I just let the smoke out of my testing setup. I designed and built a simple power supply using this guide. I built a capacitive power supply with a bridge rectifier and a final 3.3VDC voltage regulator, and it appears to have not worked. I plugged my power supply into 110 VAC and tested the voltage output at the regulator and got 3.3 VDC as expected, but there was also still a 6.3 VAC component at the output. Now, I'm not a clever man, and I assumed that this AC component wouldn't hurt anything, and boy was I wrong. I blew out my PIC32, a wifi module, and my PICkit3 — go me.
For the curious, C1 was 4.7 uF,
R1 was 4.7 ohm,
C2 was 470 uF,
and D1 was 6.8 V
What I'm trying to do is build a low cost, physically small power supply to step down mains power to 3.3VDC. Transformers are large and I'd like to stay away from them if possible. Does anyone have any experience with flyback transformer designs — they seem complex?
Any advise as to what I did wrong and what I can do to fix it? Any other design ideas?
Best Answer
Do you understand that this type of power supply provides no isoation at all from the mains? It is only intended for applications in which the circuit being powered will have no connections whatsoever to anything else.
You're lucky you didn't lose your PC as well, since you were essentially connecting 120 VAC directly to a USB port! Be thankful that the PICkit sacrificed itself to save your system.
Go out right now and invest in a real bench power supply. Used ones can be had for a few dollars, and even a good low-end new one will be less than $50.
As an aside, I don't understand why that application note has you putting the dropping resistance/capacitance and the diode D2 in the neutral side of the mains connection instead of the hot side. I think it would be at least a little bit less risky to do it the other way around. But maybe they wanted to make sure failures like this one would be more memorable.