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Choosing_the_correct_Fuse.php    13268 Bytes    14-06-2024 21:38:24

Choose the right Fuse

A Mini Guide * Tutorial * Javascript Calculator

DIY Noise Source

✈ Current Rating

This is the very first calculation for the device under maximum load. With a chosen "security factor" we get a starting value for our iteration. Most sources use a security factor of 33%, but in litearture any value from 33% to 100% can be found.

Voltage AC


Power [W]




Current [A]



✈ I2t Rating or this inrush-current thing

This value takes into account, that some devices consume a much larger current when switched on. If you use a toroidal transformer, the inrush current mayst be ten times larger. Or a switch mode power supply has a large inrush current, because the capacitor needs to be charged. You mayst select the type by using the table below :

FF0.18Super-Quick-Acting, SuperflinkResistive Load
F1.045Quick-Blow, FlinkSensitive Semiconductor Circuits
M6.00Medium-Time-Lag, Mittelträge...
T6.73Slow-Blow, TrägeToroid Transformer, SMPS
TT9.00Super-Time-Lag, Superträge...

*) in A2sec. Example data from 1 A types (Littlefuse, Eska, Schurter)

Current [A]

 under maximum load

Inrush Current [A]

 when switched on

Startup Time [s]


I2t [A2sec]




✈ Temperature Re-Rating

Fuse temperature Re-Rating Datasheets usually talk of tem- perature derating, but with fuses, this becomes a re-rating, as the curve is not horizontally below 25°C. As the temperature is usually higher than 25°C, the fuse must be re-rated to take this temperature into account. The graph on the left is from a littlefuse fuse. The one you select mayst have a different curve. Use the sketch below to calculate the necessary value.

Temperature 1 [°C]

 from datasheet of fuse

Rating 1 [%]

 from datasheet of fuse

Temperature 2 [°C]

 from datasheet of fuse

Rating 2 [%]

 from datasheet of fuse

Temperature [°C]

 actual temperature

Rating [%]

 actual rating

Current [A]

 with headroom and temperature re-rated

Current [A]

 This is the norm value you should use.


✈ Voltage Rating

Fuses are designed to work for a maximum voltage. This is defined by the geometry of the design and the resulting spark-gap. The fuse must be able to reliable extinguish any spark replacing the melted wire in the fuse.

Therefore the fuse used must have a least a voltage rating of the applied voltage, more is no problem.

✈ Share your thoughts

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t1 = 6585 d

t2 = 210 ms

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