Interupting xenon flash current ?

[Mike]

Oil or film caps have low esr, and some can be arranged for low esl.
Keeping inductance down, in the caps and wiring, is important.

Yeah, have to build it like a vhf amp.
The only cap like that that I is 100uf 1KV.

Anybody know the speed of the xenon itself?

I suspect that depends on the tube geometry, fill pressure etc.
There are xenon flashes available (EG&G 549-11 Microflash) that achive 0.5us flash duration.

Perkin Elmer has some info on their site.
for example
http://optoelectronics.perkinelmer.com/content/ApplicationNotes/APP_PFL.pdf

I built a xenon/pmt lidar when I was a kid, but the light flashes were
circa 10 usec, and the optics were terrible, so it didn't work very
well.

John

When truth is absent politics will fill the gap.

 

[GregS]

Yeah, have to build it like a vhf amp.
The only cap like that that I is 100uf 1KV.


I suspect that depends on the tube geometry, fill pressure etc.
There are xenon flashes available (EG&G 549-11 Microflash) that achive 0.5us
flash duration.

Perkin Elmer has some info on their site.
for example
http://optoelectronics.perkinelmer.com/content/ApplicationNotes/APP_PFL.pdf



When truth is absent politics will fill the gap.

My old Hnneywell strobe said it was about 1/50000 sec. it stopped
the flash by discharging the cap.

greg

 

[John Larkin]

After I get everything mounted and shorten all the wiring as much as possible.
I'll wind the transformer and see what happens. I went thru some junk and found
a current sense transformer. The data sheet with it says it's 1:200 and to use
a 200ohm resistor. It also says 35A max. It has large hole in the center to
run a wire thru. Any idea why it is limited to 35A? I don't want to run into
that problem when I make my own transformer.

It's probably laminated steel, with a lot of winding capacitance, too
slow for this measurement; but you could test it.

John

 

[Glen Walpert]

Yeah, have to build it like a vhf amp.
The only cap like that that I is 100uf 1KV.


I suspect that depends on the tube geometry, fill pressure etc.
There are xenon flashes available (EG&G 549-11 Microflash) that achive 0.5us flash duration.

The EG&G Microflash seems to be out of production, but
http://www.optikon.ca/palflash.optikon.pdf
claims 250 nS pulse width and up to 4 pulses at 1 uS intervals.

Perkin Elmer has some info on their site.
for example
http://optoelectronics.perkinelmer.com/content/ApplicationNotes/APP_PFL.pdf

Thanks, I see I have an electrode stabilized lamp (arc length / bore
dia ~ 2, which should be good for short duration flashes), and a
resonant discharge power supply.

 

[Tim Williams]

My wife worked for Edgerton while I was a student at M.I.T.

"Doc" used to have monthly steak fries in his lab, using an
old-fashioned galvanized wash tub as the charcoal grill. And he'd
play his guitar and sing ;-)

Ain't he from Wisconsin? ;-)

Tim

 

[Tim Williams]

Oh, something else comes to mind: xenon-filled thyratrons are in the
microsecond range ionization time, about 1ms deionization. These devices
are at lower pressure than flash tubes, giving a diffuse blue to white glow
discharge rather than an arc. Probably, a flash tube is somewhat faster,
due to the higher pressure bringing the plasma bric-a-brack together
faster. I don't know what cool-down time is, but if you feel adventurous,
you could calculate it from the black-body radiation formula and an
assumption of peak temperature, mass of the plasma and its heat capacity.

Tim

 

[Mike]

Thanks, I see I have an electrode stabilized lamp (arc length / bore
dia ~ 2, which should be good for short duration flashes), and a
resonant discharge power supply.

Unfortunately mine has an e/r of about 8, not so good for short flashes.

Mike

When truth is absent politics will fill the gap.

 

[JosephKK]

Mike [email protected] posted to sci.electronics.design:

After I get everything mounted and shorten all the wiring as much as
possible. I'll wind the transformer and see what happens. I went
thru some junk and found a current sense transformer. The data sheet
with it says it's 1:200 and to use a 200ohm resistor. It also says
35A max. It has large hole in the center to run a wire thru. Any
idea why it is limited to 35A? I don't want to run into that problem
when I make my own transformer.

Mike



If there is no absolute truth then nothing can be known.

That is not going to work. You need a much bigger (or lower u(r))
core. Core saturation will trash everything.

 

[JosephKK]

John Larkin [email protected] posted to
sci.electronics.design:

Yes, I've seen units that used a second flashtube.

John

OK some foolishly did. Probably to avoid a patent issue. Oh well,
all the original ones are expired now.

 

[JosephKK]

GregS [email protected] posted to sci.electronics.design:

My old Hnneywell strobe said it was about 1/50000 sec. it stopped
the flash by discharging the cap.

greg

Was that a 5600?

