For a project I'm currently working on, we're trying to drive 940nm IR LEDs (Kingbright L-934-F3C) using PWM, regulated by an Arduino Uno. We've run into several issues during our prototyping process and could use some help!
We want to illuminate a 50 x 70 cm area from above using these LEDs, to be able to take a photo with an IR-sensitive camera when it's dark. There will be several objects located on this area that need to be lighted. We'd like to use as few LEDs as possible, so our reasoning is to pulse these LEDs at their maximum current while the camera is taking the picture, to achieve maximum illumination.
The Kingbright datasheet specifies an peak forward current of 1.2A, when a duty cycle of 1% and pulse width of 10μs are used. However, it doesn't say at what voltage (the forward current vs forward voltage graph doesn't extend that far). This peak current value doesn't seem to jive with the max. specified voltage of 1.6V (according to the graph, this happens at 50 mA).
(1) Are LEDs able to take massive amounts of voltage when pulsed at such short times, as long as the peak current stays below the limit?
We currently use a Velleman current & voltage source 30V/3A(http://www.velleman.eu/products/view/?id=420136). We hooked it up to one IR LED using the PWM settings as described in the datasheet and set the current source to 1.2A. The LED instantly blew up as the voltage rose to 30V. When used as a voltage source, further experimenting found that above 15V, the LED did not get appreciably brighter. At 15V, the single LED only appears to draw 40 mA (according to the display on the source).
(2) As these voltage and current figures do not match the datasheet of the LED, perhaps the lab source doesn't display the correct current flow. How can we accurately measure this?
(3) What's the longevity of the LED when you operate it at the limit? Does it have enough time in between pulses to dissipate the heat that is generated when running at 1%?
We found that a single LED gave a fairly disappointing amount of light. We're thinking of hooking up around 15 parallel strings of 2 LEDs connected in series. If we were to put 30V into the array (2*15V), assuming that the LEDs can draw a peak forward current of 1.2A, the array would draw a current of 18A.
(4) Is it possible to get a peak current of 18A @ 1% duty cycle out of a 3A source without blowing it up?
Lastly, we're very interested to see other peoples' solutions for this kind of build. I have the feeling that PWM is mostly used to 'dim' LEDs instead of being able to overdrive them to the max, but I might be mistaken. We'd love to hear your opinion on this!
Edit June 25th
Thanks for the responses! I will try to give some background information. As you might have guessed, we're not really electronics ninjas, but that's exactly why we're asking!
The lab source is a Velleman LABPS3003SM (it's strange that the link isn't working for you, WhatRoughBeast, it works over here!)
Responding to the question of CL.: we're trying to pulse the LEDs at the 1% duty cycle for the duration of a single snapshot of the camera, at 3fps. We're homing in on the actual moment that the photo is taken but this is proving difficult. In the final product, it would be great if we would be able to only pulse the LEDs exactly when the picture is taken.
Responding to the questions of mkeith: unfortunately, we do not have an oscilloscope. I'm don't have access to our prototype right now, but the PWM circuit consists of an IRF540N, with the gate hooked up to the Arduino (5V) and the source directly attached to the negative lead of the LED, no resistors.
The source was set as a voltage source, no current limit. This appeared to work when just a single LED was hooked up at 15V. When we tried to hook up 2 LEDs in series (and the voltage source set to 30V), the first of the LEDs in the series blew up.