Can an old SMPS serve as a tablet/cellphone charger
Any Smps with well regulated 5V DC can be used as a charger. But there must be some sort of safety/ protection be in place so that the output current is limited.

A standard mobile or tablet nay demand 2Amps of charging current from a 5V smps. A standard ATX power supply may capable of delivering 25A on its DC 5V output. Designated chargers will have internal current limit to avoid excessive current flow during malfunction of the connected gadget. But having these gadgets connected to an ATX will reduce the gadget to ugly burnt thingy in case of malfunction of the battery/ charger controller. Safety is thus seriously jeopardize.

So, what can be done is build a simple current limiter and install it between the SMPS and yr gadget.



Current sense resistor of 0.33ohm will turn on the Q2 at about 2Amps and the base bias for Q1 will be removed thus limiting the current at that value.

Q1 must be rated at least two times of the intended charging current and it must be on the heatsink.