Interupting xenon flash current ?

[John Larkin]

I wonder how I might calibrate such a homebrew current transformer?

1 ohm divided by 100 gives 10 mV/amp.

Shouldn't I see some serious ringing in the current waveform if my shunt is that inductive?

No. Typically the error results in a current step having a substantial
overshoot on the leading edge of the sensed voltage, with a
complex-exponential decay tau more or less in the few hundred
microsecond range. The complex decay is an infinite sum of
exponentials as the magnetic fields soak into the shunt matarial,
eddy-current wise. It's one of those ugly diffusion things. It's hard
to make shunts or heatsunk power/wirewound resistors that have clean
transient response. Vishay makes some super-precise metal foil power
resistors, 4-wire in a TO-3 can, that have 100% overshoot on a current
step, decay in the 500 usec range. I was impressed; then I started
making my own shunts.

If you tuck the sense wires in close to the shunt intill they meet in
the middle, and then bring them out as a twisted pair, that minimizes
the effect, but most shunt geometries limit the improvement. There's
also the common-mode/ground-loop issue.

John

 

[John Larkin]

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.

The flashtube is roughly a constant-voltage device once fired, with
maybe a bit of negative resistance. So there is energy lost in the cap
esr and the wiring, and maybe adding the igbt just moves the losses
around and spreads out the pulse.

I thought igbt's latched on at high currents, but obviously the series
guy turns off.

John

 

[Spehro Pefhany]

The flashtube is roughly a constant-voltage device once fired, with
maybe a bit of negative resistance. So there is energy lost in the cap
esr and the wiring, and maybe adding the igbt just moves the losses
around and spreads out the pulse.

I thought igbt's latched on at high currents, but obviously the series
guy turns off.

John

There are "short circuit" rated IGBTs that can take hundreds of volts
while full on for a few microseconds.



Best regards,
Spehro Pefhany

 

[Robert Baer]

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.

On SCRs and other devices made for hogh current and relatively fast
circuits like this, one should know the I*I*T rating; something that
used to be published about 30 years ago and "vaporized" maybe 5 years after.
I guess that the I*I*T rating of that spec was low...

 

[Michael]

This is a very low inductance shunt. It consists of 2 straps of resistance material about 1" wide and 5" long
stacked on one above the other in brass blocks with Kelvin sense terminals. I just measured the inductance to
be 0.2 to 0.4uH. It wants to drift around a bit. I have used this same shunt to measure a 3000A pulse in an
impulse magnitizer that I built, but that was a much longer pulse. Somewhere around 8ms if I remember
correctly.
Yes the shunt is located between the capacitor neg terminal and the flash tube. The scope gnd, the charge
power neg, capacitor neg, and shunt are all connected to the same node. I connected the scope probe tip to the
that same node and the scope showed zero voltage.

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

5" long shunt????? I bet you are measuring L*dI/dt. 0.3uH with ~1kA/
less_than_1us will easily give you more than 1V. I'd use bunch of chip
resistors in parallel (e.g.: http://search.digikey.com/scripts/DkSearch/dksus.dll?Detail?name=P5.0TCT-ND)
or current transformer. Layout is not a trivial thing when you are
dealing with the stuff like this.
Just my $0.02
)

 

[Winfield Hill]

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.

There are "short circuit" rated IGBTs that can take hundreds
of volts while full on for a few microseconds.

However they are rated, most IGBTs, in my experience,
simply current limit, much like MOSFETs, at very
high currents. I'm not aware that they latch up.
But, given the typically rather high limit currents,
if the voltage drop is very high and lasts for long,
destructive die temperatures can occur. Perhaps
something appearing like a latch-up can be part of
the high die-temperature process. For my high-
current IGBT designs, I've used gate drivers that
include a Vce over-voltage-detection gate-shutoff
circuit, to protect the IGBT. Once I disconnected
that circuit, because it was tripping, improperly
I thought, but then a huge IGBT module I was using
immediately failed. It turned out an insulation-
failure short in the output cable was responsible.
I promptly reconnected the fault shutoff and only
then replaced the IGBT. The shutoff will remain.

 

[[email protected]]

Fairchild Semi has this nice app note on IGBT in series with the tube
and a 555 timer test circuit for the pulse width

http://www.fairchildsemi.com/an/AN/AN-9006.pdf

then there is this piece of marketing drivel from power electronics
magazine

http://powerelectronics.com/mag/407PET06.pdf

MIT thesis

http://via.mit.edu/documents/negrete_2004.pdf

enjoy,

I do xenon as part of my livelihood at the University.

Steve the strobemeister.

 

[Glen Walpert]

Fairchild Semi has this nice app note on IGBT in series with the tube
and a 555 timer test circuit for the pulse width

http://www.fairchildsemi.com/an/AN/AN-9006.pdf

then there is this piece of marketing drivel from power electronics
magazine

http://powerelectronics.com/mag/407PET06.pdf

MIT thesis

http://via.mit.edu/documents/negrete_2004.pdf

enjoy,

I do xenon as part of my livelihood at the University.

Steve the strobemeister.

It seems that all of the IGBTs designed for strobe use are limited to
150 Apeak, suitable only for small on-camera type flash tubes under 20
Joules, and I see the apparently typical CY25BAH-8F flash IGBT is
rated for a 5000 flash lifetime.

