How to choose Fuse ?

@shyamsundar-dZmKP4 • Oct 19, 2024

Oct 19, 2024

1.1K

Dear all,
How to choose a fuse for a particular design? I just saw one design of SMPS. In that the input voltage range for the design is 90 to 270 volts. But the fuse chosen has the rating like this:
250V,1A.
is it correct? why 250V rating is only chosen? Please let me know the answer

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@abcd-2NEyMb • Dec 20, 2012

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.
lal

@lal-R60Xjx • Dec 20, 2012

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.
Shyamsundar

@shyamsundar-dZmKP4 • Dec 25, 2012

lal
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.

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

@lal-R60Xjx • Dec 25, 2012

Shyamsundar
OK, 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.
Shyamsundar

@shyamsundar-dZmKP4 • Dec 26, 2012

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

@lal-R60Xjx • Dec 26, 2012

Shyamsundar
Rating 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.
Shyamsundar

@shyamsundar-dZmKP4 • Dec 26, 2012

lal
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.
OK, you assuming 100% efficiency right?
lal

@lal-R60Xjx • Dec 27, 2012

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