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But, you can still make use of that by using a large inductance primary and long burst lengths. The limiting factor will be dielectric heating, so place a blast shield around it and watch temps remotely.
Registered Member #1521
Joined: Thu Jun 05 2008, 10:46AM
Location: Hungary
Posts: 128
Has someone any experience with these cap? Cap I'm planning a large DRSSTC, and this cap looks almost perfect, but it's too expensive just to order one for testing.
Registered Member #33
Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971
omegalabs wrote ...
Has someone any experience with these cap? Cap I'm planning a large DRSSTC, and this cap looks almost perfect, but it's too expensive just to order one for testing.
The auction says 25 kV RMS, but the label on the cap in the picture says 25 kV peak, big difference. There's also no stated RMS current rating, considering the considerable cost I would not gamble on using it in a big DRSSTC. Especially since you can get proven caps for a lower price.
Something like a bank of these would be much better value for money.
Registered Member #1521
Joined: Thu Jun 05 2008, 10:46AM
Location: Hungary
Posts: 128
Wolfram wrote ...
omegalabs wrote ...
Has someone any experience with these cap? Cap I'm planning a large DRSSTC, and this cap looks almost perfect, but it's too expensive just to order one for testing.
The auction says 25 kV RMS, but the label on the cap in the picture says 25 kV peak, big difference. There's also no stated RMS current rating, considering the considerable cost I would not gamble on using it in a big DRSSTC. Especially since you can get proven caps for a lower price.
Something like a bank of these would be much better value for money.
This snubber is actually more expensive for the same voltage/capacitance (also any other MMC construction is). I have an idea for an ultra reliable and cheap home made cap (LDPE plates + aluminum strip+silicone oil, it works great), but it needs a good container, and it's pretty big for a DRSSTC. It would be possible to make it better, but I can buy only 1mm thick PE plates here, but actually 0,5mm thickness should be enough for any kind of DRSSTC from PE. Looks like it's better to do it myself.
Registered Member #33
Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971
omegalabs wrote ...
Wolfram wrote ...
omegalabs wrote ...
Has someone any experience with these cap? Cap I'm planning a large DRSSTC, and this cap looks almost perfect, but it's too expensive just to order one for testing.
The auction says 25 kV RMS, but the label on the cap in the picture says 25 kV peak, big difference. There's also no stated RMS current rating, considering the considerable cost I would not gamble on using it in a big DRSSTC. Especially since you can get proven caps for a lower price.
Something like a bank of these would be much better value for money.
This snubber is actually more expensive for the same voltage/capacitance (also any other MMC construction is).
Something is a bit fishy with the cap in the auction. In the eBay picture, the cap is clearly labelled KMOP754, according to the Condenser Products website the KMOP series are "Kraft/Polyester" capacitors intended for "DC filter, limited AC, and pulse discharge applications", even though it's listed as polypropylene on eBay. The peak/RMS voltage rating confusion I mentioned earlier is also a bit suspicious.
Assuming you needed 750nF at 25kV DC (which this capacitor is, assuming the Peak rating stamped on the cap is a DC rating, the actual DC rating might be lower), you would need strings of 25 of the RBPS caps in series. 10 of these strings in parallel would give you 800nF, with an unprecedented RMS current rating of 225 A, for a total cost of 350 dollars. In my experience they accept lower offers as well, so it could end up being even cheaper still.
One problem is that they only have 200 of these caps listed, in two separate auctions. But if you could relax the specifications a bit, you could probably even get by with 100 of them. For example configured as sixteen in series by six in parallel, yielding 750nF at 16kV DC. These capacitors are rated for 530V RMS each, so the bank would be rated for 8.5kV RMS, or 12kV peak, with a very good safety margin. This corresponds to 2800 A peak in the primary, if you're running at 50 kHz.
This snubber is actually more expensive for the same voltage/capacitance (also any other MMC construction is). I have an idea for an ultra reliable and cheap home made cap (LDPE plates + aluminum strip+silicone oil, it works great), but it needs a good container, and it's pretty big for a DRSSTC. It would be possible to make it better, but I can buy only 1mm thick PE plates here, but actually 0,5mm thickness should be enough for any kind of DRSSTC from PE. Looks like it's better to do it myself.
It's a nice idea, but if I calculated right, you need 37 square meters of 1mm PE to get 750nF.
Registered Member #1521
Joined: Thu Jun 05 2008, 10:46AM
Location: Hungary
Posts: 128
Oh ok the price is for 50 pieces, I didn't saw that. 16kVDC should be enough for the cap, my plan is a CM600 full bridge around 10kW.
It's a nice idea, but if I calculated right, you need 37 square meters of 1mm PE to get 750nF.
Yes, originally I made this cap for spark gap coil, where it works really good in a few 10nF range. This is why I'm looking for thinner PE sheets, 0,5mm would be the ideal thickness. Half weight, and half the needed surface area, so it would be extremely cheap. 37 square meter is also not a big deal, but it needs a lot of work to assemble hundreds of plates together. It would be big and heavy.
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DRSSTC capacitor
