Hello!

I have a problem...

I have a 450v capacitor charged to 320v, which I have to discharge across two electrodes inside a vacuum chamber. The vacuum chamber has a pressure low enough to allow plasmas to generate at about 1000v or so.

Ive gone through several attempts to solve this:

Using a HV transformer outputting 2kV to create an arc, and discharge the capacitors through said arc. (Plasma is conductive, so it shorts out the electrodes and discharges the capacitors)

The idea was to use a couple of microwave oven diodes to prevent the 2kV from popping the 450v caps, and it worked. However, upon increasing the size of the capacitor, (from 1000uF to 5000uF) it would still work, albeit destroying the diode in the process.

For this reason I tried using an old flyback to generate the spark. The flyback is isolated from the capacitor by two capacitors which dont allow DC through, and the capacitors are protected by two inductors which filter out the 25kHz from the flyback.

This again, worked, however, since the inductors have arround 35ohms of resistance each, it they dont allow the capacitor bank to discharge with a spark. (Otherwise the resistance of the path to the electrodes is a bit less than 1ohm).

The problem is, I need a method of discharging the 320v as fast as possibe, inside a vacuum chamber.


I was thinking of three possible solutions:

1 - Substituting the large inductors for smaller 10mH inductors which only have 40 milliohms of resistance each, and use the flyback method,

2 - Since the capacitor bank is composed of 5, 450v 1000uF caps, I could rig them in series to get a 2700v 160uF capacitor bank, and then charge that with the HV transformer to 2kV. (Which should be able to discharge on its own, without the need for an additional spark)

3 - Using the HV transformer outputting 2kV and relying on the fact that the capacitors can withstand 450v, and that the 2kV are only applied for arround 20ms. (The capacitors will not be in continuous operation, and they will probably only see about 50 2kV pulses total in their entire life. Maybe even less)

What do you guys think? Any ideas?

 

Perhaps you could use a low inductance HV pulse transformer such are used to ignite xenon flash lamps.
Or could you use a separate (third) electrode connected to a HV pulse generator to generate the ionization path for the capacitor discharge?

 

Have a third electrode and apply the trigger voltage to that. This is the method used to a xenon flash tube used for a camera flash or stroboscope. Sometimes this third electrode is not inside the tube. it is just some thin wire wound round the outside of the tube. You could probably get one of the trigger transformers from a disposable camera and just copy it's driver circuit..

Les.

 

Hmmmm. A third electrode? If both the HV circuit and the capacitor circuit share only one elctrode they wont damage each other?
There are some mechanical issues together with adding a third electrode tho.

The vacuum tube consists of a glass tube, and inside it there is a steel tube, which inside contains a copper tube. Both the steel and copper electrodes are arranged like if it were a coaxial cable, with the copper one inside the steel one. (Steel is 38mm diameter and copper is 12mm)
The problem would be getting ionization to occur inbetween both electrodes.

I will try using some 450v 100uF caps to make a smaller bank, 2700v and 16uF. I will use this to test if it will discharge correctly and if so I might end up making a bigger bank. One thing to note, is how would I measure the voltage in that capacitor. Im thinking 300v analog voltmeter and a voltage divider.

 

Another suggestion is to inject the high trigger voltage as an RF signal in series with the 320 volt DC supply. This metod is used it initiate the arc on TIG welders. If you Google something like "TIG welder trigger circuit" it should give you some ideas.

Les.

 

flash tubes are not in a vacuum.
the OP says its a vacuum, then says it low pressure. so which is it?

 

flash tubes are not in a vacuum.
the OP says its a vacuum, then says it low pressure. so which is it?

What is a vacuum? It is low pressure.
How low? It depends on the quality of the vacuum pump and seals in the chamber. There is no such thing as an ideal (perfect) vacuum.

One atmosphere is 760 mmHg. In the chemical industry, we called reduced pressure drying “vacuum drying” even though the pressure was about 10 to 25% of atmosphere.

