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Registered Member #4187
Joined: Fri Nov 04 2011, 08:08PM
Location: Spain
Posts: 43
Hi For an SSGTC I am between this two caps for primary MMC (from Farnell)
WIMA MKP1T032207C00KYSD 220n 1600V PP film with metalized electrodes, for "pulse duty" 1400V/us VISHAY MKP1845410204 100n 2000V PP film HV pulse applications 2600V/us
WIMA is almost double price (about 3,8€ against 2,1€)
With NST 8kVrms/60mA budget optimun would be the VISHAY (if I use as max voltage 18kV) with 3 strings of 9 caps with WIMA using 2 strings of 12 caps (19,2kV) but almost doubling the budget
18kV is enough for 8kVrms NST? anybody have used them?
Registered Member #1403
Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968
I think the reason lies with a rather early made list of good and bad capacitors. For many people there have been no reason to experiment when they could just look at the list and choose a capacitor from that.
Note: Full metal "film" caps fail at the end plates. You want "film/foil" caps like the CD 942 series. Thanks to Garry for helping dicover this (the hard way...)!!
G.E. 42L series caps are all metalized and not recommended. they have higher current rating than most but people still have trouble with them.
Registered Member #4187
Joined: Fri Nov 04 2011, 08:08PM
Location: Spain
Posts: 43
Ok, the problem is that checking ebay, Cornell Dubiliers are really expensive and the captioned VISHAY can be got from Farnell for less than 1,9€/cap. With about 100€ I can get a 40nF 20kV for primary.
Nearer "known good" cap is Arcotronics RS-114-474 but these are 3,4€/cap and half capacity, my budget can´t assume that
I have a doubt with the difference between "all metalized" "metal film" "film/foil" terminology.
These VISHAY are "Vacuum deposited aluminum electrodes" "Extended double-sided metallized polyester film, internal series connection, double-sided metallized polyester carrier film" but not sure if they comply with the recommendations given in the "good caps" list
Registered Member #89
Joined: Thu Feb 09 2006, 02:40PM
Location: Zadar, Croatia
Posts: 3145
As far as I know, "all metallized" and "metal film" designate the same thing. The film/foil caps are constructed in a way that electrodes that attach to the endcaps are thick metal foil that won't fail so easily in overcurrent conditions. These foils don't overlay each other, but are would over (insulated with dielectric ofcourse) another metallized polypropylene film electrode that is floating inside the capacitor, making the whole construction like basically two caps in series (the "soft" part of CDE caps is where the foil parts are no longer). This film electrode is to allow for self healing of the capacitor.
There are some series of caps made completely with metal foil too - they are extremely robust to overcurrent conditions but can't heal in case of overvoltage - It takes just one pop and they die.
Registered Member #30
Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706
Metallised film caps can be acceptable for small, low powered coils.
To find the peak current rating of a capacitor, multiply the dV/dt (in V/us) by the capacitance (in uF)
For example: 220n = 0.22uF. 0.22* 1400V/uS = 308A peak.
and 0.1 * 2600 = 260A peak for the other capacitor.
Use your favourite Tesla coil calculator software to see if your peak primary current will exceed this. If it won't, then the caps may well be usable.
However note that with a spark-gap coil it is easy to accidentally tune the power supply into resonance and go way above the design voltage and current.
Film/foil caps cost more per unit energy storage, because the metal foil is thicker than a metallised coating, they contain more metal per joule stored. And the extra metal also makes them bigger, so they have to contain more of all the other materials too.
The extra metal is the very thing that gives them the high peak current rating. So that's why the known good caps are always more expensive.
Vishay is one of these brands like Tyco that buys up any other company it likes. The Vishay caps were probably sold under some other brand before.
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WIMA MKP1T032207C00KYSD / VISHAY MKP1845410204 for MMC
