Palflash 500, nanosecond light source

[dexters_lab]

something that happens quickly and  repetitively like say water dropping onto fan blades would not be a problem

but the more interesting pictures seem to be one off events so i need a trigger

it does not help either that i have to be in darkness when i open the shutter on the camera while it waits for the flash

because photos like this are very quick and completely frozen in time there is little perception of speed or movement unless you have something explosively disassembling

i picked up some argon welding gas and a regulator today, i'm interested to see how this affects it.

[dexters_lab]

Will be a while before i do another update on this as i will need to investigate a proper rig with trigger, but for now here are some (cropped) test pictures.

[Yago]

Looking forward to it.

A cable release for the cam might help.
Canon are two (common ground)normally-open push to make switches, one for focus (unneeded for this), the other fires the shutter.

Shorter focal lengths, close-up should return less blurring outside of the focal plane, but puts you closer to the action.

[mikeselectricstuff]

Looking forward to it.

A cable release for the cam might help.
Canon are two (common ground)normally-open push to make switches, one for focus (unneeded for this), the other fires the shutter.

Shorter focal lengths, close-up should return less blurring outside of the focal plane, but puts you closer to the action.

IR remote is better - no cable to trip over in the darkness.

[dexters_lab]

i have found that if i stick a little led desklight over in the corner of the room i can work in almost darkness, it doesnt seem to affect the actual shot

oh, these are using argon too, it allowed me to increase the gap distance by a couple of mm at least. Before i adjusted it the argon would actually make it randomly flash over on its own.

[Yago]

Looking forward to it.

A cable release for the cam might help.
Canon are two (common ground)normally-open push to make switches, one for focus (unneeded for this), the other fires the shutter.

Shorter focal lengths, close-up should return less blurring outside of the focal plane, but puts you closer to the action.

IR remote is better - no cable to trip over in the darkness.

This is true, but the IR ones I have seen are bit crap, poor range, have to be infront of camera.
A raid of the bits box should return a cable release for free, gaffer it to the floor perhaps.
It's alright for you Mike , you bits box probably returns several night-vision systems and several robot assistants !

As a simple trigger (that might not work), could measure a current through the filament of a bulb, a pellet breaking the filament trigger the light?
The event would be half over yes, but it is a simple proposition.

[dexters_lab]

yea, trigger needs to happen before the event (this would rule out an audio trigger also), with an adjustable delay

i dont believe my D5000 has anything other than an IR remote shutter, but thats ok, i can cope with pressing the shutter button. It's triggering the flash i need to concentrate on

triggering from watching current is an idea, i wonder if you over volted a filament bulb and trigger when it goes open circuit you might be able to see the filament split, but i suspect the filament itself would be too bight to expose properly.

[Yago]

yea, trigger needs to happen before the event (this would rule out an audio trigger also), with an adjustable delay

Does the light have a trigger latency?

[dexters_lab]

i'm guessing there would be some delay, the datasheet also mentions 500ns jitter but i doubt that would be of concern to me with what i want to do.

just had a look at the trigger, it's very simple. just apply a voltage to the trigger input! The inputs are just pulled low, the test button simply applies the 24v power supply to the trigger inputs internally. It triggers on the rising edge and seems to be happy triggering from 5v.

right well i suppose i need to brush the dust off my pic development stuff

[dexters_lab]

hey everyone,

been making some progress, i expect to make an update video soon once i have worked on this a little more.

i have an optical trigger running now, here are some sample pics.

[SeanB]

Looks good there.

[TerraHertz]

I think you've adjusted the airgap way too small. You can see there's barely any tube left extending, to stick the plastic hose on. That's not their mistake, it's yours. Those gradations are probably meant to be visible above the top edge of the locking nut, to allow adjustment without taking it completely out.

With the gap correct, the centerline of the lenses will be about the centerline of the airgap. Not with the top electrode extending mostly into the optical path - because then it's blocking a lot of the light from the rear spark unit.

The airgap is supposed to be quite a lot more than you'd expect for 8 to 10 KV. Because the actual breakdown field strength is the main 8KV PLUS the HV trigger pulse, and that's going to be another 10KV or more. Also it's supposed to be acting from a needlepoint at the trigger end.

The way triggered spark gaps work, is an electric field is established between the two major electrodes, which don't have any sharp points. This never breaks down the gap.
The center trigger electrode is supposed to be a needle point, but it's down in the low static field strength area within the end of the ground electrode and the point tip is at the same potential as the surrounding main electrode, so there's little or no ionization on the needle point.

