Weird op-amp voltage increase when using AC-DC power supply

Question

I'm trying to create a PWM signal to dim an LED. I came a across this circuit: When I tried it out on a breadboard powered by a 5v supply coming from my USB Digilent Analog Discovery, it worked great. However when I tried to power it up from a 5.3v power adapter that plugs directly into the wall, I get a short increase in voltage on the PWM output signal. On the oscilloscope, it looks like this: What's interesting is that the voltages reach up to about 5.8v, 0.5v more than what I'm even supplying the op-amp.

Moreover, when I set the duty cycle to 100%, it becomes apparent that the voltage increases have a frequency of 60Hz, the same frequency as the AC coming from the wall.

I tired to see if the voltage from the power supply was fluctuating between 5.3v and 5.8v but after measuring it seems that it does output a constant 5.3v.

What could be causing this weird behavior??? Thanks!

Answer 1

The spikes in output voltage may be due to your AC/DC switching supply not being able to regulate its output completely. See: Does a switching supply always need load. Did you check the supply rail with an oscilloscope?

You can solve the poor regulation by loading the power supply more using resistors between the supply and ground. Be aware of your resistors' power ratings and that they will get warm. You should use 1/4W resistors with no lower resistance than 125 ohms (from \$0.25 W = (5.3V)^2 / R\$).