Initiating capacitor discharge with a spark

I am not a spark expert.

It is the electric field that is important. You might start here,

Wiki says 3 X10^6 V/m... but this is at atmospheric pressure... if you reduce the pressure you can reduce the field strength.

Why are you trying to do this with a spark? Seems like there must be easier ways. It's not at all clear how much of the capacitors energy is going to be deposited into your dot of metal. (Lots goes into making the spark.) How about a laser pulse to heat the dot? Or maybe an induction heater? Spot welder?

George H.

30 volts is not enough to maintain ionization. Add some zeros to the voltage or make a direct connection.

John

Yes, Marconi did that successfully. It doesn't much matter that there's a 30V capacitor, he used what we now call radio frequency energy (RF).

If you want the discharge not to quench, at 30V/mm, it has to be in something other than atmospheric-pressure air. Low pressure neon/helium mix is typical (which is why multi-kilovolt neon lights in meter-long tubes are not unknown). Cold metal electrodes next to the glow discharge are ... unhelpful, so the volts/mm number doesn't scale down very well.

It is possible, but you need to (1) have a chemical reaction to generate free ions, or (2) use radioactive materials or (3) use tricks like gas mixtures with odd power supplies (ballasts) that prevent the Geiger effect. Geiger tubes give one 'click' then it takes a while to recharge the capacitor so it can click again. In air, at anything close to thermal equilibrium, a 30V source doesn't discharge across a 1mm air gap.

"Piotrne"

** Certainly possible under the right conditions.

Eg: If you open a switch while it is passing say 25 amps of current from a

30 volt DC supply, an arc will form across the contacts at distances of about 1 mm or so.

The process involves heating of the contact surfaces to produce metal vapour which sustains the arc PLUS the fact that the distance being jumped is initially VERY small.

** Likely several thousand volts is needed to simply jump a 1mm air gap.

... Phil

(...)

I take it that once the air is energized to a plasma state, it's easy to maintain current flow across a 1 mm gap with only a 30V gradient?

My excellent Miller welder has no problem with that. :) (Note the 150 A rating at 16 VDC)

--Winston

Really? I had an entire batch of circuit boards that were arcing between the + 5V and ground traces. Some early buzzers operated on a single 1.5 volt cell and arced across the contacts.

--
Politicians should only get paid if the budget is balanced, and there is
enough left over to pay them.

Look up Plasma welding..

Small ionized gas paths do not produce the volume you most likely are looking for.. You could get a pulse signal from it maybe!. You know, RF?

An automotive ignition makes about the size of spark you're looking for. Energy is stored in the spark coil inductance (conventional) or in a storage capacitor (CDI =3D=3D capacitor discharge ignition) , and the coil boosts the input to get to ignition voltage.

Unlikely at 30 volts on the cap. 3000 is more reasonable. One configuration is to have a sharp point with the HV from the cap on it, near the work, within a distance that's close to breakdown. A trigger electrode, like maybe a washer around the tip, is pulsed to very high voltage, which causes ionization at the tip and breakdown of the main arc.

Series triggering is sometimes done, but that's a little more difficult.

John

I believe a single HV spark won't be enough to establish an ionized path in the air. You require many, many arcs to strip sufficient electrons to provide the ionized path for your D.C.

Consider using ~15 KHz sparks instead of

0.00001 Hz sparks! :)

Here is the opportunity to build that Tesla coil you always wanted!

--Winston

Higher frequency would be better. A 6 cyl engine running at 10,000 (!) RPM is moseying along at a spark rate of only ~28 Hz.

The HF used to start an arc in a TIG welder is in the vicinity of >10 KHz.

--Winston

Well, OK. 1000 Hz. But you get what I mean.

--Winston

Winston wrote: > whit3rd wrote: >> On Oct 29, 9:20 am, Piotrne wrote: >>

Well, OK. 1000 Hz. But you get what I mean.

--Winston

Thank you for all your responses. I'll consider what to do:

- use higher voltage (but not 3000V! Maybe 50V),

- use some gas,

- lower the pressure (difficult),

- ...

I'm trying to build a small welding device, a simplified version of such a thing (which costs about $6000):

It contains a battery of capacitors charged to about 36V. May change the duration of the pulse from about 2ms to 30ms, at 600A. Uses a special gas around the welding point. Requires to touch the metal with a needle-shaped electrode. I just wanted to use another way of initiating the discharge then touching. But it really seems to be difficult...

Regards Piotr

You're looking for a TIG welder. SOMEBODY should have one for considerably less than six big ones!

Can you afford a couple hundred bucks?

Good Luck! Rich

That welding that is on that link is most likely using Argon..

also, that welder looks like it's actually detecting the short when the probe touches the metal then retracts back and energizes the tip, to give you a short burst.

6000 RPM /60 seconds per minute is 100 revs per second. For a 6 cyl, there are 3 sparks per rev, thats 300 Hz

that is how a thyratron works

possibly something else.

60V is plenty to maintain an arc (most arc welders operate in that ballpark and can maintain ars of several milimeters, I've heard of people arc welding with as low as 24VDC

--
??pun u?op ?o?? s?u??????


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