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Air gap flash

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Adam Munich

Tue Jan 11 2011, 06:46PM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

I found this on hackaday, an air gap flash for high speed photgraphy. Honestly it's the coolest thing I've ever seen someone do with a capacitor. So, those lucky enough to have one of these HV caps on hand, I'd like to see you do this.

Bullet VS Brass rod

Tesladownunder

Sun Jan 16 2011, 07:56AM

Registered Member #10 Joined: Thu Feb 02 2006, 09:45AM
Location: Bunbury, Australia
Posts: 1424

I am probably speaking about things I don't understand, but I would bet that air vs Xenon is no different in spark rise time from the point of view of this practical application.

The real difference is in the discharge of the pulse cap.
In a camera flash, this is a simple electrolytic, notorious for a slow discharge rate. (eg it is very hard to do can crushing with electrolytics)
in a Q switched laser pumped with a Xenon flashlamp like in the common rangefinder lasers there is a (polypropylene) capacitor 32 uF 600V with a 0.03 mH inductor as a pulse-forming network.

If you use a pulse cap for higher voltage (10kV +) that you need for an air gap , you are probably going to get a higher discharge rate than a lower voltage (600V for a Xenon).

Hence for these reasons, it appears that an air gap is faster.

He is going to reduce his caps lifetime by the direct shorting of the output, including his shorting stick.

Steve Conner

Sun Jan 16 2011, 10:12AM

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

The secret is that atmospheric air has a higher breakdown voltage than an ordinary flash tube, so it matches a high-voltage, low-capacitance capacitor, delivering the same energy in a shorter pulse.

If you wanted to run a xenon flash tube off 10kV, it would need to be very long to avoid self-triggering, or you could use a triggered air gap to switch it. But then you might as well just use the air gap as the flash, which is where we came in.

I think several 4hv members have done this in the past, try looking through the archives.

Thomas Rapp made a high-speed flash that does use a xenon tube fired by a triggered spark gap.

Wolfram

Sun Jan 16 2011, 11:30AM

Registered Member #33 Joined: Sat Feb 04 2006, 01:31PM
Location: Norway
Posts: 971

Tesladownunder wrote ...

I am probably speaking about things I don't understand, but I would bet that air vs Xenon is no different in spark rise time from the point of view of this practical application.

There is not much difference in the rise-time as far as I know, but xenon has significant afterglow in flash tubes.

Edgerton found that the shortest flashes could be developed by discharging the capacitors in air. Gases such as krypton and xenon, which are found in conventional discharge lamps, continue to glow strongly for a substantial period of time after the electric current has ceased.

Dr. ISOTOP

Mon Jan 17 2011, 01:06AM

Registered Member #2919 Joined: Fri Jun 11 2010, 06:30PM
Location: Cambridge, MA
Posts: 652

Hey, I think I found a use for my spare 50KV 75nF pulse capacitor!

Adam Munich

Mon Jan 17 2011, 05:55AM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

Would doorknob caps be fast enough?

Adam Munich

Tue Jan 18 2011, 04:38AM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

Hello? You know, those ceramic doorknob caps.

Turkey9

Tue Jan 18 2011, 09:40AM

Registered Member #1451 Joined: Wed Apr 23 2008, 03:48AM
Location: Boulder, Co
Posts: 661

Since ceramic caps have very low ESR similar to film pulse caps I'd imagine they would work... the only danger would be dielectric degradation from fast discharges. Check the data sheet or contact the manufacturer to see. Other wise you'd just need a big enough cap to get a usable pulse length.

P.S. Be patient and wait longer before double posting something meaningless. We're trying to up the quality of the posts, aren't we?

Dr. Shark

Thu Jan 20 2011, 03:39AM

Registered Member #75 Joined: Thu Feb 09 2006, 09:30AM
Location: Montana, USA
Posts: 711

The main problem with the doorknobs it that the most common ones only store a few J of energy each, which would make for a very dim flash. Even disposable cameras have an order of magnitude more oomph in the flash circuit. For macro work where you don't need to light up a whole room I see no problem though, the discharge would certainly be fast enough seeing how these caps are used for nanosecond pulses in longitudinally excited nitrogen lasers.

Adam Munich

Thu Jan 20 2011, 05:39AM

Registered Member #2893 Joined: Tue Jun 01 2010, 09:25PM
Location: Cali-forn. i. a.
Posts: 2242

Hmm, maybe I could make my own caps...

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