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Registered Member #51
Joined: Thu Feb 09 2006, 04:17AM
Location:
Posts: 263
For my DRSSTC project, I need some capacitors. I need at least 5 of the 942 series caps that everyone uses (the 0.15uf 3kv variety) and a pair of largish 200v electrolytic caps of at least 4000uf, they will be in series in a voltage doubler. Low ESR and ESL preferred.
Does anyone have caps like these that they would be willing to sell?
Registered Member #1225
Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253
I could part with 6 of the .15uf 2kv variety, and i have 2 brand new 11,000uf 250v 39A rms CDE caps i could part with. It would be nice to have some money to put into my drsstc project, as of now it is a $0 budget project . I also have a 1.4kvdc 10uf used big*** film cap, if you need a snubber. The leads are very short though, but you can replace it with buss bars anyway.
Let me know what voltage and capacitance you expect to make your cap bank, i may have something else you could use.
Registered Member #1225
Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253
These caps are quite large. They are 3 by 5 5/8 inches.
Well, the CDE film caps are 45 bucks each, and these CDE lytics are about 400 dollars.
But seriously, just PM me an offer. What do you think the lytics are worth? . The terminals are depressingly small, but they are low ESR (18mOhms). You can get the datasheet here: Scroll to the very bottom, on the right. Decide if they are worth anything to you, if not i can still sell the tank caps.
Registered Member #1225
Joined: Sat Jan 12 2008, 01:24AM
Location: Beaumont, Texas, USA
Posts: 2253
lpfthings wrote ...
Whats the difference between the 940C and the 942C series? If not much, you can buy them for quite cheap from :)
The 940C series tends to fail at the connection from the foil to the lead. They are in Terry Blake's 'good and bad cap' list. It is under the 'bad' capacitors. They work fine for things like sstc DC blocking caps and are good for about any other circuit that is not high pulsed current.
Registered Member #2481
Joined: Mon Nov 23 2009, 03:07PM
Location: ITALY
Posts: 134
Arcstarter wrote ...
lpfthings wrote ...
Whats the difference between the 940C and the 942C series? If not much, you can buy them for quite cheap from :)
The 940C series tends to fail at the connection from the foil to the lead. They are in Terry Blake's 'good and bad cap' list. It is under the 'bad' capacitors. They work fine for things like sstc DC blocking caps and are good for about any other circuit that is not high pulsed current.
The 942C series has higher rated limits but I think that as long as you respect the rated limits (peak and rms current, voltage) the 940C series should work as well... Probably the 942 series is also more tollerant to over current/stress...
Registered Member #33
Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971
Arcstarter wrote ...
lpfthings wrote ...
Whats the difference between the 940C and the 942C series? If not much, you can buy them for quite cheap from :)
The 940C series tends to fail at the connection from the foil to the lead. They are in Terry Blake's 'good and bad cap' list. It is under the 'bad' capacitors. They work fine for things like sstc DC blocking caps and are good for about any other circuit that is not high pulsed current.
The 940Cs are fine for DRSSTC work, and much better value for money than 942Cs. Finn Hammer used 940Cs in his Thumper coil if I am not mistaken, and I've never heard of them failing in DRSSTC duty due to overcurrent. What does kill capacitors in DRSSTCs though is overvoltage, and Steve Ward lost a 942C MMC due to overvoltage a few years ago. For a given price you can get a higher voltage rated MMC with 940Cs compared to 942Cs, essentially making it much tougher.
Sorry for the rant, but I hear 942Cs recommended for big DRSSTCs all the time, and while they are one of the few caps that work with SGTCs, the situation is different with DRSSTCs. This is especially true with big DRSSTCs, a coil I am designing requires a 1µF MMC that can handle 5kV RMS. If I built this with the common 942C20P15Ks, it would end up with 670 caps; 10 in series to get the required 5kV RMS rating (500V per cap) and then 67 strings in parallel to get the required capacitance. This bank would have a peak current rating of almost 30 kiloamps, where it would never even see more than two kiloamps in practice, and an RMS current rating of over 900 amps, where it would hardly see a sixth of this in practice. This would of course make for a very durable MMC that would essentially last forever. The problem is, it would also make for a very expensive MMC, costing over two thousand US dollars at the current pricing on these caps from Richardson Electronics. I can imagine it would be quite bulky and impractical too, not to mention the work it would take to build an MMC with almost 700 capacitors.
The best approach is to find out what sort of voltages and currents the capacitors will see, and then find the cheapest alternative that satisfies those requirements, not just taking the capacitors which turn out to be ideal for SGTC use, throw enough of them in series that they don't die immediately in use, but live a very short life, and then enough strings in parallel to get enough capacitance. Using other capacitors, for example 940Cs, I can make an MMC for my coil for a quarter of the price, and I would never even exceed the datasheet ratings of the caps.
Sure, you can exceed the datasheet ratings, that's the name of the game here, but I think the best way to do it is to balance it out, not just exceed the voltage rating by a huge amount, while staying well within the current rating. That's the way you'll get the most reliable MMC for a given budget.
Nothing personally against the 942Cs, they have been shown time and time again to be the best caps for SGTCs, I just don't think they are the right solution for medium to big DRSSTCs, though for small DRSSTCs, they might make some sense.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
I also think the 940 series are fine. The failure mode is high peak current blowing the metallization off the film, and DRSSTCs have better control over peak current than SGTCs.
I've been running a bank of some random yellow caps for quite a long tme now.
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Wanted, 942 series film caps, electrolytic caps
. I also have a 1.4kvdc 10uf used big*** film cap, if you need a snubber. The leads are very short though, but you can replace it with buss bars anyway.
. The terminals are depressingly small, but they are low ESR (18mOhms). You can get the datasheet here: 
