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Mysterious Chinese HV capacitors

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LAN_403

MRMILSTAR

Tue Oct 15 2019, 07:13PM

Registered Member #62119 Joined: Sun Feb 04 2018, 04:59AM
Location: Cedar Rapids, Iowa
Posts: 136

Has anyone used these capacitors in any projects? I have a 20-stage Marx generator that uses these capacitors and it seems to work fine with 12" sparks. I have been able to find out that they are metallized film polystyrene construction so they should be self-healing. I have the 2000 pF capacitance model but I have also seen a 3000 pF capacitance model. They are claimed to be rated for 30 KV. I may use these for a Cockroft-Walton voltage multiplier. I was also wondering if they would be good for tesla coil duty. I suspect not. The reason that they are so attractive is the price. They sell for about $2 on E-Bay. Here is an example E-Bay link.

Wow! That is certainly a long link.

Sulaiman

Wed Oct 16 2019, 01:17AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

Polystyrene capacitors have a very low temperature coeficient,
that is just nice to compensate for the thermal coeficient of air-cored inductors,
so they are very useful in tuned circuits,
they are also very low loss so can be used in high-Q resonant circuits.

Their high insulation and low dielectric adsorbtion makes them useful for sample-and-hold circuits.

BUT ... they fail VERY easily if they get hot, or just a little too warm !

I have some 18nF 1.5kV ps capacitors that I use for rf power circuits,
but the connections on the capacitors that you pointed to do not look suitable for high rms currents,
so they should be ok for h.v. d.c. or low power Marx and C-W use,
I would not choose them for a TC
- but I'd be tempted to try just because they are so cheap !

I suspect that at 30kV they are as likely to suffer from external surface tracking as they are to fail internally, so keep the surfaces clean and moisture free,
e.g. do not mount them whith their bodies in contact with a pcb.

MRMILSTAR

Wed Oct 16 2019, 03:29AM

Registered Member #62119 Joined: Sun Feb 04 2018, 04:59AM
Location: Cedar Rapids, Iowa
Posts: 136

I have some spares of these capacitors. I have looked at the mounting of the leads and I can't quite see what is going on because even though the ends are clear, they are shaped sort of like a Fresnel lens which distorts what I can see. There appears to be schoopage of some sort of silvery material on each end. There is one large axial lead on each end which makes sense. There is also an additional lead on each end made of very fine gauge wire. This smaller lead is connected to the larger lead and the other end appears to connect to the foil. I can't tell if the larger lead is connected to the schoopage or not. If not, then the entire current load must carried by the small leads. Maybe the large lead is just for mechanical strength for mounting. Some of the descriptions state that the large leads are for mechanical mounting strength but they don't say explicitly that they are also connected to the schoopage so that they can also handle the current.

I've never seen a capacitor with a large lead and a much smaller lead on each end before. The construction of these capacitors are some of the reasons that these are a mystery to me.

Sulaiman

Wed Oct 16 2019, 11:43AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

I believe that the large diameter leads are for mechanical mounting purposes,
the small diameter wires are the actual electrical connections to the electrodes.
My ps caps that I mentioned above have two smaller wires per electrode.

I'm fairly sure (but not certain) that ps capacitors do not use schoopage,
the connecting wires are bonded directly to the foil.

I do not see one on those capacitors that you pointed to
but usually ps capacitors have an ink ring at one end that indicates the wire that is connected to the outer foil,
that electrode is used at the 'earthy' end of a circuit as the foil electrostatically shields the other electrode,
for reduction of emi and/or for ease of insulation of the higher voltage electrode.

MRMILSTAR

Wed Oct 16 2019, 03:34PM

Registered Member #62119 Joined: Sun Feb 04 2018, 04:59AM
Location: Cedar Rapids, Iowa
Posts: 136

If the entire current load is carried by those fine wires then that alone would disqualify these capacitors from any sort of tesla coil duty. If these caps do not use schoopage then the ESL must be fairly large.

Sulaiman

Thu Oct 17 2019, 03:09AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

The ESL need not be large if the wires connect near the middle of the foils,
half of the current flowing clockwise and half anti-clockwise they would electromagnetically cancel each other.
I believe that this is how they are made, as it also reduces the ESR.

Have a look at your capacitors, see where the smaller wires enter the windings.

MRMILSTAR

Thu Oct 17 2019, 04:00AM

Registered Member #62119 Joined: Sun Feb 04 2018, 04:59AM
Location: Cedar Rapids, Iowa
Posts: 136

One wire connects to the outer diameter of the foil. The other wire connects close to the axis of the foil.

Sulaiman

Thu Oct 17 2019, 05:05AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

OK ... clearly in this case I am wrong !

I've been >8,000 km from my hobby stuff for a months, and probably a month or more to come, so I can't check mine.

Moses

Sun Jan 16 2022, 04:15PM

Registered Member #54500 Joined: Fri Feb 20 2015, 08:19AM
Location:
Posts: 2

You know I used the 30 kV 3000 nF units. My only complaint the actual lead is not the axial lead, but a tiny thread-like wire on both sides. The Axial leads themselves crack off easily where they are attached to the molded acrylic endcaps.

As far as "Tesla Coil Duty" they seem to work fine for a 12" coil as you have seen. You just need to pot them in mineral oil or they short to each other.

Moses

Sun Jan 16 2022, 04:18PM

Registered Member #54500 Joined: Fri Feb 20 2015, 08:19AM
Location:
Posts: 2

The tiny wires carry the entire load, those thicker axial leads are just to make it easy to connect them, but the axial leads crack off at the end caps connection point real easily, just bend he leads and or trim them, solder them to the other caps and don't play with them, and they will be fine.

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