voltage in stride than one with twice the nf's


One string of 10! They're only good for 8A RMS, according to Terry Fritz's testing. If I crank my RSG up to 360-400 bps (at which point I get 3kVA throughput--the 200:1 PT's rated power), wouldn't I surely cook 'em?

What I seem to be hearing here is that folks don't believe in the "2xVpeak" think for MMCs. Perhaps one ought to simply consider the DC rating to be the peak AC rating? If I round my 33kV up to 40kV, then strings of 20 caps would be "more than enough", in which case four strings would give me 30nF with only half the caps I was planning on using (= half the $$). That's certainly not a bad thing, provided it's not gonna explode on me!

Perhaps Terry will drop by and bless us with some first-hand knowledge of the CDE 942C20P15K's...

Regards,
Aaron, N7OE

Re: MMC for 24kV: Thoughts on best construction?
HV Enthusiast, Sun Jul 23 2006, 11:50PM


I've run those capacitors at over 30A RMS with little problems. As long as you keep them cool, they should be fine.

Re: MMC for 24kV: Thoughts on best construction?
Kolas, Mon Jul 24 2006, 01:27AM

I've heard the opposite about ceramic doorknob capacitors. Granted my coil and your coil will be seeing greatly differnt types of pulses, since mine is a tube coil.

I saw the guy in China selling them too, he says, he's the manufacturer of the caps, and I will ask him for a data sheet now.
The reason the door knobs were considered is; they would actually be more cost effective then a bunch of series\parallel caps such as your thinking of.

Kolas

Re: MMC for 24kV: Thoughts on best construction?
Terry Fritz, Mon Jul 24 2006, 02:25AM

Hi,

To get numbers, you have to give numbers...

Peak voltage?
Peak current?
BPS?
Primary frequency?
Primary capacitance?
Primary inductance?

I hear 200:1 and 24kV and 15nF...

I could assume it is a 120V input 200:1 transformer for 34kV peak output volts. If you need 15nF, then two strings of 20. About $60. Each cap is 432 amps peak at 13.5 amps RMS. you can probably run them to 3000V each but I would not push the current much. They are designed with a 1.4X margin for current from CW destruct.

Each leg is 17kV above ground so corona is not a big deal.

The cap will last about 70 hours.

Don't use oil. Chances are if one caps blows up, they are "all" dead.

Testing and programs from like 4 years ago are probably out of date now since the caps have changed significantly for the better since.

Cheers,

Terry

Re: MMC for 24kV: Thoughts on best construction?
J. Aaron Holmes, Mon Jul 24 2006, 03:19AM

Hi, Terry!


Fair enough. My interest is pretty general, though. I only have a spark gap and transformer for the coil at this point, however. The rest is still being imagined! So I don't have all the numbers you're requesting. I'm basically designing for a power level that won't set my house on fire, since my garage ain't that big! I want to build a cap that'll last through a few redesigns, and am deliberately avoiding some specificity

Your guesses were correct, though: 200:1 PT; 24kV. Two strings of 20 sound good for 15nF. Definitely affordable. Regarding your question about primary inductance, you reminded me of a question I have about JavaMMC (perhaps best answered by Bart, but...): Why does it ask for "Primary Resistance", and how does one measure that? I assume this includes the spark gap, or else the sample (3 ohms) would involve one heck of a lot of wire in the primary winding!! Hmmm... I've heard "3 ohms" passed around a lot as a typical spark gap resistance, so perhaps that's sufficient for most early design work (think I heard this in a SISG thread, actually! )

Best Regards,
Aaron, N7OE

Re: MMC for 24kV: Thoughts on best construction?
Hazmatt_(The Underdog), Mon Jul 24 2006, 08:17AM

Here's my $.02

All I run all the time are TDK Ultrafast 2000pF Doorknob Capacitors. They work FANTASTIC. THEY DO NOT GET HOT. THEY DO NOT EVEN GET WARM.

MICA doorknob capacitors DO FAIL miserabily. The mica shorts out badly. DO NOT use old SPRAGUE mica doorknobs in a TC. I have 12x of these older capacitors and they DO NOT and WILL NOT survive pulse application. They are for DECOUPLING only. I have run the old Sprague's up to ~300W RF but they will not handle SGTC duty above that.