Then a HV pulse is imposed on the trigger needlepoint by the firing circuit. That will be a HV pulse transformer, with a HV cap being discharged through its primary by an SCR. The polarity should be arranged so the leading edge of the spike is the opposite polarity to the main HV (top in this case) sparkgap electrode.
Now the field strength at the needlepoint is much higher, and the air around the tip gets ionized strongly. Those charge carriers are in the main field between the two electrodes, so get swept towards the other electrode. This starts an ionization cascade, and an arc forms across the main airgap.

Ideally, the main arc should form between the big heavy rounded electrodes, not one of them arcing to the needlepoint.

It's hard to see in your video, but it looks to me like the needlepoint has been arc-burned to a flat-ish rough end? And that's why it arcs over to the main ground electrode. It shouldn't do that. Because it's not a sharp point there's not enough field concentration upwards towards the top HV electrode. When you moved the top electrode down a LOT, it got close enough to overcome the effects of the damaged needlepoint tip.

I'd try taking the bottom electrode structure apart completely, and see if you can get the center needlepoint electrode out to reshape or replace it. I'd have expected its height relative to the tip of the surrounding electrode to be adjustable too. Is there a grubscrew or something accessible via that hole at the bottom of the electrode?

Maybe you'll be lucky, and they used something common like a gramophone needle, or a standard thin tungsten TIG welding rod, ground to a point?
But in any case, it's a tiny bit of metal. Can't be too hard to improvise a replacement.

[dexters_lab]

thanks TerraHertz for the insight into the trigger mechanism

i went for approximately 6mm because that was detailed in the information on the pulse photonics website which says '- 6 mm: when operated with air or argon gas'

though i do think i need to open the gap, it was on for about an hour yesterday, as time went on it would flash over on it's own more and more, probably due to the build up or argon inside the case so i will be increasing the gap in the future.

The electrodes are similar to your drawing but not exactly as mine is, your drawing is more like i expected it to be before i started looking at it's detail, i remember mentioning in the original video it wasn't quite what i expected. This seems to be arranged so the ground and needle are reversed. I have attached a picture and detail it's arrangement here for clarity:

A: HV+ electrode, connected to the + side of the main cap bank, is a hollow stainless steel tube with a domed end with a small hole, hole is maybe about 1-2 mm to allow the gas to flow from the top over the gap.

B: Part of the ground electrode, connected to chassis ground and the other side of the main cap bank

C: Plastic insulator, i believe this fills the cavity between F and D, it's also visible between F & D at the top (marked G)

D: Ignition + electrode, you can see the red wire connected to it at the back with a spring loop. When triggered a spark will jump from this electrode to the centre F ground.

E: This just shows the extent of D the ignition electrode as it was hard to see in the picture.

F: Ground electrode, is a small metal rod about 1-2 mm located in the centre which is part of B.

[TerraHertz]

It's really interesting that they seem to have reversed the usual trigger topology.
I wonder why? Perhaps it was just easier to make, but still works. Or maybe it results in a more uniform position of the arc - in the center, not at random positions around the rim of the bottom electrode. So better uniformity of the light focusing.

That spring loop attaching the red wire, can that slide off easily?
If it does, have you tried to see if the  outer end piece unscrews somehow from the white insulator?
It really might be worth taking that whole lower electrode apart, to see if there's anything possibly misadjusted.

Given the general style of the rest of the construction, I bet it does all come apart.

Another point is you have two of those spark gaps, and it's possible the rear one wasn't used as much as the front one. Have you compared them? Do they look exactly the same?
Have you tried firing the 2nd one, and does it behave the same?

[dexters_lab]

i think your right about the centre ground electrode being there to keep the spark in a repeatable position

it does disassemble but it's horrible to work on, there wasn't much thought about how it all goes together, many things dont come out until you take something else out, there's inaccessible screws etc

sections C & D simply pull off the B ground electrode, leaving a long prong if you move the + electrode up out of the way. B looks like its stainless steel bar that's been machined down to size. When i'm next poking around inside i will take a picture so you can see.

the rear gap looks identical to the front one and works in the same way and does work, although i have opened the gap and disconnected the gas supply so it doesn't fire, while i'm experimenting with it i thought i would just keep it simple and only use the front gap. I am tempted to re-wire it so i can discharge the full 12J into the front gap and place some white plastic at the back to act as a basic reflector.

The cap banks are 'isolated' in there must be something between the banks to stop one gap drawing charge from the other cap bank when it fires. There is something in some heat shrink on the HV supply cable but it's really thick heat shrink that has glue in it so i'm not inclined to open it up, maybe a charge limiting resistor which would also help stop the discharge of both banks when one gap fires?

[TerraHertz]

The cap banks are 'isolated' in there must be something between the banks to stop one gap drawing charge from the other cap bank when it fires. There is something in some heat shrink on the HV supply cable but it's really thick heat shrink that has glue in it so i'm not inclined to open it up, maybe a charge limiting resistor which would also help stop the discharge of both banks when one gap fires?

