What sort of question is that to ask of a guy with a lightning charger lead dangling over the arm of the settee immediately next to him?


Being an Ipad lightning lead the terminals are exposed to touch and every so often I feel a tingle when I put by bare elbow on it, guess what it is plugged into?


Yes I do think about what protects the electronics and I have it in mind there’s a standard which is being or has been rewritten, so maybe the older USB wall sockets aren’t as good as they could be.


I am suspecting the answer you are looking for is, not a lot.

Isn`t it the old adage Graham?

You put a fuse in to protect a tansistor and the transistor blows protecting the fuse!

I am not sure that the physics does not apply Graham.

There are several variations on the protection, so here goes.


Wall Warts with 5W transformer supplies: The TX is would on the mains side with the wire as thin as can be wound at a reasonable price, say 46SWG or smaller. This will fail at a few hundred mA as a fuse. The output voltage regulator if fitted will have electronic current limiting, otherwise the transformer winding fails open circuit (Experience!).

Ditto but electronic and USB sockets: Output overcurrent protected by electronic current limiting (Part of the USB spec). Electronic failure will either stop any current flow or the low power parts used will fail open circuit. In all cases fire is extremely unlikely due to the low power levels. Electronic parts may not look like fuses but are very effective all the same. Some may have small fuses in the circuits, but I have not seen many.  From another post, this is about foreseeable faults, and shorts between pins or to the usual plastic class 2 Earth pin are impossible due to the construction. Is someone suggesting these devices are not safe?

If they are so safe why do so many cheap clone power supplies burst into flames?


I realise that cheap components used in a design based on lowest possible price rather than safety or longevity will be primarily to blame but my question is a genuine one.

Small plug top power supplies are MEANT to incorporate a small fuse or other device giving similar protection. I suspect that most units incorporate no such protection and can therefore fail with considerable violence and fire risk.


I avoid plugging such items directly into a fixed socket outlet on a 30 amp circuit, preffering an extension lead with a 3 amp fuse in the plug.

As one who designs electronics at least some of the time, some thoughts.

The electronics cannot be protected against everything, so instead it is intended to fail to safe when it does

In good designs by reputable makes the PCB will often have a link designed to melt, or the inrush resistor is a fusible one. The glued case should be strong enough to contain all the flying parts during the highest energy operation of these mechanisms.

A rigorous failure mode analysis is a large part of the design for a responsible manufacturer, of anything.

(when you do the tests the largest bang is not normally the semiconductor, but it may be a consequence, for example of diode failure, and that is explosive failure of the mains side smoothing capacitor - if that is electrolytic then the good ones have deliberate casing weak spots so they unzip at a defined pressure. If we can we prefer to use ceramic in the smaller high end designs, but only available up to a few uF at a few hundred volts in surface mount.)


I am aware that some designers are not responsible, and the cheaper devices do not met the requirements for double fault or reinforced insulation between primary and secondary, (as generally these are supposed to be class 2, so the output has to float) nor are all cases adequate blast containment, and not all fail safe.

regards Mike.

Recharging Ecigs could overload the USB socket.

A well designed USB socket should current limit and collapse the voltage before anything bad happens, and recover from short circuits and so forth gracefully.


The unintended problem of fail safe is that because most do, it engenders a carelessness  in users that is quite dangerous when real power is involved.