There ya go for doorknobs.

Matt

Re: MMC for 24kV: Thoughts on best construction?
Steve Conner, Mon Jul 24 2006, 09:28AM

Like I've said before: there are two kinds of ceramic capacitors: ones meant for bypassing and DC blocking, and ones meant for RF power in resonant circuits. Doorknobs are mostly ceramic so they also come in these two flavours. The RF power ones (surprise) work in TC tank circuits and the DC ones fail. You can usually tell the RF ones because the data specifies a kVAR rating. The DC ones have a DC working voltage.

Re: MMC for 24kV: Thoughts on best construction?
J. Aaron Holmes, Mon Jul 24 2006, 03:45PM


I'll be interested to hear what he says. Still, 1100pF/50kV at $18 a pop works out to about $250 for a 15nF MMC. 4x40 CDE 942 150nF caps at $2.25 a pop works out to $180 for a *30nF* MMC (that people think can reasonably tolerate my 34kV peaks). So it would seem as though you literally get more "bang" for the buck that way, not the other way around as you suggest.

Though it occurs to me that the doorknob route, by virtue of being far more parallel than series, may take the high currents a bit better, enabling higher breakrates to more than make up for the smaller capacity (though some people have reported that higher breakrates only get you so far). Plus, with only a single "layer" of, e.g., 50kV doorknobs in your MMC, if one of them fails short, you aren't overvolting any of the others as you would be with series strings of caps. I don't know what the "typical" failure mode of such caps would be in a coil, but...

Regards,
Aaron, N7OE

Re: MMC for 24kV: Thoughts on best construction?
J. Aaron Holmes, Mon Jul 24 2006, 04:15PM

Also, perhaps I could clarify the peak string current calculation: This is just the usual peak primary current calculation divided by the number of MMC strings, yes? I.e.:

Istring = Vpeak * Sqrt(Cpri / Lpri) / #strings

So, imagining a primary of Lpri = 50uH, a 2x20-cap 942 MMC of 15nF, and a Vpeak of 34kV, I get:

Istring = 34000V * Sqrt(.000000015F / .000050H) / 2
Istring = ~290A

...which seems like plenty of headroom.

Regards,
Aaron, N7OE

Re: MMC for 24kV: Thoughts on best construction?
EDY19, Mon Jul 24 2006, 06:27PM

You might check out this auction, too. It is a 35kv 30nf pulse capacitor- looks OK to me.

Re: MMC for 24kV: Thoughts on best construction?
J. Aaron Holmes, Mon Jul 24 2006, 07:07PM


I blew one of these to pieces about two weeks ago (seem my original post above). General opinion on the TCML seems to be that you have to be gentle with these (keep V down, breakrate low) or they croak. The problem I have with them is, as I have experienced, they're "all or nothing". If one cap blows, that's the whole cap!!

Regards,
Aaron, N7OE

Re: MMC for 24kV: Thoughts on best construction?
Steve Ward, Mon Jul 24 2006, 07:19PM

Doorknob caps can and do work (strontium titanate types), but they only fail *one* time. Maxwells have this same problem. If you use either of these types, be very careful of the voltage reversal rating. Maxwells are often 20% reversal, which means that if you discharge one at its max rating, the negative excursion (due to LC oscillations) should only be -20% of the input voltage. With a tesla coils, you will experience basically 100% voltage reversal, so this means to be safe, you must only charge the capacitor to 50% of its rating. So even 50kV doorknob caps wont last long on a TC with 34kVDC peak charging voltage. You would need at least a 70kV rating on the doorknob caps. The maxwell's will likely need a 50-70kV rating depending on the reversal ratings.

So i say with years of experience with using doorknob caps and PP MMC caps, use the PP caps! You can even run them at their DC rating (even with 100% reversal!) and they last a long time. So for CDE MMCs, DC peak charging voltage can equal DC peak rating of the caps, but for maxwells and doorknobs, double their voltage rating.