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Registered Member #1321
Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843
Does anyone know how TDK UHV series caps are physically constructed? (And I assume the outwardly similar Chinese caps on ebay are also of the same type construction).
I'm trying to get an idea how rugged these caps are and how they would behave in a fast, high-power pulse situation.
Unfortunately, the data sheet doesn't give any kind of current spec or dv/dt or anything...yet it implies that the caps are suitable for laser power supply applications involving "high speed pulses".
Also I think I've seen these caps being used in a commercial Marx generator, so they must be fairly robust and reliable, but what kind of stray inductance are we talking about here for example?
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
The inside of these caps is simply a disk of ceramic dielectric with silvering/metalization on the ends where the threaded terminals are soldered to the metalized ceramic before encapsulation. The inductance is *very low*, id guess in the 10-20nH range depending on the capacitors physical size. A sizable fraction of the capacitor volume is epoxy, as this is needed to prevent flashover.
I have never croaked one of these caps, but at the same time i tend not to even run them at their rated voltage. They can probably withstand short circuit currents just fine if the rep-rate is low enough that average heating isnt an issue.
Registered Member #2431
Joined: Tue Oct 13 2009, 09:47PM
Location: Chico, CA. USA
Posts: 5639
Steve Ward wrote ...
The inside of these caps is simply a disk of ceramic dielectric with silvering/metalization on the ends where the threaded terminals are soldered to the metalized ceramic before encapsulation. The inductance is *very low*, id guess in the 10-20nH range depending on the capacitors physical size. A sizable fraction of the capacitor volume is epoxy, as this is needed to prevent flashover.
just out of curiosity, when you say "thin metalization" what about the outer plate circumference? is it graded or just a thin sharp foil edge coverd with epoxy? i try to curl or round over my thin plates to avoid a high field blowing through my epoxy.
(for instance my HV probe has a round ring conductivly epoxied to the plates for 40+ kV, and submerged under oil)
High voltage probe for O-scope, 2 of 2, ( FABRICATION ).
A single HV RC element seen here in my hand.
The resistor's V drop is linearized with the cap plates, and grading rings.
Registered Member #1321
Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843
Wow...would that be a single disk? I suspected as much (based on the fact that the higher capacitance values have a bigger diameter), but it's nice to have confirmation. If so I guess that would explain the apparent ruggedness and the very low inductance of these types of caps. I think I'm going to try to save up some money and get some of these things.
Steve Ward wrote ...
The inside of these caps is simply a disk of ceramic dielectric with silvering/metalization on the ends where the threaded terminals are soldered to the metalized ceramic before encapsulation. The inductance is *very low*, id guess in the 10-20nH range depending on the capacitors physical size. A sizable fraction of the capacitor volume is epoxy, as this is needed to prevent flashover.
I have never croaked one of these caps, but at the same time i tend not to even run them at their rated voltage. They can probably withstand short circuit currents just fine if the rep-rate is low enough that average heating isnt an issue.
Registered Member #146
Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055
When i used to work at Fermilab we ordered custom doorknob caps without the epoxy, they were just metalized right to the edge without any special grading, i think they just rely on the epoxy to smooth out the field, though im sure its less than ideal.
Yes, just a single slab of ceramic with 2 plates, as simple as it gets.
Registered Member #133
Joined: Fri Feb 10 2006, 10:27PM
Location: Pensacola, Florida
Posts: 47
The bare ceramic and metal capacitor (without epoxy encapsulation) is able to transfer heat more readily to its surroundings. Epoxy is less thermally conductive than the bare component materials themselves.
Registered Member #135
Joined: Sat Feb 11 2006, 12:06AM
Location: Anywhere is fine
Posts: 1735
I can attest to RTV silicone, its a total B@#%%ch to remove. These are TDK 40KVDC caps, 5mm thread. I'm not sure what they were used for in the past other than for some kind of laser supply, I use them for my TC bank and they work very well. The only real purpose I can see for encapsulating them in RTV is to reduce corona, which would be enormous for a large bank of these.
Registered Member #1321
Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843
I just rediscovered a paper I'd seen a while ago where a bank of 32 of these caps (50 kv, 1.7nF) was charged and discharged at rates up to 600 Hz...as the primary energy store of a system designed to create and radiate very high power impulses.
Note the construction of the capacitor bank shown if Fig 6: they stuck 32 caps in an aluminum box and filled it with silicone potting compound. And that arrangement was apparently good for a power throughput of up to 40 kw.
On a side note, the spark gap switch used with the cap bank is pretty amazing itself:
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What's the internal construction of TDK UHV series ceramic "doorknob" capacitors?
