fuses
I have a 5V power supply which needs protection. I only have a 24V, 1A fuse. Can I use this fuse for the 5V power supply.
As Watt = R * I * I = V * I.
As rated W is constant for a given fuse, will the current at which the fuse blows increase if Voltage decreases?
The current rating of a fuse represents the minimum sustained current the fuse will blow at ... eventually. A 1A fuse will take 1A for a very long time without blowing, and if the fuse can dump some heat into the PCB or has airflow across it, may never blow at 1A.
The critical parameter is the \$I^2 \cdot t\$ rating, which gives you an idea of the energy (power and time) needed to blow it. (Remember that fuses are really meant to protect circuits when there are catastrophic failures.)
It's crucially important to match \$I^2 \cdot t\$ ratings, since if you replace a fast acting fuse with a slow-blow type, even thought they both say 1A, it's going to take radically different energy levels to actually blow them.
When the fuse is intact, you only have an \$I \cdot R\$ voltage drop across it. This drop is going to be nowhere close to the voltage rating of the fuse (else it acts like a big resistor and limits the energy available to your circuit.) Once the fuse blows, the voltage rating comes into play, which represents how much voltage potential the open fuse can withstand without flashing over and re-energizing the compromised load circuit.
simulate this circuit – Schematic created using CircuitLab
For the input fuse, how do you feel about a couple more components? R2 and D1 form a cheap 5.1V regulator for the LED to assure a more constant brightness.
Best Answer
The 24 V printed on the fuse is only relevant after the fuse has blown. There is no need to calculate the power (\$P=I^2R\$).
Why is that so?
When the fuse is intact, it will have a low resistance so the dissipated power will be low.
When the fuse has blow, no current should flow so then the power is zero.
So the case where both current and voltage are high (like 1 A, 24 V) should never happen.
After the fuse has blown, it must be able to keep the circuit open (not connected). The fuse can do this up to a certain voltage. For a 1A / 24 v fuse, that voltage is 24 V.
So as long as there is no more than 24 V across the fuse when it is blown, there is no issue. Your 5 V application has a low enough voltage so this fuse is suitable.
You would not want to use a 1 A / 24 V fuse in a mains voltage ( 120 V or 240 V AC) as then the voltage is too high and the fuse might not be able to keep the circuit open after it has blown.