Maybe it's some HV diodes in series, and also resistors.
The resistors to limit loading on the supply when the caps are discharged, and diodes to keep the two banks isolated.

That heatshrink with hot-melt glue lining is impossible to do anything with once it's shrunk on.

[mikeselectricstuff]

Just saw this on kickstarter - LED based 500nS flash
https://www.kickstarter.com/projects/vela/vela-one-the-worlds-first-high-speed-led-flash?ref=category

[dexters_lab]

looks a nice bit of kit, though i like the fact mine makes sparks and wants to kill you 

speaking flash speeds, if i wanted to setup something to check the duration of the actual light output of mine what would be the best setup for my scope to measure it?

[miguelvp]

Photodiode or LED?

But maybe they don't respond fast enough, I don't know.

[mikeselectricstuff]

looks a nice bit of kit, though i like the fact mine makes sparks and wants to kill you 

speaking flash speeds, if i wanted to setup something to check the duration of the actual light output of mine what would be the best setup for my scope to measure it?

Maybe photodiode, but how to know whether you are measuring flash duration or photodiode speed?
You could maybe calibrate using a LED, which you can easily pulse accurately with a function gen or MCU.

[Yago]

I know I can be a sceptical b*****d, so just to check...

Anyone else find that LED light claims and photos a bit iffy?
IE: " the Vela One will stop a supersonic, high velocity rifle bullet in its tracks."
Then all the photos are of a BB gun pellet! (Very low velocity).

[mikeselectricstuff]

I know I can be a sceptical b*****d, so just to check...

Anyone else find that LED light claims and photos a bit iffy?
IE: " the Vela One will stop a supersonic, high velocity rifle bullet in its tracks."
Then all the photos are of a BB gun pellet! (Very low velocity).

If you read the comments and FAQ you'll see this is because he's not yet got access to real guns  - being in the UK this is somewhat less simple than dropping in to a local Wal-mart.

So, if I'm confident the quoted pulse widths are correct, how does that translate into stopping power? Let's do the maths. For our example, let's assume a muzzle velocity of 1000 m/s, which is approximately the muzzle velocity of an M16 rifle. That translates to 3280 fps or Mach 2.94. 1000 m/s is equal to 1mm/microsecond. Our shortest pulse is half a microsecond, meaning that an M16 bullet will travel half a millimetre during the exposure. This would give negligible motion blur. I hope that clears things up! I'll post in more detail about my testing setup in a later update, including waveforms from the scope. I am also trying to work out a way of testing this with real guns. My best bet is to work with a gun club, where a member with a firearm certificate could work with me. This is something I'm working on, so hopefully I should have some shots to share eventually.

[krivx]

Then all the photos are of a BB gun pellet! (Very low velocity).

Some googling says that velocities of a pellet can be pretty close to a bullet from a small handgun, bought around 1000 feet/s 

[SeanB]

Look at Taofledermaus, he got pellets up to a lot more than a bullet can reach, using simple methods and stuff he bought at Wal Mart, and not from the gun section.

[Yago]

I know I can be a sceptical b*****d, so just to check...

Anyone else find that LED light claims and photos a bit iffy?
IE: " the Vela One will stop a supersonic, high velocity rifle bullet in its tracks."
Then all the photos are of a BB gun pellet! (Very low velocity).

If you read the comments and FAQ you'll see this is because he's not yet got access to real guns  - being in the UK this is somewhat less simple than dropping in to a local Wal-mart.

So, if I'm confident the quoted pulse widths are correct, how does that translate into stopping power? Let's do the maths. For our example, let's assume a muzzle velocity of 1000 m/s, which is approximately the muzzle velocity of an M16 rifle. That translates to 3280 fps or Mach 2.94. 1000 m/s is equal to 1mm/microsecond. Our shortest pulse is half a microsecond, meaning that an M16 bullet will travel half a millimetre during the exposure. This would give negligible motion blur. I hope that clears things up! I'll post in more detail about my testing setup in a later update, including waveforms from the scope. I am also trying to work out a way of testing this with real guns. My best bet is to work with a gun club, where a member with a firearm certificate could work with me. This is something I'm working on, so hopefully I should have some shots to share eventually.

Why not use a real air rifle, and not a toy BB gun.
Even UK air rifles are much nearer(than a BB gun) the velocities claimed, FAC UK, you can hop-up a PCP air rifle to around and possible above supersonic speeds.
It is easy to control too, just fire it into a block of soft clay.
There is no need to chucking military lead about to test, that is the root of my misgivings.