# High Voltage Capacitor Charge/Discharge Circuits

Discussion in 'General Electronics Chat' started by Confuzzled, Mar 18, 2009.

1. ### Confuzzled Thread Starter New Member

Mar 16, 2009
6
0
Howdy,

I was making a small capacitor bank and had them all wired up in parallel. I built a dc-dc boost converter to charge it up. I discharged it in parallel also, but noticed the voltmeter only read 300V. At first I thought it was only reading one capacitor, but then I got to thinking about how batteries work when they are in series vs. parallel. (I don't plan very far ahead as you can see). So now I realize to get a high voltage discharge I need a series circuit. The problem with this is that I can't charge the bank to 3kV.

I'm thinking maybe I can charge them in parallel and discharge in series, that works right? So if that is right, I'm not sure exactly how to do that. I made a picture to show you how I am considering it, but I'd like your advice first.

Thanks

Last edited: Mar 18, 2009
2. ### SgtWookie Expert

Jul 17, 2007
22,202
1,793
What you are attempting to do is extremely dangerous.

Storing high voltage in large capacitors represents a LOT of energy, that can very quickly and easily kill you.

High voltage experiments should only be conducted after a good deal of learning and experience has been accomplished in low power, low voltage circuits.

I much prefer for "newbies" to restrict their experiments to below 50v.

3. ### Confuzzled Thread Starter New Member

Mar 16, 2009
6
0
OK, understood, I have some questions though. If the capacitance of the capacitor bank is low, wont the discharge be low current and not overly dangerous? I know some tasers have very high voltage, but virtually no continuous current, so they aren't fetal, right?(These aren't rhetorical questions, I'm really wondering.)

I see how you are saying this can be dangerous, I know I'm far from being even called proficient with circuits, but to be honest I'm pretty bored with beginner circuits and that's not really what I'm interested in.

Regardless,

4. ### beenthere Retired Moderator

Apr 20, 2004
15,815
293
It is not the magnitude of the current that is dangerous. The driving voltage is the hazardous part. A potential across your body that can cause a few tens of milliamps current can stop your heart. DC is hard to let go of, as well, which magnifies the danger.

5. ### Confuzzled Thread Starter New Member

Mar 16, 2009
6
0
What is the difference between a potential that can cause tens of miliamps verses a potential that cannot? Simply how great the potential is?

The capacitors I'm using are only storing 60J all together.

6. ### beenthere Retired Moderator

Apr 20, 2004
15,815
293
Current equals voltage divided by resistance. That's Ohm's law.

7. ### Confuzzled Thread Starter New Member

Mar 16, 2009
6
0
Obviously. How can a 2 million volt taser not kill someone, and a 3000 volt capacitor bank be very dangerous? That's what I'm not understanding. Could it be because the taser never sustains a current for long?

8. ### SgtWookie Expert

Jul 17, 2007
22,202
1,793
The taser by design, outputs high voltage at extremely low current.

You are storing power in capacitors over time, and then wish to multiply the voltage by reconnecting them in a different configuration.

Please review the "sticky" messages at the top of this forum.

What you are asking about has a very likely potential of being lethal. This is why we are so hesitant to help you in this quest; a person whos' abilities we have not had a chance to evaluate as of yet.

While members on this Board make good attempts to share knowledge, safety is always the primary focus.

9. ### Confuzzled Thread Starter New Member

Mar 16, 2009
6
0
Well, you've managed to scare me a bit, which I appreciate; you just have to understand it's always hard to tell when people are being annoyingly prudent or they are very serious. I will set this project aside for now, and I would like to ask if you have suggestions for projects in relation to this that are maybe less dangerous? Or even just an experiment to demonstrate something cool.

10. ### SgtWookie Expert

Jul 17, 2007
22,202
1,793
You might consider constructing a Van de Graff generator.

Very high voltage, negligible current.

Plasma balls are related.

Just don't try storing that energy in capacitors.

11. ### Confuzzled Thread Starter New Member

Mar 16, 2009
6
0
Cool! I might try and build a van der graf, those are fun.

Thanks

12. ### Camp Kohler New Member

Feb 14, 2011
1
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First: The warnings about the dangers of HV cap banks cannot be overemphasized. Dead is dead! Some HV circuits produce only low current, but when you accumulate charge, a large and lethal current can be delivered. Even a low current can startle and cause injury. If you are to proceed, an old and experienced person should supervise to prevent death and other inconveniences.

Charging caps in parallel and then arranging them in series was described by Marx in 1924 (Google it). By using a Marx generator to switch the circuit automatically, you eliminate the need to get your hands (or other body parts) involved. It uses high-resistance resistors to supply the charge in parallel and spark gaps that fire and discharge the caps in series. The resistors are so high that they are ignored at discharge time, but low enough at charge time; the gaps are open at charge time and, fired, are shorts at discharge time. To sum up, the charge resistors are always there, but the fired gaps override them and rearrange the circuit until the voltage drops to the point where the gaps stop firing.

My experience with this was at the Keesler AFB amatuer radio club in 1965. An officer left school and came to the club with this circuit. He constructed it on a large wooden table using a supply of a couple of hundred volts feeding the Marx circuit made of 4uf oil-filled caps and some resistors we had on hand, all supported on Coke bottles for insulators. After arranging the gaps, he dropped a screw into the biggest gap to reduce it to the firing point, and POW! We were rewarded with a satisfying snap. Of course, this was entertaining only for a while and it was torn down shortly thereafter to make sure nobody wandering by became a Silent Key.

Now most of us there that day were well familiar with H.V. stored in large oil-filled caps (and the tendency for the charge to reappear by itself if left setting out without a shorting wire applied), so there was no real danger. The more the experience, the greater the safety factor.

Jul 7, 2009
1,585
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When I was a student, I used to do a demo for one of the basic classes. They'd be studying capacitors and I'd bring in a bank of about 12 big capacitors (they were about 3 inches in diameter and about 5 or 6 inches tall), all rated to 600 V and connected in parallel with heavy bus bars. Then I'd use this big old Sorenson power supply (it was so heavy I had to wheel it in on a cart). I'd charge things up to about 550 volts, then short the bus bar with my Xcelite screwdriver. It made a blue flash and quite a bang -- enough to cause a few girls in the first row to scream. The teacher and I would comment that this was certainly not something that beginners should be fooling with, as the thing would be lethal if accidentally touched. This was a number of decades ago; nowadays, people probably wouldn't do this for fear of being sued for making a loud bang...

Whenever I use that screwdriver, I see the pock marks on it from those demos and instantly remember them.

14. ### RRITESH KAKKAR Senior Member

Jun 29, 2010
2,829
91
good post ...!!