EEVblog Electronics Community Forum
Electronics => Repair => Topic started by: dexters_lab on November 08, 2014, 08:18:55 pm
-
i picked up this pulse light source a while ago and want to figure out why i get no flash out of it.
its a simple device with two air-gap spark flash devices inside with a capacitor bank.
would like some ideas on getting this working, i think the problem might be more mechanical than electronic but we will see!
Teardown: Palflash 500 - Nanosecond Light Source Part 1 (https://www.youtube.com/watch?v=O8viMG_Wlsc#ws)
-
Did you try firing it from the trigger ports, do they require a voltage or just short to trigger. I guess each trigger port is to fire the 2 lights at different times or together?
Probably a sticky relay :)
-
no, not tried the trigger inputs. i am (foolishly?) assumung that if the test doesnt work then the trigger inputs wont either
i will check the input circuit closer to see what kind of input it might be
sadly there is no documentation i can find for this :(
-
Looks a very interesting device, would love to see it fire.
Do you have gas you can feed it too?
-
Beefy!
Gap seems a little wide for 9kV, which should be the case; you don't want that sparking just any time. And there should be a spark plug or trigger electrode or something like that nearby, which I'm guessing is coming from the black boxes on the board. So, I'd check those first, see if you can get those to do anything.
I'd want to guess the inputs are TTL level or something like that, but whatever it is, you should probably be able to find out from tracing the circuit?
Tim
-
I would say it isn't going to discharge without a low dielectric strength gas feed. Likely a rare gas Argon, Neon, Xenon etc and possibly a mixture with air.
It is a flash tube without the tube.
-
Hi,
I did a little searching. The palflash 501 will work with Argon or Air.
I suspect that you will have to reduce the size of the gap if you are using air. I think if you loosen the big nut, you slide the silver coloured rod up and down to adjust the gap.
This is a triggered spark gap design. The small arc will ionize the gas and cause the main arc to fire.
You may have to feed the unit with air from a compressor to extinguish the arc.
Regards,
Jay_Diddy_B
-
You will need to reduce the gap or pump the unit down internally to a low pressure to fire that large gap. Probably 0.2 to 0.5bar suction will do to fire that gap, and if you have some argon to feed into the supply port as well it will do a nice bright flash. Likely the original configuration was that the one nipple was the gas suction, fed to both top ports of the flash gap, and the other tube went down below to the lower posts to provide the gas supply. The suction would also purge the case.
Likely there was the mod done to reduce gas usage, but this contaminates the case. The suction removes the ionised gas so it will not contaminate the gap. You can see from the deposits in the pipe that the top was the suction side, the lower was supply.
-
thanks guys :-+ :-+
I will pickup a argon regulator for a mig welder and rig an argon supply up to it shouldn't cost more than £25 or so to do this, machine mart are only 20 mins away, hmmm!
A friend also commented that the gap looks too big so i will get my spanners out and have a go at adjusting it and see what happens for part 2!
@SeanBZA: i dont think there was ever a piped gas supply to the bottom electrode, there doesn't seem to be any evidence of that and there is no way you could route the pipes to the underside because of the way it fits into the case. I think they just used the gas flow from the top electrode to do all the work. The case is not designed to be pumped down either, nothing is remotely air tight!
I will have a look at it soon and post results!
-
Some random info from the web:
http://www.pulsephotonics.com/palflash.htm (http://www.pulsephotonics.com/palflash.htm)
http://www.photonicsonline.com/doc/dynamic-schlieren-imaging-palflash-501-0002 (http://www.photonicsonline.com/doc/dynamic-schlieren-imaging-palflash-501-0002)
I didn't register so I didn't download the datasheet:
http://www.photonicsonline.com/doc/dynamic-schlieren-imaging-palflash-501-0001 (http://www.photonicsonline.com/doc/dynamic-schlieren-imaging-palflash-501-0001)
Chinese site translated by google:
https://translate.google.com/translate?hl=en&sl=auto&tl=en&u=http%3A%2F%2Fwww.hrtlaser.com%2Fcn%2Fproduct-detail-52.html (https://translate.google.com/translate?hl=en&sl=auto&tl=en&u=http%3A%2F%2Fwww.hrtlaser.com%2Fcn%2Fproduct-detail-52.html)
it has the datasheet, it was too big so I made pictures but not too exciting, I attached them
Some uses of the device:
http://hiviz.org/hsi/ss/schlieren/index.htm (http://hiviz.org/hsi/ss/schlieren/index.htm)
http://hiviz.org/hsi/ss/rem222/index.htm (http://hiviz.org/hsi/ss/rem222/index.htm)
But I wonder if the inner tube in the sparkgap is supposed to move or not, or if it was supposed to have something in the inner part that broke off. But I'm just guessing.
