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High-output, low-duty cycle LED strobe circuit

J

Jan Panteltje

Jan 1, 1970
0
'swhy I like I2C so much -- wiring up discrete chip select logic (not
to mention bus lines) is a PITA.

--mj

Yes, i2c is cool, I was one of the early adaptors...
Made the i2c external too, the i2c is cool as you can drive strings of logic
remotely from say a par port of a PC, while the task switching of the
OS has no effect, because basically i2c is static, and a delay makes no difference.
I had 50 units is a building with clock and sound all controlled by external i2c.
Designed and sold video equipment with i2c interface to PC running Linux too...
and I use it in my house for control.
For very long cables you need to draw up a driver :)

I have noticed the modern motherbourds still have a par port.. but no RS232...
So it is still very useful as interface to embedded too.
 
J

Jon Kirwan

Jan 1, 1970
0
<snip>
To try to get a bright enough flash, I got some 0.5W white LEDs that
can take a max DC forward current of 150 mA, and have about a ~3.6
forward drop, producing an intensity of 130k mcd. (Not too clear on
the mcd part.) And dem suckers is bright when you're pumping even 100
mA through them. Like squint-to-look-at-it bright. They're in a
standard 5mm package (T-1 3/4, what is up with that package name?),
though it's sturdier than most you've probably seen. The LED looks
like it's been lifting weights, and the leads are shorter and fatter.
Anyway, I have two 2n2222's hooked up as a Darlington, with +5 Vcc,
driving the front Q's base with ~15mA (with a microcontroller pin).
There's NO current-limiting resistor on the back end, where the second
transistor's collector is attached to +5v, and the emitter goes
through the LED to ground.
<snip>

Some thoughts about the 2N2222.  They probably can handle the pulses
of 1.5A for 200us and .4% duty (I am assuming, for now, that your
20-50 RPM is really 20 RPS = 200us/0.4%.)  However, the Rc, Re, and Rb
plus the 26mV*(1+ln(1+I/Is)) are going to suggest something on the
order of 1.9 to 2.0 volts at the base.  (I see Re=0.2, Rc=0.3, and
Rb=10 for one model I have, with a blind Is=1e-14, and I don't think
you can expect to get better than beta=30 here.)

  Vbe = 26e-3*(1+ln(1+(1.5/30)/1e-14))+1.5*(.2+.3)+(10+.2)*(1.5/30)

Which reads out at about 2V.

Since this is a darlington, I'm assuming something like this:

                             |
      about 2.7V             | <--- about 2.2V
              |     ,--------+
              v     |        |
          R1      |/c Q2     |
   ON----/\/\-----|   2N2222 |
                  |>e        |
                    |      |/c Q1
       about 2V --> '------|   2N2222
                           |>e
                             |
                             |
                            gnd

That's Q1, I'm talking about.  The Vbe of Q2 is about 0.7V and, if Q2
is considered saturated at about Vce=0.2V, I'd expect to see a Vce on
Q1 of 2.2V or so.  [Note that Q2's base is about 0.5V above Q2's
collector, so that is going to conduct a little (.2V/60mV, about 3
orders, or maybe 0.1% of the base current -- nothing to get excited
about.)]

With a 5V source above, that leaves about 2.8V for your LED.  Not the
3.6V you were talking about at 150mA.  And you want 1500mA, not 150mA!
So your circuit probably won't get there.

Here's the same circuit with 150mA as the estimate:

                             |
      about 1.45V            | <--- about 1V
              |     ,--------+
              v     |        |
          R1      |/c Q2     |
   ON----/\/\-----|   2N2222 |
                  |>e        |
                    |      |/c Q1
    about 0.85V --> '------|   2N2222
                           |>e
                             |
                             |
                            gnd

This provides about 4V compliance (not sure if you are using a
resistor on the collector leg) for your LED.  So at 150mA, it probably
has just about enough for your 3.6V requirement.

