Triggered spark gaps / High Voltage / Forums

Forums

4hv.org :: Forums :: High Voltage

« Previous topic | Next topic »

Triggered spark gaps

1 2 3 next

Move Thread

LAN_403

The Wumpus

Fri Mar 24 2006, 02:19PM

Registered Member #325 Joined: Fri Mar 17 2006, 12:42AM
Location: Turku, Finland
Posts: 55

I'm using a homemade triggered spark gap as a light source for short-duration flash photography. Before building I searched these forums for ideas, and found the following schematic:
http://dave.4hv.org/site_imgs/tsg.gif

I implemented essentially the same schematic, but with some extensions (added cap charger). I connect an automotive ignition coil to the trigger electrode. The gap is triggered by discharging a small photoflash cap into the coil. Here are pics of the unit (discharge capacitors not shown) and a closeup of the gap with trigger electrode:

Tsg1

It is being used in a low power mode. I'm switching no more than max. 150-200 Joules at about 25kV with this gap, and just a few firings a minute max. The gap seems to work very reliably. However, I have three questions:

The schematic says that the high voltage cap bank charger must have positive ground and negative supply. However, my gap seems to work just as fine with a positive supply with respect to ground (I have tried both). Any idea about the rationale of the comment in the schematic?
What kind of limetime (how many firings) should one expect from a thing like this? The electrodes are currently iron but I am planning to replace them with brass electrodes. Is it worthwhile to go for more expensive materials like tungsten, etc?
I am going to build a more compact unit with a real flashead, and obviously I want to use a smaller trigger transformer (the ignition coil was used because it is cheap and seems to do the job). Any idea what kind of transformer is good for this purpose. Should be cheap and easily obtainable. I have been thinking about winding a 1:100 transformer on a small ferrite core from a computer PSU, would this do the job?

the_anomaly

Fri Mar 24 2006, 03:51PM

Registered Member #19 Joined: Thu Feb 02 2006, 03:19PM
Location: Jacksonville, FL
Posts: 168

I think that the gap can survive for a very long time if it is cleaned regularly. I would say a dc flyback would be a good option instead of an ignition coil. I don't think you'll get 1:100 turns on a computer psu core not to mention how long that will take to wind. I would also lower the trigger electrode to lower than the surface otherwise it will get eaten up fast.

Dr. Shark

Fri Mar 24 2006, 04:50PM

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

I have triggered small flashtubes reliably with a pulse transfomer I wound on a ferrite rod from an inductor. It had a few 100 secondary turns and just a few turns for the primary, and I powered it by a 1uF 300V MKP capacitor. That's really all that is needed.

The Wumpus

Fri Mar 24 2006, 06:13PM

Registered Member #325 Joined: Fri Mar 17 2006, 12:42AM
Location: Turku, Finland
Posts: 55

joe doh wrote ...

I have triggered small flashtubes reliably with a pulse transfomer I wound on a ferrite rod from an inductor. It had a few 100 secondary turns and just a few turns for the primary, and I powered it by a 1uF 300V MKP capacitor. That's really all that is needed.

Yes, though I think it is easier to trigger flash tubes than air spark gaps (they can be somewhat finicky). I will give it a try, though.

On a related note: Anybody played around with using a small mass of a radioactive isotope inside the spark gap to help ionization? Would it be possible to use the radiation source from an old fire detector (Americium I think) for this, and how should it optimally be placed in the spark gap?

Bjørn

Fri Mar 24 2006, 06:44PM

Registered Member #27 Joined: Fri Feb 03 2006, 02:20AM
Location: Hyperborea
Posts: 2058

I have tried Americium 241 and it had little effect on the ignition voltage of NE-2 bulbs. A LED with some UV output was much more efficient in igniting them. In air it made a 74HCT14 chip oscillate so it might have a measurable effect in a spark gap. It seemed to ionize a cloud of air up to 3 cm in front of it.

The Wumpus

Fri Mar 24 2006, 09:30PM

Registered Member #325 Joined: Fri Mar 17 2006, 12:42AM
Location: Turku, Finland
Posts: 55

BjÃ¸rn BÃ¦verfjord wrote ...

I have tried Americium 241 and it had little effect on the ignition voltage of NE-2 bulbs. A LED with some UV output was much more efficient in igniting them. In air it made a 74HCT14 chip oscillate so it might have a measurable effect in a spark gap. It seemed to ionize a cloud of air up to 3 cm in front of it.

The UV LED idea sounds interesting. Do you think it could even be used as the sole trigger for a spark gap, if one used very smooth round electrodes and precise voltage, perhaps in an evacuated vessel to have constant pressue/humidity?

The Wumpus

Mon Mar 27 2006, 01:01AM

Registered Member #325 Joined: Fri Mar 17 2006, 12:42AM
Location: Turku, Finland
Posts: 55

I put up a page with some recent images here. I also describe the equipment (all home built

), but this section is not finished yet.

Anyway, the spark gap works quite well but is very noisy. I plan to take many more pictures but my neighbours are getting tired of all the bangs (I live in an apartment)

. Any ideas how to soundproof an air spark gap?

Dr. Shark

Mon Mar 27 2006, 10:41AM

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

Hey, great pictures! You seem to have some artistic skill in addition to your engineering abilities. Are you in Finland? I'd like to meet some people who are into this high voltage thing in person, so if you are anywhere near Vantaa, please give me a PM.

Finally, a few thoughts on your sparkgap: Depending on the pulse shape you are after, and the voltage you are using, a series trigger transformer could work well. This is often done for laser flashlamps, but will probably not work for a pulse shorter than 100us.
I think the americium 241 would ionise the air in the gap, the radiation just does not pass through glass, so no wonder it doesn't work with neon bulbs. I don't see how this would help you though, you would just have to increase the gap distance, or is that what you want?
A fairly simple triggered spark gap (without need for an additional HV source) can be made by bringing a wire between the two electrodes, and have it floating at midpoint potential. To fire, you connect the wire to one of the electrodes via a large value resistor (via a relay or whatever), so the full voltage is between the wire and the other electrode. This breaks down one half of the gap, and consecutively the second half.
Enclosing the gap should not be too difficult, I have done it a sipmle as having the gap inside a short length of aquarium tubing sealed with silicone. Of course if you need the spark gap as a light source (but you should really get a xenon flash!!), this will not work as the tubing gets very dirty after a few shots.

Steve Conner

Mon Mar 27 2006, 11:54AM

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

Wow, beautiful pictures! I'm glad I'm not a fruit or an egg in your kitchen

I think to soundproof the spark gap all you need to do is enclose it in a sturdy container that is sealed to be almost airtight. If the container is big enough, the walls will be far away from the discharge, and shouldn't get burnt or blackened for a long time. Maybe a glass or plexiglass tube with putty or modelling clay packed around the electrodes to seal it. Be careful though as the container might shatter with a big enough bang!

WaveRider

Mon Mar 27 2006, 12:00PM

Registered Member #29 Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500

Wow! Great pictures! The colors are crisp and the photos are very sharp. I've been wanting to do a similar thing with triggered spark gaps... You've offered me a bit of inspiration.

1 2 3 next

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