Teardown of a cheap UK style USB power supply.04:33

I’ve sussed out one of the purposes of that little capacitor in the feedback circuit. It’s to block the feedback winding and resistor from the 0V connection so the high value start-up resistor can drive the power switching transistors base. Otherwise they would just shunt it’s current almost completely.
I got this little USB PSU from Banggood for a measly $2 purely to examine its circuitry and internal electrical isolation. I thought it was going to be a bit more crowded inside than it is, but was very surprised to see that it uses a single sided board with standard through-hole components, but still manages to provide a tolerable level of electrical separation on the PCB. I’m not going to go as far as saying that it’s a safe device because it’s track isolation isn’t good enough and there are two other critical components that couple the low voltage side to the high voltage side. The transformer itself could potentially just have a layer of tape between the primary and secondary windings, and the capacitor used between the primary and secondary could also pose a shock hazard if it failed.
But aside from these potential weak points the unit is quite intriguingly simple. It doesn’t use any non-standard components other than the custom would transformer and that includes common TO92 NPN transistors. The use of a proper opto-isolator for feedback is very impressive. Even if the voltage varies a bit under load it at least makes an effort to limit the upper threshold.
The case is clipped together, but is pretty solid and I wouldn’t anticipate it coming apart accidentally even if it was put into a fairly tight socket.
In summary, it’s very neat externally and internally, but I wouldn’t trust it with USB items that had lots of exposed metal to touch or were too expensive to trust with a cheap power supply.