....

All this suggests to me that you aren't going to see a lot more than
150mA.  I don't have the LED model, but since it is white the LED is
blue with some phosphor used for the white appearances.  Guessing a
simple model has me taking about 2.8V as the minimum on-voltage (which
I get from (700nm/400nm)*1.6V, extrapolating from a red led) and a
model where R=(3.6-2.8)/150mA or in the area of 5 ohms.  Actually,
that seems too high to me, so that 2.8V estimate is probably wrong.
Let's assume it is closer to about 2 ohms or so (3.3V minimum.)  At
1.5A, that's still 3V all by itself.  Adding that to the 3.3V figure
gives you well more than 5V.  So again, a problem even assuming your
darlington arrangement could support a Q1 Vce of 0V, which it cannot.

Your darlington arrangement creates one problem.  Q1's Vce will be on
the order of 2V at those high currents and you don't have that kind of
headroom to spare.  Your LED itself creates another.  It's not likely
to allow 1.5A with only 5V of drive, under any circumstance.  These
are guesses, admittedly.  But what this seems to say is that you
probably need another voltage rail, at a minimum, if you want to get
up to 1.5A on your LED.

If you can live with less than that but still want a lot more than
150mA, the perhaps your LED will allow ... hmm ... say (4.4V-3.3V)/2
... or about 500mA, let's call it, assuming you can limit your BJT
switch to a Vce of 0.6V or less.  And no more than (4.8V-3.3V)/2 or
about 700mA.  I think that's the best you can hope for.  But I'd plan
no more than 500mA, after some testing first.  In this case, you can
keep your rail, maybe, but you have to lose the darlington.

Jon

Thanks, Jon that's a very helpful analysis.

No problem. I'm a hobbyist, though. So be warned. I'll probably
catch some flak from someone on the above. But I'll learn from it, if
I do. So that's fine.
I'll have to scope it out in the next couple of days.

It's not hard and it can be a great help in hand-computing a few
important things that will help you in a realistic design. One thing
is to drive different currents through the LED (assuming you have a
desk power supply that lets you set current limits and monitor
voltages, as I do) and list a few interesting data points on a sheet
of paper. From there, you can develop a very simple and workable
guess about the ON voltage and the intrinsic resistance (which isn't
realistic across all possible circuit design needs, but is very simple
for planning purposes and will cover your needs here.) I'll talk
about this model more, below. Also, it will tell you a lot about what
your 2N2222s are really doing for you.
And I'll consider a higher voltage rail
(that just means soldering that 7805 on the MCU board, where I'd left
it out before... oh, yeah, and probably decoupling caps if I'm
snapping off pulses like that...)

Well, I imagined you already had a 5V rail. A higher rail would be
more than that and the 7805 probably isn't right.
If 500-700 mA gives me the brightness I want, fine.

Well, I don't think it will. I'm guessing that you are already seeing
a little more than 150mA. Tripling that will definitely help, but as
our eyes are logarithmic against that change, so I'm betting you will
imagine more like about 30-40% brighter. Not 3X. You need to really
crank hard on the LED or else use a longer period of time for your
pulse -- but I think you already mentioned that causes its own
problems in your application.

I'm betting you need a higher rail. Question will be, will the LED
last long or stay as bright with this kind of abuse? There are a lot
of failure mechanisms, but you are probably looking at delivering a
couple of milli-Joules in a very short time. Optical output degrades
with higher pulsing -- for example, HP's book on LEDs suggests that
30% loss of brightness occurs when driving at 5X the rating for 1000
hours, but where less than 3% loss occurs driving at 1X for that long.
That's direct gap LEDs. The indirect gap LEDs are worse. There is
moisture creep into the package and if that vaporizes from some pulse,
it's not so good. Plus thermal fatigue due to different expansions of
lead frame, plastic, junction coating, die, bond wires, and so on.
Then there is annealing of metals, which is strongly temperature
dependent. And at higher temps, the plastic, which normally has a
fairly stable expansion coefficient, can see it's coefficient
amplified crossing over some unknown T. Chemical degradation... well,
the list goes on. In any case, if you really do manage to whack them
with 1.5A per pulse, you may see some significant reduction in overall
lifetime. Just keep it in mind, assuming the thing doesn't just blow
up, of course!
I'm not looking to
laser etch anything. I just want LEDs that aren't pitfully dim. Your
discussion gives me hope that I can get there.