-
That gap looks way too big to fire at 9Kv in air.
My guess would be that it wants to breathe argon.
Xenon & other inert gases are too expensive for use in an open-flow system.
Maybe Nitrogen?
It looks like it uses a cheap gas, otherwise you'd expect to see a gas control valve to turn the flow on when required.
I wonder if the second tube is simply to allow the main one to be blocked to avoid wasting gas.
Is the unit sufficiently well sealed to allow pressurisation? It probably isn't designed for negative pressure as I see no precautions against outgassing from components.
The 2 gaps are probably to allow measurement of speeds - look at distance moved between 2 images at known intervals.
-
You may have to feed the unit with air from a compressor to extinguish the arc.
No - the cap will discharge until the arc extinguishes.
-
Nice light Dexter, looking forward to see the results of your work.
This mean you are moving to stills work now?
Do you mind if I ask how much you paid for this unit? (I am a bit of a photog too).
-
Reminds me I have an ancient 1000fps flash unit gathering dust somewhere - must dig it out sometime.
I have a suspicion that it may not be much brighter than a modern white power LED...
-
thanks for the tips guys,
i just had it open on the bench while i mused over the comments and noticed the gap is very easily adjustable and in fact i have made it fire after adjusting the gap :-+ Didn't half make me jump when it flashed though! I just need to redo it on camera so you can see. >:D
@mikeselectricstuff
will be trying some argon as it's easy to get hold of and cheap. There is no sealing whatsoever on the enclosure so i'm sure it's not meant to be under anything other than atmospheric pressure. 1000fps flash? sounds interesting maybe you should make a video on it, isn't it about time we saw some of your glassware ;)
@yago
no, i'm far more interested in high speed video, but it might be fun to so something with it if i can rig up the right trigger based on light or sound. I paid £30 for it including postage, i could have saved a tenner and picked it up as the seller was local to me but i CBA. :-[
@miguelvp
thanks for the links, i had found them already but i still appreciate your effort! I did wonder about the inner part of the ground electrode being adjustable but i dont think it is. It seems to be set into the plastic insulation that surrounds the trigger electrode.
more to come...
-
Wow! glad you are making progress. Looks bright! hope you don't get arc eye or put your finger in there lol!
-
Might be hard to find something fast enough for a 500nS flash to be useful.
The only thing that comes immediately to mind is an exploding glass prince Rupert's drop.
I wonder if a DSLR has any significant sensitivity to exposures this short?
-
Might be hard to find something fast enough for a 500nS flash to be useful.
The only thing that comes immediately to mind is an exploding glass prince Rupert's drop.
I wonder if a DSLR has any significant sensitivity to exposures this short?
I didn't heard of Prince Rupert's Drops before Steven Fry shattered one on QI this week.
-
Smarter every day did a great video on them a while back...
Mystery of Prince Rupert's Drop at 130,000 fps - Smarter Every Day 86 (https://www.youtube.com/watch?v=xe-f4gokRBs#ws)
-
Might be hard to find something fast enough for a 500nS flash to be useful.
The only thing that comes immediately to mind is an exploding glass prince Rupert's drop.
I wonder if a DSLR has any significant sensitivity to exposures this short?
should be fine i think, long exposure in darkness then fire the flash
will try it when i do a an update on it in a few days
-
Reminds me I have an ancient 1000fps flash unit gathering dust somewhere - must dig it out sometime.
I have a suspicion that it may not be much brighter than a modern white power LED...
