Hello everyone. I created an old thread that describes some of what I am doing http://forum.allaboutcircuits.com/showthread.php?t=36791. Now my problem is understanding why my LEDs seem to randomly not turn on when I driving them with a larger current? I know this is a very long post, but I wanted to include everything that may be relevant to why this is happening. Also I am at a complete loss on what is causing this problem, so I would be very grateful for any suggestions that might be causing this!
First let me remind everyone what I am trying to do. I have ~40 NIR LEDs that I need to rapidly turn on/off individually (i.e. only one LED is on at a time), and I take an image only when the individual LEDs are on. I use a digital output from a USB data acquisition board to "switch" a reed relay that completes the LED circuit allowing a specific LED to turn on/off. This digital output signal is also used to send a trigger signal to the CCD camera, so it only exposes an image when the signal is "high". The 40 LED image sequence needs to be completed in the shortest amount of time possible. Therefore, I use very short pulses to turn the LEDs on/off (e.g. ~5ms). The NIR LEDs I use can be pulsed up to 1A for very short times (see the attached L850-40series.pdf from last thread).
During the previous thread I took some good advice from Bill Marsden and others and I decided to just use a single resistor and a 5V power supply (see http://www.powerconversion.com/assets/ssl20c_1191534424.pdf for a description of the 5V power supply). During that time I ran tests to verify that the LEDs performance was not changing over time. During the test I turned on/off an individual LED many times (e.g. 500) and I recorded the average pixel intensity for each of the 500 images. The idea was that if resistor was too small (i.e. current too high) it would damage the LED and it would degrade in performance over the 500 images, and it might not come back to the original intensity. From my tests I decided that a resistor of 5.6 ohms was best. With this resistor I should have a current of 5V-2.5V=I*5.6ohms => I~450mA. At that time with this resistor I showed that that the LED brightness degraded over the 500 images (not significant though see image in last thread), and the when I repeated the test the pixel intensity came back to the original value so it wasn't permanently damaging the LEDs. Also from the manufactures data sheet (see attached in last thread) it was clear that it is safe to do this as long as duty cycle was taken into account.
This brings me to my current dilemma on why higher currents seem to cause the LEDs to not turn on sometimes... Even before when I posted the old thread I noticed that if I used a smaller resistor (3.3 ohms) sometimes the individual LED would not turn on. At the time I didn't give it much thought because I planned on using the larger 5.6 ohm resistor, because I didn't want the current that high anyways. Since then I built a prototype device with a customized circuit board that takes the sequence of ~40 individual LEDs. Now I noticed that this problem seems to occur more frequently, and it occurs more often with some of the LEDs? Although when I use a very large resistor (e.g. 100 ohm) the LEDs seem to always turn on when they are supposed to (see attached image). I am attaching 4 images that illustrate what I mean. 2 of them used a 100 ohm resistor and the other two used a 5.6 ohm resistor, and I tried it with 2 different LEDs. The only difference is that I replaced the resistor! Why does the LED sometimes not turn on when I use the smaller resistor (larger current), but everything seems to work just fine with the larger resistor (smaller current). You can see when the LED is not on by looking at the figures when the average pixel intensity is ~ 37 counts.
Some of the ideas that I have that might be causing the problems include: 1) I need a different power supply, but the current can output up to 4A and it is 5V (see http://www.powerconversion.com/assets/ssl20c_1191534424.pdf for a description of the 5V power supply). I also use the 5V power supply to power some IC chips (transistor arrays, OR logic gates, etc., but I don't think they take too much current especially up to 4A? 2) For some reason when I created the customized circuit/ prototype there are some type of cross talk because the higher currents lead to higher electrometric fields? 3) The reed relays that I use might have a problem (part number HE3621A0510, and spec sheet is located at http://www.hamlin.com/specsheets/HE3600 revised.pdf). 4) For some reason the LEDs themselves might be the problem, but from my understanding they should be fine with a larger current (please see attached PDF spec sheet), and if they are overheating because of the larger current why is it so random?
