I am using the IC Max7221 together with an Arduino for a Word Clock in a White LED Array Formation. It is quite similar to a dot matrix. The Max7221 will impose a duty cycle of 20% on each row on the 5x5 matrix. At the current state, the LEDs are underdriven (8mA) on each when averaging over time, the voltage is at about 3.3V.
From time to time, some LED went out and I replaced them accordingly. After three LEDs I measured the voltage on the respective broken LED, which showed about 2.2V when connected to it, cutting off the LED and measuring the open connection it reverts to about 3.3V. Connecting the supposedly broken LED to a 3V cell battery, it shone up again, but not when putting in back in the original node on the matrix (or anywhere else on the matrix that is)
On the working LEDs in the matrix I could for, now at least, still measure 3.3V on each single node.
What could be the reasons that a single LED will revert the voltage on a node from 3.3 to 2.3 and therefore be off?
Is it still a damaged LED in respect to some other characteristics than simply due to heat damage (which is unlikely due to the low amperage mentioned above)
Could it be that the minimum wattage specification for current-limiting resistor with the Max7221 is higher for the load I am using (am using 1/4 W SMD resistors)?
Could missing decoupling caps on the IC cause this malfunction and damage the LEDs?
Or could I have exceeded the temperature/duration during the soldering of the LEDs and therefore some are giving in much faster than the lifetime would be?
Thank you!