Well, time will tell.

There are other considerations in terms of what humans perceive. A
lot of it is about contrast. I don't know exactly what your setup
looks like, or how it is expected to be used, but there often are a
lot of things you can do to improve the contrast and thus improve
visibility instead of relying only on hammering your LEDs with huge
currents. You might want to investigate those aspects before planning
on using an electronic jack-hammer on your poor LEDs.
I actually don't have a current-limiting resistor on the collector
leg.

I gathered that. I didn't include them in the schematic, either.
When occasionally a software glitch leaves the LED on (I switch
it off quick), it's bright. Once it even started going a bit green,
but I shut it down just in time. (That color change is unmistakable.)
And it gets hot after just a few seconds.

I bet. ;)
As for the LED analysis, I'm only slowly figuring out how to model
LEDs in my head; I'll go through your discussion above in detail when
I have time.

A very simplified LED model is:

V(I) = R*I + Von

You just need to supply Von and R. That's why I wrote 3.3V and 2
ohms, as one of my guesses. More realistic models will use the y-axis
cross point for the 'saturation' current (it's a theoretical offset,
usually not directly measured but instead taken using a straight line
slope from some set of measured points back to the y-axis current) and
an Rs value, a funky N factor (emission coefficient, I think I recall)
which shows up in the power of an expression using e, and a few other
values. But the above equation is often good enough for "government
work." It just says, you "need Von voltage at least to get anything
out and R*I more than that for any chosen I."
Does anyone here have a favorite resource or book about circuit design
with LEDs?

Yes. The one I keep handy is HP's old (old enough that they cover a
whole lot and don't expect you to run around finding other references,
yet not so old that it doesn't have a lot of very useful ideas that
remain useful today) "OPTOELECRONICS: FIBER-OPTICS APPLICATIONS
MANUAL," 2nd edition. Get it.
I've found a few good websites, but many of them are mostly
app notes for somebody's IC. I've had a great time with TLC5940s, but
they're overkill if all I want to do is flash.

Best of luck with all this.

Jon
 
N

Nobody

Jan 1, 1970
0
14 amps, well now that ought to do it. :{)

Can someone explain to me why a MOSFET would be better here than a
bipolar?

He wasn't suggesting using a MOSFET per se, but a MOSFET *driver*.

These are designed for supplying (and sinking) short pulses of high
current, in order to charge (and discharge) the gate capacitance quickly.
 
T

Tim Williams

Jan 1, 1970
0
555 timer could do it, but I have a load of those 12Fsomething PICs, and
no 555s, so why not use the PIC?

*NO* 555's!? How is that possible? Don't they like... give you a
lifetime supply with your degree or something? (Maybe they
should! :) )
I do not see what people have against PICs, programmable logic, you see
people here make huge complex circuits with 7400 series or even CD400 series,
or whatever, as solutions to some question.
Was it you who did a z80 setup to just display some 7 segment data while
a PIC<-- could do it faster cheaper and better?