Thanks again for any suggestions/comments on what might be causing the LEDs to not turn on sometimes when I use more current to drive them.
Kevin
First let me remind everyone what I am trying to do. I have ~40 NIR LEDs that I need to rapidly turn on/off individually (i.e. only one LED is on at a time), and I take an image only when the individual LEDs are on. I use a digital output from a USB data acquisition board to "switch" a reed relay that completes the LED circuit allowing a specific LED to turn on/off. This digital output signal is also used to send a trigger signal to the CCD camera, so it only exposes an image when the signal is "high". The 40 LED image sequence needs to be completed in the shortest amount of time possible. Therefore, I use very short pulses to turn the LEDs on/off (e.g. ~5ms). The NIR LEDs I use can be pulsed up to 1A for very short times (see the attached L850-40series.pdf from last thread).
During the previous thread I took some good advice from Bill Marsden and others and I decided to just use a single resistor and a 5V power supply (see http://www.powerconversion.com/assets/ssl20c_1191534424.pdf for a description of the 5V power supply). During that time I ran tests to verify that the LEDs performance was not changing over time. During the test I turned on/off an individual LED many times (e.g. 500) and I recorded the average pixel intensity for each of the 500 images. The idea was that if resistor was too small (i.e. current too high) it would damage the LED and it would degrade in performance over the 500 images, and it might not come back to the original intensity. From my tests I decided that a resistor of 5.6 ohms was best. With this resistor I should have a current of 5V-2.5V=I*5.6ohms => I~450mA. At that time with this resistor I showed that that the LED brightness degraded over the 500 images (not significant though see image in last thread), and the when I repeated the test the pixel intensity came back to the original value so it wasn't permanently damaging the LEDs. Also from the manufactures data sheet (see attached in last thread) it was clear that it is safe to do this as long as duty cycle was taken into account.
This brings me to my current dilemma on why higher currents seem to cause the LEDs to not turn on sometimes... Even before when I posted the old thread I noticed that if I used a smaller resistor (3.3 ohms) sometimes the individual LED would not turn on. At the time I didn't give it much thought because I planned on using the larger 5.6 ohm resistor, because I didn't want the current that high anyways. Since then I built a prototype device with a customized circuit board that takes the sequence of ~40 individual LEDs. Now I noticed that this problem seems to occur more frequently, and it occurs more often with some of the LEDs? Although when I use a very large resistor (e.g. 100 ohm) the LEDs seem to always turn on when they are supposed to (see attached image). I am attaching 4 images that illustrate what I mean. 2 of them used a 100 ohm resistor and the other two used a 5.6 ohm resistor, and I tried it with 2 different LEDs. The only difference is that I replaced the resistor! Why does the LED sometimes not turn on when I use the smaller resistor (larger current), but everything seems to work just fine with the larger resistor (smaller current). You can see when the LED is not on by looking at the figures when the average pixel intensity is ~ 37 counts.
Some of the ideas that I have that might be causing the problems include: 1) I need a different power supply, but the current can output up to 4A and it is 5V (see http://www.powerconversion.com/assets/ssl20c_1191534424.pdf for a description of the 5V power supply). I also use the 5V power supply to power some IC chips (transistor arrays, OR logic gates, etc., but I don't think they take too much current especially up to 4A? 2) For some reason when I created the customized circuit/ prototype there are some type of cross talk because the higher currents lead to higher electrometric fields? 3) The reed relays that I use might have a problem (part number HE3621A0510, and spec sheet is located at http://www.hamlin.com/specsheets/HE3600 revised.pdf). 4) For some reason the LEDs themselves might be the problem, but from my understanding they should be fine with a larger current (please see attached PDF spec sheet), and if they are overheating because of the larger current why is it so random?
Thanks again for any suggestions/comments on what might be causing the LEDs to not turn on sometimes when I use more current to drive them.
Kevin
Attachments
-
144.4 KB Views: 24
-
129 KB Views: 19
-
75.2 KB Views: 18
-
144.9 KB Views: 20
-
84.7 KB Views: 17