How to choose Fuse ?

the device cannot get spoilt below that voltage and ampere rating. So, to safeguard the device at higher voltages and currents, a fuse is used like that.

A fuse works melting by its own when a higher or over rated current flows through it. Fuses are selected based on the Ampere rating for the purpose of protection but not the voltage because a fuse is connected in series with the line and it breaks depending on the heat generated by the over current. In the 1A fuse mentioned here, currents higher than 1A will generate enough heat energy (I^2xRxt) for the fuse wire to melt. Higher the current, lower will be the breaking time.

Well, then what exactly is that voltage rating for! When a fuse blows, there will be a spark between the broken ends of fuse wire and current keeps flowing till the gap becomes large enough to extinguish the spark. As the gap gets larger, the voltage cannot ionize the air between the broken ends to keep the spark alive. A higher voltage needs larger gap to extinguish the arc.

In this case 250V is chosen because the supply voltage is not expected to rise any higher than that even though the SMPS is rated to withstand up to 270V.

lalA fuse works melting by its own when a higher or over rated current flows through it. Fuses are selected based on the Ampere rating for the purpose of protection but not the voltage because a fuse is connected in series with the line and it breaks depending on the heat generated by the over current. In the 1A fuse mentioned here, currents higher than 1A will generate enough heat energy (I^2xRxt) for the fuse wire to melt. Higher the current, lower will be the breaking time.

Well, then what exactly is that voltage rating for! When a fuse blows, there will be a spark between the broken ends of fuse wire and current keeps flowing till the gap becomes large enough to extinguish the spark. As the gap gets larger, the voltage cannot ionize the air between the broken ends to keep the spark alive. A higher voltage needs larger gap to extinguish the arc.

In this case 250V is chosen because the supply voltage is not expected to rise any higher than that even though the SMPS is rated to withstand up to 270V.

OK, but why 1A fuse is only chosen in 70% of smps design? why not 2A,3A....

ShyamsundarOK, but why 1A fuse is only chosen in 70% of smps design? why not 2A,3A....

Check the rating of SMPS, there will be power or current mentioned. You can see it as less than 1A.

Rating of SMPS is 9.6V,1A and power is 9.6Watts...(10 Watt flyback converter topology is used)

ShyamsundarRating of SMPS is 9.6V,1A and power is 9.6Watts...(10 Watt flyback converter topology is used)

Well, assuming there is only one output for the SMPS, the output power rating is 10W. So input current will be (output + losses)/input current.

Neglecting losses, Input current = 10W/230V = 0.0435A
That is way too lower than 1A, the fuse rating.

lalWell, assuming there is only one output for the SMPS, the output power rating is 10W. So input current will be (output + losses)/input current.

Neglecting losses, Input current = 10W/230V = 0.0435A
That is way too lower than 1A, the fuse rating.

OK, you assuming 100% efficiency right?

ShyamsundarOK, you assuming 100% efficiency right?

I neglected the losses because it won't be much for a 10W converter. Well, yeah assumed 100% efficiency.