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Strange cap charging issues

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LAN_403

Coherent

Sat Feb 28 2009, 10:18AM

Registered Member #1886 Joined: Sun Dec 28 2008, 02:55AM
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Posts: 73

I just got a maxwell 50uf 3.5kv pulse cap from ebay and I've been having a weird problem charging it and some other caps with my HeNe psu. The power supply originally powered a helium neon laser and now I use it for charging caps because the laser died. It puts out around 2.5kV 5ma DC with 12V DC input. Before I got the big caps I used it to charge little HV disc caps and it worked fine. I could hook it up directly to the cap and it would charge normally. If I took a screwdriver and shorted the cap while charging it I would get a little spark gap effect.

For some reason it just won't work normally with my larger caps though. It's not just the huge cap that it has problems with. I have a 4kV .45 uf MMC bank and a microwave oven cap and it won't charge them normally either.

If I hook the psu directly up to the terminals of the caps it simply won't charge them. If I take a screwdriver and short the terminals I get nothing. Not even a small spark. Now onto the really odd part. If I hold the psu output a few mm away from the cap terminals and let it arc to them it will charge them. I charged up the big 50uf 3.5kV cap this way and blew up a small diode like a firecracker. I can also charge the MOC and MMC with this method. This is really annoying me because I have no idea why this is happening. The caps are good and the psu is good. What could be the problem?

Conundrum

Sat Feb 28 2009, 10:43AM

Registered Member #96 Joined: Thu Feb 09 2006, 05:37PM
Location: CI, Earth
Posts: 4062

Could be overvoltage causing a punch through..

the spark gap is acting as a voltage limiter as below 2kv/mm it will not arc across preventing charging beyond this point.

Could also be overloading the supply. FWIW its not generally a good idea to charge any capacitor directly from a supply, you should always use a series resistor to limit the current, also all HeNe tubes have an internal cathode resistor for this reason (or the tube melts down TMI-style)

regards, -A

#include "$0.02.h"

Proud Mary

Sat Feb 28 2009, 12:12PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Try charging them through a large resistance.

Download

Sat Feb 28 2009, 01:22PM

Registered Member #561 Joined: Sat Mar 03 2007, 02:46AM
Location: Adelaide Australia
Posts: 230

It could be an internal discharge resistor in the cap?

Proud Mary

Sat Feb 28 2009, 01:31PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

Surely, it's just that the capacitors look like a dead short to the tiny little supply, so try charging them up through a resistor.

Coherent

Sat Feb 28 2009, 07:29PM

Registered Member #1886 Joined: Sun Dec 28 2008, 02:55AM
Location:
Posts: 73

After messing around with some 3M ohm resistors I finally got the 3.5kv cap to charge normally. First I tried a string of 7 in series and the cap charged, but it took beyond forever to even get a small charge. Then with a string of 3 at around 9M ohms I managed to get the cap charging at a decent rate when charged through the resistors. Logically I tried using only two next and suddenly the cap wouldn't charge anymore. One resistor didn't work either. Apparently anything under 7,000,000ish ohms and the cap doesn't charge unless I arc the psu to it.

Proud Mary

Sat Feb 28 2009, 07:43PM

Registered Member #543 Joined: Tue Feb 20 2007, 04:26PM
Location: UK
Posts: 4992

There's nothing mysterious going on here, lad!

50uF is quite a big capacitor compared with your 'little disks' , so of course it will take a long time to charge through a 9M resistance, and look like a dead short to a small power supply.

It's not for us to do your homework for you, so Google RC time constant and you'll soon understand what's going on!

PS: A 50uF capacitor charged to 2.5kV contains 156.25 joules of energy, so could kill if you touch it.

Coherent

Sat Feb 28 2009, 08:13PM

Registered Member #1886 Joined: Sun Dec 28 2008, 02:55AM
Location:
Posts: 73

Harry wrote ...

There's nothing mysterious going on here, lad!

50uF is quite a big capacitor compared with your 'little disks' , so of course it will take a long time to charge through a 9M resistance, and look like a dead short to a small power supply.

It's not for us to do your homework for you, so Google RC time constant and you'll soon understand what's going on!

PS: A 50uF capacitor charged to 2.5kV contains 156.25 joules of energy, so could kill if you touch it.

Well this mystery is solved. The ps cuts off when dead shorted by clamping the two leads together. I didn't expect that because the thing should already be current limited since it was used for a HeNe tube. I feel like an idiot now

I know about the danger btw. I always short the cap with an insulated screwdriver before touching the terminals. Now if only I could charge it up to 3.5kV for a full 306 joules. I wonder if a rectified 3kv NST would do a better job.

Fraggle

Sat Feb 28 2009, 08:17PM

Registered Member #1526 Joined: Mon Jun 09 2008, 12:56AM
Location: UK
Posts: 216

I wouldn`t discharge it with a screwdriver when it`s fully charged to 3.5kV!

Coherent

Sat Feb 28 2009, 08:35PM

Registered Member #1886 Joined: Sun Dec 28 2008, 02:55AM
Location:
Posts: 73

Fraggle wrote ...

I wouldn`t discharge it with a screwdriver when it`s fully charged to 3.5kV!

What I meant is that I do that after the main discharge just to be sure there isn't any residual charge remaining.

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