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When constant current isn't constant.... on LED driver.

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nctnico:

--- Quote from: 1sciguy on March 22, 2019, 05:07:18 pm ---I agree with the point that the LM317 has gone out of regulation and it slams the diodes with 24V.  I have heard that it is bad to run multiple LED's in series with a constant voltage source and a current limiting resistor because differences in forward voltage will cause one led to burn brighter and the others go dimmer.

--- End quote ---
This isn't true. The current through each LED in a series string will be equal. If one LED is brighter/dimmer than the other you have to buy better matched LEDS. I think the others already pointed out the problem with the circuit. A better solution however would be to use a simple transistor circuit to create a constant current sink for each string. A +/-30% tolerance in current probably isn't noticable for the user.

See the zener diode current source (actually it is a sink) from here:
https://en.wikipedia.org/wiki/Current_source
The output from your microcontroller can serve as a constant voltage at the base of the transistor.

Gyro:

--- Quote from: 1sciguy on March 22, 2019, 05:07:18 pm --- I think the 18V  or 19V zener to ground would be the best solution. The zener would never fire unless the diodes are off...

--- End quote ---

I think that would work fine... at least for now.

1sciguy:
Thanks guys.. sounds good.  At least I see my original fatal flaw more clearly.  Now I probably have months worth of warranty replacements to start working on..  Aaarggh!
Probably would have worked forever had I not wanted to do the fancy PWM dimming.  Like I said, the red LED's are not failing in the field and they have an even lower stack voltage drop.  Lots of options now..  Since I just bought a huge reel of 317's, I guess I'll turn them into voltage regulators on the board to get a constant 20V and then use current limiting resistors in the next board design.

mariush:
You could just replace your Lm317 with something simpler like this : https://uk.farnell.com/on-semiconductor/nsi50010yt1g/ic-led-driver-50v-0-01a-sod123/dp/1794979

10mA +/- 30% , up to 50v ... cheap in quantity. You could probably solder it between the IN and ADJ in the lm317 footprint so you don't have to throw out the boards

Or this one (10mA +/- 10% preset, up to 350mA with external resistor), up to 40v : https://uk.farnell.com/diodes-inc/bcr420ufd-7/led-driver-linear-1khz-udfn-6/dp/2748559
This could also work but you may have to put it on a tiny pcb and then solder pcb to lm317 footprint on old board.

1sciguy:
I think this will be the final fix for the long term.  I have reels of BSS123, so easy to make current mirrors.  R1 sets the current for the green string based on 5V out of microcontroller.  R4 sets it for the red string.  No voltage regulator on top of the strings.. just rely on the current mirrors to set the currents.  Anybody see anything wrong?  Switch 1 is there to turn off some lettering on a display by shorting some red LED's out.  Have not checked the proper value of R1 and R4 yet, but I think I'm in the ballpark.  I have traditionally tried to minimize component count because I was hand placing parts, but now I have a pick-n-place machine, so part count isn't a big deal.

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