Electronic – trying to understand erratic LEDs behavior

led

I have a small PCB that holds 6 LEDs (model, fV 3.2V, 30 mA), 2 parallel lines of 3 LEDs in series. The PCB has wires soldered onto it that go to a connector about 10cm down the line.

circuit showing 6-LED pcbs and connection points

In an installation with several of those PCBs, I've experienced that after a while a few of them only light up half the LEDs.

In the proper setup those LEDs are driven with a LED PWM chip. Outside the setup, i test them with a 9V power supply.

What I now want to understand is the behavior I observe when debugging those faulty boards:

  1. I test connect 9V in connection A (at connector) and only half the LEDs work
  2. Then I test connect 9V at connection B (directly on the solder joints on the PCB) and now all LEDs work
  3. I go back to connect the 9V in connection A (at connector) and suddenly all LEDs work from here as well

There is no visible sign of a broken solder joint or lose wire connection. (Obviously, there could be, as the boards are handsoldered. Also, most connection points are covered with conformal coating and I am now suspicious if the coating puts stress on some of the solder joints)

What makes LEDs sometimes turn on and sometimes not?
Does this behavior indicate that either one or more of the LEDs are actually damaged, or is it more likely that there's a lose connection somewhere?

Best Answer

The problem you have is one that is a design problem. You need to realize that the three LEDs in one string may very well have a higher total voltage drop than the three LEDs in the parallel string. The ones with the lower voltage drop may light while the others do not.

Since you do not have any intentional current limiting scheme to control the current flow through the LEDs in your testing scenario you are depending upon the forward voltage drops across the LEDs to be more than the sourcing power supply. In this scenario the effective internal resistance of the LEDs and the resistance of the wiring harness are being used to limit the current. Operating in this mode the V / I characteristics of the LEDs can shift with time, aging of the LEDs and the current operating temperature of the diode junctions within each LED.

The solution is to provide for a separate driver for each string of three LEDs or to place a resistor in series with each of the three LEDs that are already in series.