# All LEDs died yet rest of board fine; why?

I had a board with 3 LEDs, two driven by an AtXmega325e and one simply connected to the rails. The rails are 3.3V, the series resistors 70ohms.

simulate this circuit – Schematic created using CircuitLab

All LEDs stopped working at once, yet the rest of the board (OpAmps, ADC, UART) seemed completely functional. After replacing the LEDs, the new ones worked as intended again, so I must have damaged the LEDs themselves somehow. I am not sure whether they were oriented correctly, but they did not light up (as they normally do (albeit dimly) when measuring the diode drop) in either direction. The diode drop function of my multimeter measured the following (disconnected from the power supply):

forward   backward
1.7V      0.6V      driven by uC
1.6V      0.6V      driven by uC
0.5V      1.2V      power rails directly


The most confusing thing: After dismounting the LEDs they actually worked again. The two IO pins have been in all possible combinations (both on, etc.) but the LEDs never lit up. I lost the datasheet/name, but they are generic orange surface mount LEDs, with a max. current of 20mA (the names in the schematic are wrong). The solder joints looked completely fine, I also find it improbable that all three would be defective.

Do you have any idea what could cause this strange behaviour? How should I investigate this further?

• Do you think I should post pictures of the schematic/layout themselves? I thought they must be rather irrelevant, as the components are only connected to rails and uC. – caconyrn May 3 '16 at 17:26
• With the information provided there should not be any problem. Unless the actual connections are not matching it... – Eugene Sh. May 3 '16 at 17:31
• "All LEDs stopped working at once" suggest a power failure of some sort. Assuming D2 and D1 are fed from the same rail that connects to your microcotroller, I would look for the problem there. Could be a simple bad solder joint... – JvO May 3 '16 at 17:32
• Any flexing of you pcb whilst setting up? That can break a track. maybe re-soldering bridged the break so it now works.. – Spoon May 3 '16 at 18:11
• Do not use the diode function on your DMM when testing in-circuit diodes. Your backward voltages are the meter driving current through the power supply. – WhatRoughBeast May 3 '16 at 18:32