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ifryd

Thu Nov 17 2011, 07:43PM

Registered Member #4215 Joined: Wed Nov 16 2011, 09:45PM
Location:
Posts: 32

Hi

This is my first post on this forum so I would like to say hi and welcome you all. I am happy that I can be part of this community.

I need your advice concerning my new project. I want to make let`s call it "discharger".
Basically what this circuit has to do:

Charge bank of electrolytic capacitors to the desired voltage (15 - 20 KV). Sum of capacitance of these capacitors could be let`s say 2.2uF or less.
Then I want to rapidly discharge these capacitors but what is more important I want to discharge them with precise frequency of intervals. I would like to adjust this frequency from 250hz up to 1200hz. (+-1hz)
Now after this short description I have questions concerning it:

1. What would be the best approach to the above problem?
2. How to switch with precise frequency (+-1hz):
a) by adjusting PWM of the high voltage supply?
b) by using rotary switch?
3. What is the maximum switching frequency of the static and rotary switch?
4. How loud can be these discharges?
5. How to get rid of the ozone problem completely. How do you deal with it? My first approach was to drill holes in jar and the make discharges in jar. Would be a good idea to build case for rotary switch ?
6. what source of power supply would be the best? Flyback driver or high voltage transformer ? Maybe I should consider also tesla coil but can secondary widing be tuned to a desired frequency (+- 1 hz)?

Adam

Sulaiman

Thu Nov 17 2011, 08:26PM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3141

Adam,
2.2 uF at 15 kV is 247.5 Joules, at 20 kV it's 440 Joules.

So 250 Hz = 61.875 kW to 110 kW

at 1200 Hz that is 297 kW to 528 kW

... maybe a re-think is in order ?

Forty

Thu Nov 17 2011, 08:44PM

Registered Member #3888 Joined: Sun May 15 2011, 09:50PM
Location: Erie, PA
Posts: 649

edit:
apparently sulaiman was typing out the calculations at the same time i was.

A Marx generator might be what you are looking for.
its discharge frequency could be controlled by the input current.

although for 250+Hz repetition rate you'd probably want a cw multiplier or a switched hv transformer.
an ignition coil fed by a square wave might work.

Steve Conner

Thu Nov 17 2011, 08:44PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

To paraphrase the quote from Jaws: You're gonna need a bigger jar.

ifryd

Thu Nov 17 2011, 09:03PM

Registered Member #4215 Joined: Wed Nov 16 2011, 09:45PM
Location:
Posts: 32

It is not a problem I will change this capacitance from 2uF to 2nF or even less. Nothing is predetermined here. You concentrated about capacitors and what about other questions ?

jpsmith123

Thu Nov 17 2011, 10:13PM

Registered Member #1321 Joined: Sat Feb 16 2008, 03:22AM
Location:
Posts: 843

It's not clear exactly what you want to do. If the capacitor is being fully charged and fully discharged hundreds of times per second or more (as your description seems to imply), then as Sulaiman pointed out, you're talking about a ridiculous amount of power. You need to clarify; are you trying to drive some kind of load?

klugesmith

Fri Nov 18 2011, 02:13AM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

ifryd wrote ...
It is not a problem I will change this capacitance from 2uF to 2nF or even less. Nothing is predetermined here. You concentrated about capacitors and what about other questions ?

We have to respond to your other questions with more questions.

What discharge time do you require?
Where is the C*V^2/2 energy supposed to go when you discharge the HV cap?

You sort of implied a spark in air. Is that necessary, for example as a stroboscope lamp?
Then why not say so? It would direct us toward spark-gap quenching and triggering ideas.

Do you want it to be relatively quiet and dark?
How about discharging the capacitor into a resistor, perhaps switched by a high voltage thyratron? (now the circuit is starting to look like a radar pulser). Then you can change the resistor value to trade between peak current and discharge time.

Could your application use a pulse shaping network?

Newton Brawn

Fri Nov 18 2011, 05:51AM

Registered Member #3343 Joined: Thu Oct 21 2010, 04:06PM
Location: Toronto
Posts: 311

Ifrid:

Ia...
You must explain the purpose of your project to have the chace of receive good answers.

Make a schematic, a drawing, clarify your proposal

Regards

Newton

ifryd

Fri Nov 18 2011, 09:41PM

Registered Member #4215 Joined: Wed Nov 16 2011, 09:45PM
Location:
Posts: 32

Hi I want to make device to test so called "disruptive discharges". So basically I want to trigger rapid discharge of high energy. Now switch can have delay time of triggering it is not a problem but should switch high energy as fast as possible.

Thanks Klugesmith for giving me idea about thyratron. I searched internet and I found this good article about high power switches http://205.243.100.155/frames/pasley.html

I also found manufacturer of various switches http://www.rell.com/Pages/Product-Category.aspx?productCategory=12

What do you think about triggered spark gap switches. I think this is basically what I need.

What discharge time do you require?

I want to send as much energy as I can in the shortest time. As far as I read triggered spark gap switches have time of switching equals 2ns.

Do you want it to be relatively quiet and dark?

What I want from switch is to stop giving ozone and stop giving UV radiation. So in other words should be safe for operation. Yes it would be good if switch could be quiet. Is it possible when switching high energies ? What would be better thyratron or triggered spark gap? What voltage is needed to switch on triggered spark gap switch?

Below I am attaching figure:

GrantX

Sat Nov 19 2011, 06:44AM

Registered Member #4074 Joined: Mon Aug 29 2011, 06:58AM
Location: Australia
Posts: 335

From a cost, practicality and expense viewpoint, you should look into using one or two PIO pulse capacitors, rather than an electrolytic bank. First of all, I don't think an electrolytic bank can do sub-micro second discharges, secondly, to build up a reliable 20kV bank you will have to spend big money.

For example, in the pic below I have 2 capacitors. The big pulse capacitor is 50kV 100nF (perfect for your application), and cost me only $50 dollars on ebay. It's in perfect condition, very tough, and can discharge a lot quicker than electrolytic caps.
The blue electrolytic is 450V 1900uF and cost $39. You would need 112 of these to match the 50kV rating of the pulse cap.

As for switching, perhaps just a shielded rotary spark gap using a synchronous motor driven by a VFD?
like this:

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