High Voltage Forum

Pulse power => Capacitor banks => Topic started by: MRMILSTAR on October 15, 2019, 09:12:09 PM

Title: Mysterious Chinese HV capacitors
Post by: MRMILSTAR on October 15, 2019, 09:12:09 PM
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.
Title: Re: Mysterious Chinese HV capacitors
Post by: davekni on October 16, 2019, 03:33:52 AM
These look exactly like the polystyrene capacitors I purchased on EBay 3 or 4 years ago.  They are very-low leakage, great for voltage-multiplier use.  However, they are foil, and have random tiny bubbles included, which form corona discharges in AC or pulse (Marx) use.  For low-frequency Marx use, they work OK.  My Marx generator recharges up to 120 times per second (plays low-register tunes).  In that use, they last a few minutes at most.  Same for AC (Tesla coil) use.
Title: Re: Mysterious Chinese HV capacitors
Post by: MRMILSTAR on October 16, 2019, 05:40:55 AM
That's sort of what I thought. My Marx generator only produces a 12" spark about every 2 seconds although it is a very juicy spark. None of the caps have ever failed.

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 which I assume is a conductor. 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.

Since they only cost $2 apiece, I'm tempted to sacrifice one of my spares by cutting those small leads to find out if I can charge the capacitor through only the large axial leads.

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.
Title: Re: Mysterious Chinese HV capacitors
Post by: ElectroXa on October 16, 2019, 08:35:30 PM
I've seen them on EBay, ant these caps were interesting for me, for a voltage multiplier
But partial discharges are a problem as davekni said, because  I planned to use them at 75 kHz and ~15 kV .
Does anyone already tested them at high frequency, to have a feedback before buying them ?

thanks a lot  :)
Title: Re: Mysterious Chinese HV capacitors
Post by: Experimentonomen on October 16, 2019, 08:41:05 PM
Looks like inductive wind, so no good for anything above the audio range.