I have used many of them for year for my electronic projects. I stripped off
the plug on the end of the wire and plugged the wire directly into my
breadboard. A couple times the wires came out and shorted together which
blows the internal diode bridge or the primary winding opens. The primary
winding is made of fine wire on these small transformers and acts as a fuse.
Never any smoke, fire or oders from a failed one.
Some of the new ones have switch mode power supplies. The secondary is still
isolated via a transformer and feedback to the primary switching circuit is
through an optoisolator.
Nothing is 100% safe
The best suggestions I Have heard, both from the standpoint of
safety and economy is to plug them into a power strip and turn them
off when not using them.
On the the biggest problems seems to be related to the
batteries in many of the new electronic devices. They can be fire
hazards even when disconnected from the wall.
I have some wall-warts that are 20+ years old and they still work ok
when I pull them out of my junk transformer box for some odd project.
But, I treat them reasonbly carefully. The odds on a UL or CSA
approved unit giving 120 V (Assuming in the USA or Canada) shock is
very unlikely, unless the output wire is caught between the wall
outlet and the wall-wart itself.
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