'Twas I. But I have a load of those Z084something Z80s, and no PICs,
so why not use the Z80? ;-)

My other excuse is, this is actually my first fairly large
breadboarded digital project (no, I haven't already breadboarded an
IBM compatible or anything!). So I have the double excuse of
experience to use this. :)

As far as PLDs and such, they're fairly nice. I've used a MAX7k CPLD
before (just for pissy labwork stuff- VHDL 7 segment decoder and that
sort of thing), and it's certainly as programmable as it's supposed to
be. Looked it up on Digikey though- still going for 50 bucks each.
I'm not entirely sure just how much can fit inside a CPLD (I know it
has a good 60-some odd I/O pins, and a fairly good combinational logic
capacity, but not many flip-flops in this one), but for any of the
logic purposes I've developed so far, I can't at all imagine using one
instead of maybe 10-20 logic chips at a tenth the price. There's just
no comparison there. (No size / placement costs in homebrew,
remember!)

Tim
 
J

Jan Panteltje

Jan 1, 1970
0
*NO* 555's!? How is that possible? Don't they like... give you a
lifetime supply with your degree or something? (Maybe they
should! :) )


'Twas I. But I have a load of those Z084something Z80s, and no PICs,
so why not use the Z80? ;-)


Yes Z80 was, and is, a very nice processor to program,
and also simple to use from a hardware POV, with it's own I/O
chips..
I still have the old Z80 system I designed in the eighties, with my own CP/M clone
OS, but I think the EPROMs are dead, if some museum wants it, fine with me,
email me.

My other excuse is, this is actually my first fairly large
breadboarded digital project (no, I haven't already breadboarded an
IBM compatible or anything!). So I have the double excuse of
experience to use this. :)

Doing the Z80 thing got me into a big multi million project at that time.
The educational value far exceeds the cost of the parts :)

As far as PLDs and such, they're fairly nice. I've used a MAX7k CPLD
before (just for pissy labwork stuff- VHDL 7 segment decoder and that
sort of thing), and it's certainly as programmable as it's supposed to
be. Looked it up on Digikey though- still going for 50 bucks each.
I'm not entirely sure just how much can fit inside a CPLD (I know it
has a good 60-some odd I/O pins, and a fairly good combinational logic
capacity, but not many flip-flops in this one), but for any of the
logic purposes I've developed so far, I can't at all imagine using one
instead of maybe 10-20 logic chips at a tenth the price. There's just
no comparison there. (No size / placement costs in homebrew,
remember!)

Tim

Yes, for the hobbyist, maybe these days, even to avoid soldering very fine
pitch packages... if you like experimenting with digital design get
a FPGA board, less then 100$ will get you one.
No more breadboarding 150 TTL chips or so together....
You can then use that FPGA board for all sorts of things, I have one,
added FLASH, ADC, DA, LCD display, card reader, some other stuff...
You can use it as scope, video processor, frequency counter, anything you
want without much soldering.... simply by reprograming the FPGA.
ftp://panteltje.com/pub/2h/alles2.jpg
Now the 'breadboard' is just added to the digital logic via a more home brew
friendly header....
 
R

Rich Grise

Jan 1, 1970
0
Not yet. It's the beginning of May, for gosh sake. Finals week is
probably towards the beginning of June.
OK, then - mid-terms. ;-)

Cheers!
Rich
 
R

Rich Grise

Jan 1, 1970
0
Yes, i2c is cool, I was one of the early adaptors...

I once drove myself nuts trying to find an I2C "standard" - the best I can
figure is, you make up your own!

Cheers!
Rich
 
S

Spehro Pefhany

Jan 1, 1970
0
I once drove myself nuts trying to find an I2C "standard" - the best I can
figure is, you make up your own!

Cheers!
Rich

You're probably thinking of SPI. I2C is well defined.
 
J

Jon Kirwan

Jan 1, 1970
0
<snip>
I think you can get flash tube and a trigger transformer at radio shack for
a couple of dollars.

I suspect, but don't know, that there is a benefit to the dispersion
angle/lens of the LED that the OP may prefer. A flash tube is great,
brighter than all heck, but it may be hard to find one of the right
size and with the right reflector/baffles for it. Or maybe all this
is fine. I don't know. I like flash lamps a lot -- particularly
their ability to be triggered very reliably simultaneously. But I
wonder about this application. Plus getting that 300V charged back up
may be a minor bother for a row of these and at up to 50Hz.

Jon
 
J

Jon Kirwan

Jan 1, 1970
0
Firstly, I'd put a trickle through the LED at all times (maybe half a
milliamp) so the storage capacitance doesn't have to be reloaded each
flash.

Then, with a blocking diode, connect to a flyback switched inductor.
The current in the inductor builds, when it reaches Ipeak you
turn off the input, and the current continues to flow through the
only other connection, the LED with its blocking diode now forward
biased.

Just a quick, off-cuff thought. With an existing 5V rail, and my
expectation that a 1.5A pulse through the LED will require something
on the order of 7-8V total, we're talking about 2-3V across the
inductor while the LED is lit up. (In fact, let's assume 3V to start,
dropping to 2V.) I'm assuming for a moment that the Ron is about 3
ohms or so with a Vfwd of 3.3V. That suggests a dI=(3V-2V)/3 ohms or
(1/3)A. The dt is 200us. So the dI/dt, or V/L, is a little more than
1500. With a mean V of about 2.5V, this is on the order of 1.5mH.
Once the 200us has expired, it would be desirable to ramp up the lost
(1/3)A of dI. But over the much longer period of something on the
order of as long as 50ms (the OP mentioned 20Hz.) This suggests a V
across the L of about 10mV, to gradually get it back up there. (The
same as the 2.5V average times the .4% duty cycle.) The other option
is to just dissipate the energy, in between, I suppose. Or use an
even higher peak and just let it go flat yeilding an average of 1.5A.
Taps on the inductor will allow impedance matching to both the
charging supply and the LED. It's just about the same kind of
circuit as an old auto ignition.
<snip>

Could you expand a little on this? I have some very vague things in
mind, but I'm falling short and could use a few pointers to consider.

Jon
 
N

Nobody

Jan 1, 1970
0
Bank Switching is Evil.

Well every mainstream CPU architecture in existence does it.

Oh sure, they all hide it, performing the bank switching automatically
and pretending that you have "random access" memory, but underneath it all
you still have cache lines, RAS/CAS and even swap.
 
M

mj

Jan 1, 1970
0
I suspect, but don't know, that there is a benefit to the dispersion
angle/lens of the LED that the OP may prefer.  A flash tube is great,
brighter than all heck, but it may be hard to find one of the right
size and with the right reflector/baffles for it.  Or maybe all this
is fine.  I don't know.  I like flash lamps a lot -- particularly
their ability to be triggered very reliably simultaneously.  But I
wonder about this application.  Plus getting that 300V charged back up
may be a minor bother for a row of these and at up to 50Hz.

Jon

I used to work on instant cameras at Kodak (before the Polaroid
lawsuit that shut down Kodak's entire instant camera business.) We had
something called a "quench flash". The shutter would trigger the flash
via an SCR, illuminating the scene. In the camera was a photodiode
that charged a capacitor. When enough reflected light had come back
from the scene (that is, when the sense cap had charged to a certain
point), a comparator would trigger another charged cap to apply a
negative-going spike to the SCR, shutting off the flash immediately
and maintaining the remaining charge on the cap. So you got just
enough light for good exposure, but charge recovery time was greatly
improved because the flash cap only partially discharged.

The original design wasn't mine, but I understood how it worked at the
time. Alas not well enough that I could repeat the idea. Maybe some
day. So a big fat cap and an array of SCRs could probably do it, but
not by me.
 
C

Charlie E.

Jan 1, 1970
0
You're probably thinking of SPI. I2C is well defined.

Well, I spent two months trying to talk to a I2C color sensor, before
finally giving up on it! Found one major bug in the PIC I2C
libraries, and the sensor kept giving nonsense data back...

Charlie
 
T

Tim Williams

Jan 1, 1970
0
When enough reflected light had come back
from the scene (that is, when the sense cap had charged to a certain
point), a comparator would trigger another charged cap to apply a
negative-going spike to the SCR, shutting off the flash immediately
and maintaining the remaining charge on the cap. So you got just
enough light for good exposure, but charge recovery time was greatly
improved because the flash cap only partially discharged.

The original design wasn't mine, but I understood how it worked at the
time. Alas not well enough that I could repeat the idea. Maybe some
day. So a big fat cap and an array of SCRs could probably do it, but
not by me.

They do that with IGBTs nowadays. In fact, I have some parts to a
Canon S1 IS laying right here. The unusually small 220uF 330V
electrolytic is wired to a circuit board with some fairly small parts,
including a small ferrite transformer, something SOT-223, an axial
diode shaped like a six amp axial rectifier but a good four times
smaller, and on the back side, an array of ceramic capacitors, a
diode, SOT-8 and I'm not sure what else (where'd I leave that
screwdriver...). The SOT-8 is the IGBT, rated for something like
400V, 8A average, 150A peak. It drops 1V at 8A, I don't think I'd
even like to put that much on the thing.
http://www.iele.polsl.pl/elenota/Toshiba/en_gt8g132_20030219_datasheet.pdf

Tim
 
D

Don Klipstein

Jan 1, 1970
0
Firstly, I'd put a trickle through the LED at all times (maybe half a
milliamp)
so the storage capacitance doesn't have to be reloaded each flash.
Then, with a blocking diode, connect to a flyback switched inductor.
The current in the inductor builds, when it reaches Ipeak you
turn off the input, and the current continues to flow through the
only other connection, the LED with its blocking diode now forward
biased.

Taps on the inductor will allow impedance matching to both the
charging supply and the LED. It's just about the same kind of
circuit as an old auto ignition.

That's how the old flashlamp strobes worked; of course, the dynamic
range of a discharge lamp allows lower duty cycles, at really
large peak currents.

I think you can get flash tube and a trigger transformer at radio shack for
a couple of dollars.

It appears to me that Rat Shack most recently sold trigger transformers
sometime in the 1980's. My memory is that they did not sell such in the
mid 1990's or ever since.

Maybe still available as "special order item" RSU-11996667, maybe was as
recently as mid-late 1990's or 2001 or so, but the search box in their
website can't find for me anything RSU-11996667 now.

Mouser Electronics appears to me to be a source of these at least as
recently as late 1990's. Electronic Goldmine sold them at least as
recently as mid or late 1990's.

I have a web page on xenon strobe parts suppliers, which may be out of
date due to latest update as of this posting being in 2001:

http://members.misty.com/don/flashsrc.html

I also mention how to build one, should you be unable to get one or hack
a usable one out of something that is cheap and has one:

http://members.misty.com/don/trigcoil.html

- Don Klipstein ([email protected])
 
S

Spehro Pefhany

Jan 1, 1970
0
Well, I spent two months trying to talk to a I2C color sensor, before
finally giving up on it! Found one major bug in the PIC I2C
libraries, and the sensor kept giving nonsense data back...

Charlie

Hey, I didn't say everyone implements the standard* perfectly.

The PIC series has a long-standing hardware bug in slave mode.
Libraries are often imperfect.

Master mode is usually not a problem, and starting with bit-banged
master mode usually works. Hardware devices such as I2C SEEPROMs are
done properly, IME. Not surprisingly, NXP (nee Philips) micros are
quite solid in I2C support.

SPI, OTOH, is a dog's breakfast.

* http://www.nxp.com/acrobat_download/literature/9398/39340011.pdf


Best regards,
Spehro Pefhany
 
B

Bob Larter

Jan 1, 1970
0
whit3rd said:
Firstly, I'd put a trickle through the LED at all times (maybe half a
milliamp)

That's enough to dimly light up a really high efficiency LED.
 
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