With PWM value of 0, LEDs are off, and there is a sinusoid voltage with +7V max, -57V min, frequency of 50 Hz, between points A and B. Is this expected?
Within two weeks, about 9 LEDs.
Based on the sound coming from your power supply, I think it has bad capacitors.
without any changes in wiring neither circuit, I replaced the power supply. Things got better:With PWM value set to 0, oscilloscope measurement between points A and B shows sinusoid voltage again, but it is about Vmax = +5V, Vmin = -5, f = 50 Hz. Multimeter shows 90 mV AC. Much better!
(I tried to connect old power supply without disconnecting oscilloscope probes and it was the same situation as described in my first post.)
Gentlemen, in this situation, can I safely connect my LEDs to circuit without worries about burning them out? I think so...
While the electrical mains could be inducing an AC current in the circuit, I really doubt it in this case because there's such a large and uneven swing in voltage. If the power supply in question is hooked to the mains, any induced AC shouldn't have an DC offset of over 40 V from the DC circuit's reference voltage. This is only reinforced by the description of the sounds comming from the power supply, most likely something is wrong with the power supply.
The unevenness is easily explained by the diodes in the circuit! LEDs are still diodes and rectify AC.
Yes, I used oscilloscope to see what is happening. Ground probe is OK as it behaved good for all other measurements without touching probes. I measured AC voltage between points A and B with a common multimeter as well.