choosing the right fuse of Power supply

[kgavionics]

HI
I want to build a dual channel variable power supply.I've chosen the Sharp PQ30RV31 which is a voltage regulator can output 1.25 to 30V @3Amp max.so what would be the fuse rating i use for this dual channel power supply?is it 3Amp fuse?thanks in advance

[Zero999]

The regulator has current limiting built-in. A fuse is still a good idea to protect the transformer, if something before the regulator goes short circuit, such as the rectifier. A 5A slow blow fuse will probably be sufficient. A primary fuse is also a good idea but might not be necessary, if the transformer has a built-in thermal fuse.

Note that the voltage regulator has a maximum power dissipation of 20W on a perfect heat sink. In real life it will be less than this. If the power dissipation is exceeded, then the regulator will shut down. This means the 3A will only be available if the voltage, between the input and output terminals of the regulator, isn't high enough to cause the regulator to overheat and shut down.

[KL27x]

You probably want a fuse on the live mains wire. You can calculate the value to be closer to what you need, but there's no need to get crazy exact. It needs to be small enough to save your circuit breaker from tripping. Large enough to handle your PSU's needs. 30V x 3A x 2 channels is 6A @30V. Where's that coming from? Transformer with a 35v secondary? A Switchmode supply?  A 3A fuse on the mains should be more than enough unless your PSU is particularly inefficient.