AC/DC PCB Module Testing

[kkessler]

I’m working on a project where I will be putting a microcontroller inside a wall switch, and I’m investigating 3.3V power options.  I  have a Mean Well AC/DC module from Mouser, a Hilink from Aliexpress, and I’ll possibly be testing a capacitive transformerless power supply.  I started looking at efficiency, to get an idea of how much power loss and heat I’m going to get from these devices, so I setup a kind of naive test with a DC load on the output and multimeters measuring voltage and current on the input.  I realize, though, that to get a measure of input power, I need to take the power factor into account. 

The way I can think of to measure PF is simply to use an Oscilloscope to measure the voltage across the input, and for current, maybe just measure the drop across the ammeter.  I have my test setup connected through an isolation transformer (how does that affect the PF calculations?), although I’ll still probably pull out my old Rigol scope instead of risking my new one. 

I have a relatively inexpensive device that plugs into an outlet to monitor power usage, and it gives the PF, but I don’t know how accurate they are.  How do they work?  I suppose they could just be using a microcontroller’s ADC to poll drops across shunt resistors, but maybe they are using a better way to measure PF.

I guess I am just asking for any input on characterizing my power supply options, especially where it pertains to power loss and heat. 

[kkessler]

I plugged in my scope, and what I saw was not was what I expected.  Googling around I learn a great deal on how these switch mode power supplies work.

[ebclr]

Do you know this tool?

https://webench.ti.com/webench5/power/webench5.cgi?origin=ti_panel&lang_chosen=en_US&VinMin=14.0&VinMax=22.0&O1V=3.3&O1I=2.0&op_TA=30

[kkessler]

That link certainly sent me down some very interesting rabbit holes that I'm still exploring.  It's obvious to me that the question I should have ask was:

I have an application to put a microcontroller inside a wall switch.  I need a 3.3V power supply that only needs to supply about 20mA.  What options exist that 1) won't burn my house down, 2) won't require me to rip open my wall to install it, and 3) won't break the bank?