Transformerless Power Supply Safe?

[bryan]

Hello:

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.??

 

[Nigel Goodwin]

They aren't transformerless, they use a switchmode transformer.

 

[Hero999]

Don't assume that it's regulated just because it's a SMPS, many cheap switchers aren't regulated, espcially the small wallwart variety.

 

[bryan]

Thank for the replies. I also note it is not very clean either, a lot of noise showing on my scope.

 

[Nigel Goodwin]

Thank for the replies. I also note it is not very clean either, a lot of noise showing on my scope.

Well it's a charger, not a PSU.

 

[Leftyretro]

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.

Lefty

 

[Hero999]

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.