I have a military RPM stroboscope, using a xenon filled vacuum tube for the flash. GenRad 1531, tube is 1538-P1. Interesting tube, if simple; a shame there's no data on it.
Tim
-
quick update with some video of it firing and some test shots
the last picture looks good which is shown at the end. I will need to experiment with the setup some more. Of course you have no control over the exposure time so you have to control exposure with the aperture. I was typically around F8 to get a better DOF. I will look next time at upping my ISO to get a higher F and trying different camera angles.
i just need to sort out some triggering mechanism and some things to try with it
It's a bit of a crap thing to work on though, as in they didn't think about how it went together, screw heads hidden under things etc, so i will have to set aside some time to get the control board out and figure the trigger circuit.
Project: Palflash 500 Light Source Part 2 (https://www.youtube.com/watch?v=yL_5pdjwvsY#ws)
-
photodiode triggering of a water droplet, or you could use it for a time of flight distance measurement with a camera but you will have to synch it with the camera refresh rate to get an accurate measurement, 2nd delay trigger might help if the camera sensor has persistance.
Get a bicycle wheel or anything that rotates and freeze it in time, 2nd flash to determine the angular velocity. I bet there are a million uses for that equipment.
-
i was thinking about it last night and i think the best setup would be a simple beam break trigger.
I am thinking that having a tube that i can drop a big ball bearing though with the break sensor at the bottom. This should be usable for dropping other things like water as well.
So i can drop it onto things like glass objects might prove practical, interesting and reusable for various objects.
this guy has some amazing photos using a airgap flash
https://www.flickr.com/photos/mauriceribble/sets/ (https://www.flickr.com/photos/mauriceribble/sets/)
-
It's alive! :)
The shot with the blot and solder reel was a really good indicator of scale.
Close up or even macro only.
It is going to be difficult (impossible?) without being able to sync the light to the event.
Wonder if you could capture ripples across the surface of something oscillating (a membrane?), or a travelling wave on a wire (mechanical).
Cyclic stuff might help zone-in on the event until the trigger is sorted.
Looking forward to more Dexter, cheers :)
-
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.
-
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.
-
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.
-
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.
-
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.
-
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.
-
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.
-
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?
-
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 :-/O
-
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.
-
Looks good there.
-
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.
-
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.
-
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?
-
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?
-
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.
-
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 (https://www.kickstarter.com/projects/vela/vela-one-the-worlds-first-high-speed-led-flash?ref=category)
-
looks a nice bit of kit, though i like the fact mine makes sparks and wants to kill you :bullshit:
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?
-
Photodiode or LED?
But maybe they don't respond fast enough, I don't know.
-
looks a nice bit of kit, though i like the fact mine makes sparks and wants to kill you :bullshit:
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.
-
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).
-
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.
-
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 :-//
-
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.
-
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.
-
He's claiming the 500ns pulse was measured with an optical sensor with 1ns rise-time, I don't really see why it matters if he has captured a bullet or not? Surely an optical measurement is more useful than an image of a bullet?
-
Ok, I am that curmudgeon, under-a-cloud, test engineer! :P
If someone says "it's bulletproof" etc.
I'll leave it alone then, with the irony of; if it's any good I might buy one (well, try making one :P)
-
from what i understand 500ns should be good enough to capture bullets from guns and stuff.
the reason i ask is in the data sheet it says the 2J model is rated at 250ns, i can change mine by removing caps so i just wondered about measuring it to see how much of a difference it made,
i just shoved a 3mm red led in front of it directly to my scope, and get a good rising edge off it in the light beam but the falling edge faded slowly over 100s of microseconds so wasn't useful so put a 10k resistor across it and got the following which looks better, but 4uS is still no where near what it should be so i'm guessing i need to do better. Maybe there is some strange physics happening with that much energy in a short period?
-
To do good sub-microsecond photodiode work, you really want the diode reverse biased (so the photocurrent is cleared quickly and the junction capacitance is small), and you want it returned into as low an impedance as possible -- you can use a very low value resistor, but the signal is very weak. (For a freaking flash, this is probably an advantage -- it's probably more than bright enough to saturate the diode, so the current will be relatively large.)
