Current-driving a powerful IR-illuminator array

A simple current mirror might work for parallel/series strings of LEDs. You may need to thermally couple the tansistors to keep matching reasonable.

Don't take it too seriously, John. I've been killing threads for years. ;-P

Cheers! Rich

I saw it. Noted that it works... as is usual for your posts.

So I left it to the dweebs to ask dumb questions ;-)

...Jim Thompson

-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | |

Have I ever criticized your circuits other than professional-level questions?

...Jim Thompson

Nope.

...Jim Thompson

I used the idea with the op-amp and power transistor, and it worked fine! Of course, afterwards, I recognized the circuit, but it was buried in my brain since it was about 10 yrs ago I studied this.. The op-amp circuit has the advantage of reducing the dependence on Vbe and Rb*Ib, and temperature (within sane limits).

I downloaded Eagle, and drew a schematic and made a PCB. Really good program!

Now, for the next version of this, I'd like to reduce the number of LED chains by making some kind of step-up power supply which can take the

I wonder how high pulsed current these LED's can sustain. In the datasheet it only says max 500 mA pulses of 100 us duration. I need to do at least 1 ms duration, but there are no curves showing max current as a function of pulse-duration (I've seen those curves in other IR LED's datasheets). So I put it at 300 mA arbitrarily :) BTW anyone know any IR LED's that are more powerful than the Agilent HSDL-4230 ?

Regards,

Bjorn

I've done some tests with the HSDL-4400 recently. At a 1 kHz rep rate, on time = 2 us, current = 9 Amps, I ran the LED for 1.8 million pulses without problem. The same LED went through a battery of tests from

Interesting. So what you are saying is that if I hook up a sufficiently linear (or calibrated) photodetector, I could measure how the LED behaves when scanning over duty-cycles and/or currents, and where it starts going non-linear is probably where I don't want to be because it goes non-linear as an effect of heat buildup at the junction (that's what kills it right) ?

/Bjorn

Mark,

Has your E-mail address changed?

I sent you an Excel question but have seen no response.

...Jim Thompson

-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona Voice:(480)460-2350 | | | E-mail Address at Website Fax:(480)460-2142 | Brass Rat | |

Sent you an email with updated info. Should have been stable for the past 10 months.

Your main concern isn't really non-linearity, but overheating of the die. But yes, by monitoring the LED voltage, current, and light output you will come to some sort of conclusion of an appropriate drive level. Running the LED at currents over 6A you can see the light output sag with time after about 1 us. Since your using longer on-times, you need to operate the diode at a saner current level.

Seems my PCB worked, driving all 50 LEDs pulsed at the required current, and I didn't even get any fat jitter on the 12VDC feed thanks to the big honking capacitor I put to handle the excharge. Thanks guys for the hints now I have something to play with over easter :)

I did realize however that it's very clumsy to have to have 10 copies of the same circuit (opamp + resistor + power trans) so I was wondering if you who have suggested a higher voltage to run more LEDs in series have any hints on what to base a DC-DC converter on (say 12 VDC to

Of course, putting more LEDs in a single string means if one dies, the whole string is caput. But maybe that's not probable, what happens first to a LED with age ? Does it retain its v-i curve but gets less light-efficiency, or does it change v-i (potentially breaking the circuit) ?

Regards, Bjorn

Your main problem is being sent on a wild-goose chase by a well known newsgroup pest, troll, and ignorant pretentious idiot, "Rich Grease"- the "dreaded" this and that. You will notice the complete fake offered no further suggestions to handling your quandary as is usual with that kind. At 12V you end up with bunches of current source strings, each requiring a bunch of parts overhead and delivering poor accuracy.

John