I just hauled a big flash unit from a ~1970 copier breadboard in from
the barn to see how they did it. This flash was designed to fire
around 5 times per sec continuosly 24/7 for about a year at around 10
or 20 J per flash IIRC; it has a 5" long 1/2" dia tube in a fan cooled
lamp housing and a 19" rack mount power supply with a big
ferroresonant transformer and bridge rectifier charging a 4" x 4" x 7"
capacitor with inacessible labeling. The capacitor does not discharge
very much per flash. In series with the tube is a 0.1 ohm wirewound
power resistor with 27 turns of edge wound metal strip 1.25" dia 7"
long, I calculate about 4 uH. This resistor drops enough voltage to
quench the arc very quickly after it fires, but without any adjustment
posssible.

I might be able to fire it up and measure the flash current sometime.

Hey John, do you have a core material recommendation for that 100:1 CT
idea?

Regards,
Glen

 

[Mike]

It seems that all of the IGBTs designed for strobe use are limited to
150 Apeak, suitable only for small on-camera type flash tubes under 20
Joules, and I see the apparently typical CY25BAH-8F flash IGBT is
rated for a 5000 flash lifetime.

I noticed that to.
Even in the TO247 pkg it's hard to find one good for more than 300A.

I just hauled a big flash unit from a ~1970 copier breadboard in from
the barn to see how they did it. This flash was designed to fire
around 5 times per sec continuosly 24/7 for about a year at around 10
or 20 J per flash IIRC; it has a 5" long 1/2" dia tube in a fan cooled
lamp housing and a 19" rack mount power supply with a big
ferroresonant transformer and bridge rectifier charging a 4" x 4" x 7"
capacitor with inacessible labeling. The capacitor does not discharge
very much per flash. In series with the tube is a 0.1 ohm wirewound
power resistor with 27 turns of edge wound metal strip 1.25" dia 7"
long, I calculate about 4 uH. This resistor drops enough voltage to
quench the arc very quickly after it fires, but without any adjustment
posssible.

I might be able to fire it up and measure the flash current sometime.

If you do, please post your results here. I'd sure like to know.

Hey John, do you have a core material recommendation for that 100:1 CT
idea?

Regards,
Glen

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

 

[Mike]

1-turn primary, maybe 100 turn secondary on a ferrite toroid, 1 ohm or
less non-inductive burden resistor. We've done this in the 100 amp, 50
ns sort of range, on a node that kicks a kilovolt, and it worked well.

John

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.

 

[Tim Williams]

35A --> 175mA in the winding. It's probably made with fine wire (around
28AWG).

A 200 ohm resistor sounds absurdly high.

What V.s is it rated for?

Tim

 

[Winfield]

It seems that all of the IGBTs designed for strobe use are limited to
150 Apeak, suitable only for small on-camera type flash tubes under 20
Joules, and I see the apparently typical CY25BAH-8F flash IGBT is
rated for a 5000 flash lifetime.

No, they go to 10kA and higher. My circuit went
to 1.5kA and 1.2kV = 1.8MW. Look for IGBT modules
made by Powerex, Mitsubishi, Eupec, ABB and others.

 

[JosephKK]

Jim Thompson [email protected] posted to
sci.electronics.design:

I did that in the '60's, but used an SCR, read my patent at...

http://analog-innovations.com/SED/Pat-3496411.pdf

...Jim Thompson

So basically you are commutating the flashtube with the SCR instead of
another SCR?

 

[JosephKK]

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

That was my frst reaction also since I expected to see something
less than 400A, but I can't find any reason for such a major error.
I have a 200mv 200A shunt in series with the tube and am simply
grabbing the voltage waveform across the shunt with an HP 54502A
storage scope. So with 1mv/A and reading 1.9v peak that's 1900A. I
connected the 1V scope calibrator output from an analog scope to
54502s input and it reads right on amplitude and time.

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

Mike. Re: your sig, see Godel's theorem. But don't give up it is
just a theorem.

 

[JosephKK]

Spehro Pefhany [email protected] posted to
sci.electronics.design:

Well, if that's what the 'scope shows, it's *got* to be true.


Best regards,
Spehro Pefhany

My tongue would hurt after that.

 

[JosephKK]

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

I wonder how I might calibrate such a homebrew current transformer?
Shouldn't I see some serious ringing in the current waveform if my
shunt is that inductive?

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

There is no calibration needed for a current transformer, just the
terminating resistor. Just use big enough core to not have a
saturation problem. A company called "Ion Physics" makes them to
handle many kA in sub microsecond square topped pulses. The rest of
the instrumentation must be up to the task as well.

 

[JosephKK]

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

Maybe they didn't exist?

The older controlled-flash units had two flashtubes, one to actually
flash, and a second shunt tube, inside, that shorted out the cap
when enough light had been generated.

John

No, they commutated the flashtube with an SCR. Google SCR
commutation.

 

[John Larkin]

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


There is no calibration needed for a current transformer, just the
terminating resistor. Just use big enough core to not have a
saturation problem. A company called "Ion Physics" makes them to
handle many kA in sub microsecond square topped pulses. The rest of
the instrumentation must be up to the task as well.

Pearson makes CT's that work over a million:1 frequency range, down to
0.15 Hz. Beautiful stuff.


http://pearsonelectronics.com/


John

 

[John Larkin]

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


No, they commutated the flashtube with an SCR. Google SCR
commutation.

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

John

 

[John Larkin]

Fairchild Semi has this nice app note on IGBT in series with the tube
and a 555 timer test circuit for the pulse width

http://www.fairchildsemi.com/an/AN/AN-9006.pdf

then there is this piece of marketing drivel from power electronics
magazine

http://powerelectronics.com/mag/407PET06.pdf

That mag is pitiful. I may rant on it one day soon.


John