 

[JosephKK]

John Larkin [email protected] posted to
sci.electronics.design:

In a typical xenon strobe, roughly how much of the cap electrical
energy becomes light?

John

Guestimate about 2 to 3 percent.

 

[GregS]

GregS [email protected] posted to sci.electronics.design:


Was that a 5600?

I would have to look at it.

greg

 

[Don Klipstein]

I always tought a SCR, not an IGBT, was fired parallel to the flash tube.
And the flash tube is connected via a small inductor to the cap,
the inductor limiting the current rise.
But it has been at least 20 years ago I looked....
Anyways SCRs can have huge peak currents.

That inductor is sometimes used, mainly to shape the current pulse to
reduce peak current and make the pulse closer to a square (more constant
in current) than it would be without the inductor. They do that in laser
pump strobes and elsewhere that stress on the flashlamp is greater.

I have picked into plenty of photographic flashes, including a couple
larger ones, and I never saw that inductor being used there.

DISCLAIMER - I only dissected photographic flashes without means to
adjust the flash intensity.

- Don Klipstein ([email protected])

 

[Don Klipstein]

I believe there is a mistake with your measurement. In the common
cameras, the flash current is at the order of 100A and the duration is
~tens of milliseconds. They handle it with IGBT of SOT-223 form factor.

I have used photodiodes and oscilloscopes on camera flashes, and found
flash duration to be more in the 1 millisecond ballpark. Sometimes a few
milliseconds. Some very large ones I suspect to get into the 10
millisecond ballpark; I have yet to do that measurement.

- Don Klipstein ([email protected])

 

[JosephKK]

GregS [email protected] posted to sci.electronics.design:

I would have to look at it.

greg

If (when) you do please post back here. I think i would like to have
one. I am not comfortable withe Eprey.

 

[Don Klipstein]

They show a flash-tube current of 60A peak (with
a short little extra bit to 80A) decaying with an
approximately 70us time constant. They suggest a
CY25BAH-8F IGBT, which is a 400V "150A" Renesas
part in a tiny tssop-8 package.

Not that I looked at the datasheet, but a pulse with a peak of 60 amps
and decaying exponentially with a 70 microsecond time constant has a
charge of only 5.6 millicoulombs, doesn't it?

Is not energy stored in a capacitor half the charge times the voltage?

In my experience, most small to smallish medium photoflash capacitors
are rated 330 volts. 1/2 times .0056 times 330 is is about .92 joule.
If I should have used 80 amps rather than 60, then the energy is about
1.23 joules.

That sounds like an awfully small photoflash to me. I have seen about
3-4 joules in a couple digital cameras, 6-8.5 joules in 1-time-use
cameras, and generally even more in other photoflash units.

- Don Klipstein ([email protected])

 

[Don Klipstein]

Yes, if we assume Vcap starting at 275 volts, and
60 volts across the tube, about 3.5 ohms of flash-
capacitor esr should do it. Hmm, I'd expect a 22uF
photoflash capacitor to have lower esr than that.

In those "disposable" cameras, the capacitor is 110 to 160 microfarads
and usually charges to about 300 volts or slightly more. In photoflash
units better than those, the capacitor is even larger.

I have measured the voltage across the flashtube exceeding 250 volts
at the beginning of the main somewhat-exponential decay of capacitor
voltage when the flashtube starts discharging the capacitor.

- Don Klipstein ([email protected])

 

[Don Klipstein]

Naa, The fools said I was crook and threw me off. Funny thing is I wasn't
even selling anything. Trust me, don't try to talk to them either!

Anyway, Those really big IGBTs seem a little on the slow side to shut off
the tube current down in the 1us range.

I think 10 or even 20 us range should be just fine! Flash duration is
usually in the 1 millisecond ballpark!

- Don Klipstein ([email protected])

 

[Don Klipstein]

You don't need an igbt, and they're slow anyhow. Just pick a capacitor
size, and arrange the parasitics, to dump the energy in less than a
microsecond.

I don't know how fast the xenon will quit making light after the
current is gone.

It appears to me that this would be around 50-100 microseconds in usual
smaller photoflash tubes.

I have made a xenon strobe with entire flash duration not much over 50
microseconds. Then again, I had the xenon plasma running hotter than
usual for a photoflash in that one.

- Don Klipstein ([email protected])

 

[Tim Williams]

That inductor is sometimes used, mainly to shape the current pulse to
reduce peak current and make the pulse closer to a square (more constant
in current) than it would be without the inductor. They do that in laser
pump strobes and elsewhere that stress on the flashlamp is greater.

I have picked into plenty of photographic flashes, including a couple
larger ones, and I never saw that inductor being used there.

I've seen exactly one so far. Was a stack of copper turns, bank wound air
core about 24AWG. In series between the cap and tube.

Nothing to do with the SCR, which discharges a film cap into the trigger
transformer.

Tim