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I have a scanner which I'm pretty sure has a dead board full of IR LEDs (see image, ignore missing D4) enter image description here

I believe D4 and D7 are defective as I measured the voltage drop across each LED and found the drop on these two LEDs was much much higher than all the others (about 4v vs 0.8v on the rest).

So I am trying to find the correct replacement LEDs, but I don't really know how to determine the correct specifications. I thought to measure the power being supplied to this board, which came out as 12.4v on open circuit and 19.3mA short circuit (there is no resistor on the LED board so I assume there must be one in the board it is connected to).

Anyway given there are 9 LEDs in series, with a 12.4v and 19.3mA (short circuit) supply, is there any way to determine the correct LED specifications? Or are there any other readings I can take to try and figure it out?

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  • \$\begingroup\$ Are you sure they're all IR???? I see that D2 and D6 appear to be slightly different color plastic package, suggesting they're not the same as the others. Maybe D4 & D7 are blue or white LEDs (which could give around 4V instead of 0.8 for an IR) \$\endgroup\$
    – Kyle B
    Apr 27, 2022 at 22:58
  • \$\begingroup\$ @KyleB I did notice that colour difference but wasn't sure if it meant anything, it was manufactured in 2005 so could it just be burned out/worn LEDs causing the colour shift? This board is part of a 35mm film scanner that has a cold cathode light for the main scan and then does a secondary IR scan for dust removal. So I wouldn't imagine any non-IR LEDs would be required as it would give an inconsistent scan, the fact remains that power is being supplied and none of the LEDs are illuminating, so I assume at least some of these must be defective. \$\endgroup\$
    – protango
    Apr 28, 2022 at 0:04
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    \$\begingroup\$ If they were IR, you wouldn't be able to tell if they're illuminating ;) Seriously though- Unless you can actually measure the output, you can't really "know" you've found a suitable LED. I would probably, given your situation, go find some "middle of the road" emission level IR LED's that fit, and replace ALL of them. Possible (likely?) the scanner could compensate for the almost certain change in brightness so long as it's uniform. If you only replace a couple, the new ones almost certainly will be something (very?) different than the originals. \$\endgroup\$
    – Kyle B
    Apr 28, 2022 at 1:50

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Electrically you can replace those dead diodes with any modern, similar package size LED with the same forward voltage and a current rating of at least 20mA (which should be simple to find). As long as the forward voltages are nearly the same the current will be the same and the circuit will work. If that circuit is driven by a constant current driver, then even getting the forward voltage the same isn't so important. Digikey/mouser/etc should have hundreds of inexpensive options.

Optically it is a little trickier. You probably want to replace the dead diodes with ones of the same wavelength, or at least reasonably close. Getting diodes with similar beam patterns and brightness would also be good if you can find them. Unfortunately, you probably don't have a spectrometer to measure the exact wavelength or a good way to measure the actual radiant intensity of the diodes, so I would buy a couple different wavelengths that have approximately the right forward voltage, try one, and see if it works. If the beam pattern is way off or the wavelength bleeds through into the main visible image, then try another diode until you get good results.

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  • \$\begingroup\$ Thanks for the help! The problem is have no idea what the forward voltage of the existing LEDs are, is there any way to find out? \$\endgroup\$
    – protango
    Apr 28, 2022 at 1:21
  • \$\begingroup\$ @protango The forward voltage is the value you measured (0.8V). \$\endgroup\$ Apr 28, 2022 at 1:49
  • \$\begingroup\$ a consistent color mismatch could be easily corrected in post (photoshop). Same for brightness, within reason. 0.8v indicates they are indeed IR LEDs. \$\endgroup\$
    – dandavis
    Apr 28, 2022 at 8:59

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