calculate the current of a marx generator / High Voltage / Forums

Forums

4hv.org :: Forums :: High Voltage

« Previous topic | Next topic »

calculate the current of a marx generator

Move Thread

LAN_403

IÃ±aki

Sun Aug 10 2008, 02:01PM

Registered Member #1631 Joined: Sat Aug 09 2008, 04:36PM
Location:
Posts: 1

Hi,
I am a postgraduate student and I am using an impulse generator (Marx generator) for my dissertation. I would like to know if there is any way to calculate the current of the discharge in a theoretical way. In all the books and articles I have checked I haven't found anything.
Thank you for your time, regards,

IÃ±aki.

Antonio

Sun Aug 10 2008, 02:37PM

Registered Member #834 Joined: Tue Jun 12 2007, 10:57PM
Location: Brazil
Posts: 644

The current is limited by resistance and inductance of the discharge path, including the output spark, the gaps, and the capacitors. The equivalent resistance is difficult to determine, but is probably low. The inductance is probably the limiting factor. Consider an approximation of 1 microHenry per meter, and consider that the stored energy in the capacitors 0.5*C*V^2 will become 0.5*L*I^2, and you will obtain the maximum possible current.

lightt

Thu Aug 14 2008, 01:57PM

Registered Member #1636 Joined: Thu Aug 14 2008, 01:24PM
Location:
Posts: 1

Hi,

I am a newbie in this forum and this is my firs post.
There is a possible and realy effective method for crren t viewing in impulse systems. mostly there is an current viewing resistor (shunt resistor) connected serial between ground and ground terminal of the tested object. Mostly shunt resistors are very low resistance for example I used one of them rated 0.00075 ohm, so they didn't affect shape of signal critically.

Myke

Thu Aug 14 2008, 05:54PM

Registered Member #540 Joined: Mon Feb 19 2007, 07:49PM
Location: MIT
Posts: 969

I would think that most equipment that would be able to measure that short of a pulse might be damaged by it if it were connected to a marx. Otherwise I don't see why not.

...

Thu Aug 14 2008, 10:05PM

Registered Member #56 Joined: Thu Feb 09 2006, 05:02AM
Location: Southern Califorina, USA
Posts: 2445

You could use a current shunt with a marx, but be sure that it has a very low inductance, or the output results will be excessivley high. Also make sure that you shunt is capable of surviving very high pulse currents, although you will have a hard time blowing up a .75mOhm shunt with a hobby sized marx...

Also, be sure to ground one side of the shunt through a thick/low inductance connection to the bottom of the marx generator, or else you will risk generating high voltages on the ground side of the shunt and blowing up your test equipment. Particularly because in order to view the output of a marx you are going to need a very fast digising scope...

Steve Ward

Thu Aug 14 2008, 11:43PM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

Yes, its called a wide-bandwidth current transformer. Check out Pearson, or Ion Physics. CT's offer galvanic isolation between the measurement equipment and the circuit being measured. You must get a high-bandwidth type transformer to measure these high frequency pulses. There are even methods for measuring DC pulsed signals with a CT, so this is the proper solution for your problem.

Terry Fritz

Thu Aug 14 2008, 11:55PM

Registered Member #393 Joined: Tue Apr 18 2006, 12:30AM
Location:
Posts: 297

Beware that those currents might be very high and the bandwidth might be "very" high too!! Possibly into the GHz!! As those large air breakdowns take place, it is just a matter of how fast electrons can move in a dV field.

Just make a spark and look at the extreme frequencies that are generated. Hard to say where the most "power" is as a function of those frequencies. But I have seen a lot of power going down far faster than I can imagine measuring.

If the spark was repetitive then a spectrum analyzer would be useful. But it is hard to get good P vs. F data off a single event at super high frequency. Maybe there are better toys now to explore such things.

Terry

Steve Ward

Fri Aug 15 2008, 07:08AM

Registered Member #146 Joined: Sun Feb 12 2006, 04:21AM
Location: Austin Tx
Posts: 1055

Beware that those currents might be very high and the bandwidth might be "very" high too!! Possibly into the GHz!!

With the capacitors being "lumped" and the loop inductance being several uH, do you really think so? I mean, given the actual length of the spark itself, could it really be in the GHz? Seems a bit extreme Terry

. Rise times on the order of nanoseconds is probably likely. Then again, my theory of air breakdown is not complete, so maybe the actual transition from leader to having a high channel current is extremely fast, but id still think its limited by the length of the spark?

Shaun

Tue Aug 19 2008, 12:28AM

Registered Member #690 Joined: Tue May 08 2007, 03:47AM
Location: New Jersey, USA
Posts: 616

Everyone seems to be talking about "measuring" the current, but the original post asks for a way to "calculate" it based on known values.

I agree with antonio; all that's really needed is R, L, and C, and once you have those you can calculate the maximum current, ringing frequency, etc. Or hell just plug it into Barry's RLC sim!

The problem with that is actually getting those values, though. Especially the inductance. Plus there will be various losses in the circuit, and other things of that nature. But you should absolutely be able to get close to the actual value.

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob