<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