Is it safe to use a surplus transormerless/switching power supply for a small project. I am referring to the small ones commonly supplied with cellphones etc. I assume they must somehow isolate the mains from the output somehow.??
Well I found a small 5vdc 800ma wall wart switcher supply that looks like it was used for a cell phone. I put my scope on it and it measures 5vdc even unloaded and has about 24mv AC ripple on the DC output.
It's very small and light so I assume it's a switcher. I used an ohm meter and DC output seems to measure isolated from AC terminals.
The output will be isolated from the input - it's the law.
The only connections between the input and output will be a transformer, an opto-isolator and there might be some low value (4.7nF typical) high voltage (Y2 rated, 2kV) RF suppression capacitors.