Electronics > Projects, Designs, and Technical Stuff

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

(1/6) > >>

1sciguy:
I have a circuit I designed in 2010.  I am starting to get reports of failures of LED's in the field, and I believe this is way too short of a lifetime.  Strangely, the LED's are failing shorted because there are six in a series string and two or three in the string will fail but the others remain lit.  I have used an LM317 as a constant current source to the string providing 12mA.  The LED's are rated at 20mA constant and allow 50mA pulsed.  I have two sets that can be on depending on when my microcontroller switches them via the ULN2004 7 channel switch.  I normally select the 6 green, or the 7 red LED string.  I'm trying to figure out why the green LED's are failing, but the reds never fail.  I think I figured it out, and seek further advice.  The green LED's are sometimes pulsed at a very high rep rate of varying duty factor to make them brighten and dim in a slow undulation, just like the Apple logo in a Macbook.  The forward voltage on the green LED's is 3V at 12mA, so the total string of green is 18V.  I believe that when I place them in the undulation mode, the LM317 doesn't have time to stabilize its 12mA output setting, so I am spiking them with much higher currents.  The open circuit voltage on the string would go to 24V, so at the very beginning of switch closure on the ULN2004, I have 24V on the series string.  My immediate thought is to add a series resistor to drop the remaining 6 volts (24-18V), but I also realize that sometimes the 24V supplied to the LM317 may sometimes be 28V.  Also considered an 18V Zener across the string.  I'm in the middle of a move and packed up my scope, so I can't measure the size of the current spikes, but it must be substantial.   Should I add another path for the LM317 output so it can stay in regulation mode at times when neither the red string or the green string are switched on?

chemelec:
You should Add Filter Caps to the Regulator's Output.

1sciguy:
But a capacitor in the circuit could make things worse if that cap charges to 24V when all LED's are off.  This would be more joules to dump into the LED's.  I'm leaning toward putting a 1.6K ohm resistor to ground so when the LED's are off, the regulator will simply rise to a new compliance voltage of 19 volts to stay in regulation.

duak:
The LM317 has a 10 mA minimum current requirement.  I'd bet there's a high current pulse just after the LEDs are enabled and before the current stabilizes on the desired value.  I'd probably consider a zener diode first.  A parallel resistor will bypass current from the LEDs whereas the zener will only conduct when the LEDs are disabled.  A capacitor might help but it would also store energy that would be discharged into the LEDs when they were enabled.

Since green LEDs have a greater forward voltage drop, they'll dissipate more power for the same current.  Since the LEDs fail by shorting, I'd expect to find each die to have a very small melted area. This is caused by current filamenting where current is concentrated in the hottest areas and finally runs away.

Best o' luck

Benta:
As duak says, the LM317 needs a minimum output current of 10 mA to regulate. If the ULN part is off, there is no regulation at all until you suddenly slam it with a load. This will result in ~22...23 V directly on the LEDs until the LM317 settles into regulation.
A quick and very dirty solution could be a parallel 10 mA sink and increasing output current to 22 mA, but it's not nice.
A better solution would be a discrete solution with an NPN, two diodes and two resistors. This will not have the issue, but will not be as exact as the LM317.

Navigation

[0] Message Index

[#] Next page

There was an error while thanking
Thanking...
Go to full version
Powered by SMFPacks Advanced Attachments Uploader Mod