 

25atm on one side and 12atm on the other side, is a "vacuum"
but in physics, "vacuum" means no particles inside that will affect the process.

automotive/house AC system vac gets pretty darn close to 0atm, but that is not what a flash tube is.

low pressure to me is not near 0atm. almost anything under 1atm is "low" pressure.

i assume OP means the tube is evacuated like a AC system is. if its 0.7atm of air, then that would not be a vacuum unless the air is part of the experiment to get cap charge to flow from one side to the other.

 

25atm on one side and 12atm on the other side, is a "vacuum"
but in physics, "vacuum" means no particles inside that will affect the process.

automotive/house AC system vac gets pretty darn close to 0atm, but that is not what a flash tube is.

low pressure to me is not near 0atm. almost anything under 1atm is "low" pressure.

i assume OP means the tube is evacuated like a AC system is. if its 0.7atm of air, then that would not be a vacuum unless the air is part of the experiment to get cap charge to flow from one side to the other.

Well, I’m sure everyone thanks you for adding confusion to the thread. Unless you want to point out where you see someone taking about low pressure AND vacuum or 25 atm vs 50 atm, please refrain from posting until you have something constructive to add.

 

What is a vacuum? It is low pressure.

hmmm, i do distinctly see here it was you who said "What is a vacuum? It is low pressure."
so i believe your statement is true, you did talk about low pressure AND vacuum

and its absolutely critical to know if the cap inside the tube is in a physics vacuum, or as you say, in a low pressure atmosphere.

 

and its absolutely critical to know if the cap inside the tube is in a physics vacuum, or as you say, in a low pressure atmosphere.

Why? What are the tolerances of your components?

Relative permittivities of some materials at room temperature under 1 kHz[citation needed]

Vacuum 1 (by definition)
Air 1.00058986 ± 0.00000050 (at STP, for 0.9 MHz),[1]

 

How do you get ionization in a vacuum if there are no ions to ionize?

Perhaps this will help:

https://en.wikipedia.org/wiki/Vacuum_arc

 

Ok ok people, don't get confused.

I mean vacuum as in a "low pressure vacuum"

However, that low pressure is about one millionth of an atmosphere, so yes, it is basically a physics vacuum, but there are still enough air molecules to ionize. However, it is way different from a xenon lamp or anything of sorts, as those usually have much higher pressure. (Still less than atmosphere of course)

 

Okay, back on track.
So could you ionize the path for the discharge with a third electrode?

 

I think I won't be able to do so. I tried it, and it does work on a small test chamber I made, but only if the third electrode ionises a path through one of the other two electrodes:

With three electrodes arranged like so:
A B C
I I I

B and C only fire if the HV arc goes from A to C, passing through B

However I am testing the series arrangement of the capacitors to charge them to 2kV, and see if they fire.

As an additional question...
Could I use a 500v max analog voltmeter, with a potential divider, to measure 0-2kV? (Divide voltage by 10 and measure in the range of 0-200v)

 

how are the electrodes formed? are they nice and pointy & pointing at each other?

 

Nope

They consist of a 38mm steel pipe which contains a 12mm copper pipe inside it.

 

Ok ok people, don't get confused.

I mean vacuum as in a "low pressure vacuum"

However, that low pressure is about one millionth of an atmosphere, so yes, it is basically a physics vacuum, but there are still enough air molecules to ionize. However, it is way different from a xenon lamp or anything of sorts, as those usually have much higher pressure. (Still less than atmosphere of course)

What do you mean by "low pressure vacuum"?

Do you mean 1*10^-6 torr?

 

Yep, somewhere about in that range

 

Quick thought...

Remember when I said that the problem I had with using the microwave diodes method, is that the diode would be destroyed??

What if I buy a bunch of 1300v 3A diodes (BY255) and hook three in series to get 3900v which is more than enough, and then take those strings of three and wire lots of those strings in parallel to get a much higher current capability?? (say 20 strands for 60A capability)

I bet those wouldnt pop, and they are way cheaper.

If they whithstand the discharge, they would be a solve all my troubles!