For ordinary signals, you use a TIA (transimpedance amplifier), which has (ideally) zero input impedance, and converts input current to output voltage (thus, a gain of V/I = impedance, but gain, so it's 'transimpedance'..).
Tim
-
A phototransistor might be quicker, but not sure if they have any available under 1 us
What's the fastest optocoupler and what do they use inside? darlington configured phototransistors?
-
They don't get faster than photodiode + TIA.
No combination of PD, BJT or darlington will get anywhere near close enough, being limited by recombination time of the B-E junction(s). Darlingtons in general are slow as molasses.
For example, this circuit uses a 4N35 as its own TIA (plus some outboard bits, of course); analog bandwidth is a few MHz, versus maybe 20kHz in saturated operation, or a few 100kHz with a small load resistor.
http://seventransistorlabs.com/Images/4N35_TIA.png (http://seventransistorlabs.com/Images/4N35_TIA.png)
Compared to an LED, any small junction PD will probably not improve much. I could even think of reasons why an LED would be better: the junction potential is higher (wider bandgap semiconductor), so the capacitance is smaller than that of a similar size silicon part. PV efficiency is always crap with LEDs, but to measure a flash, that shouldn't be a big deal with the sheer excess of intensity available.
Tim
-
IR LED driving the virtual ground of a beefy opamp with very low gain ( try 100R at first and increase it if it is not a big enough pulse, but it will probably work) and you should get a good pulse. IR diode because it will be silicon only. Otherwise use a metal can transistor like a 2N2222 and take the top off to use the CE junction, though you will have capacitance of the case degrading the rise time, the base will be grounded.
The laser rangefinder used a different system with a large area silicon photodiode ( because it had a 20km range it needed the light capture) with 200v bias so that it would break down and avalanche with the pulse, used because this gave a very fast response to the initial edge of the pulse, and the recovery time was not important.
-
ahh, yea... lets leave that measurement for the moment then, i'll stick to making pictures :o
-
ahh, yea... lets leave that measurement for the moment then, i'll stick to making pictures :o
Well it would develop film directly, mostly by cooking a hole through it. That was the easy way to see if the optics were aligned by seeing spot size in the office on a test bench. Centre burnt and around it a dark developed blotch giving light levels.
Had a similar size capacitor bank and discharge methods using a large GDT.
-
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.
As a young lad I had a reasonably powerful quality but subsonic air rifle which was envied by my friends. One day by accident I discovered it could be made to "diesel". Now having fired center fire hunting rifles and also being fired at by the same* I am pretty sure the dieseling pellets were making a supersonic crack . Not healthy or safe for the pellet gun and hard to make happen consistently although I think some cold start (ethylene gas) sprayed in there would fix that. Don't try this kids!
*mushroom picking in Canada's north had some rounds make that whistling-crack just past my head.
-
i will buy an air gun of some sorts to play around with the flash, probably a .177, i will need something better than the 25mm ball bearing i'm dropping on stuff
been a small delay on the trigger project though, i ordered some 10 turn pots from china and one of them is faulty (lol, surprise!) so its not worth doing any updates at the moment, i want to build it up off the dev board in one go
-
i picked up this pulse light source a while ago and want to figure out why i get no flash out of it.
its a simple device with two air-gap spark flash devices inside with a capacitor bank.
would like some ideas on getting this working, i think the problem might be more mechanical than electronic but we will see!
Teardown: Palflash 500 - Nanosecond Light Source Part 1 (https://www.youtube.com/watch?v=O8viMG_Wlsc#ws)
That thing looks like it could have a lot of magic smoke trapped in there.
-
has been a while, but i finally made an update to the flash project.
You should expect to see some interesting pics soon now i have a controller that i can just pickup and use.
Project: Palflash Air Gap Flash Part 4 (https://www.youtube.com/watch?v=oeLwtrtExTw#ws)
-
Have been playing with my air-gap flash today, dropping a 25mm ball bearing onto a microscope slide.
Attached pictures are 1:1 crops
Next step is to make some kind of rig for the impact, and probably paint the enclosure so you can see more of the detail of the subject