I have a faulty car (bad central locking engines) and a fuse is blowing every time I open/lock the it. I'm looking to do a quick fix, and just replace it with an automatic fuse. The car runs on ~12V, and the fuse is at 15 ampere.
Are fuses designed to operate on specific voltages, and have their ampere "tolerance" rated for the specific voltage?
As I understand it, a 15A fuse for 12V is designed to blow at 180W (the heat produced in the fuse wiring is what makes it melt, right?).
So, if I use an ordinary 16A/220V fuse in my car, it will blow at 3520W, and should really be rated 293A for a 12V circuit…
If my reasoning is right, that fuses blow at effect and not ampere, what I'm looking for is a 0.8A fuse that is designed for 220V (I=180/230), and it would work as a good replacement…?
Best Answer
The 180W reasoning is wrong. You don't have the full 12V across the fuse, only a fraction of it. Say the fuse has a 0.1\$\Omega\$ resistance, then 15A will cause a power dissipation of
And that 22.5W in the thin wire is just too much, and the fuse will blow.
Note that the 12V doesn't appear anywhere in this, and you could do the same calculation for 15A at 230V AC. There's a "but", however. Fuses do have a rated voltage. Not for the blowing, as we've seen, but to make sure the remains of the fuse won't cause sparks when at a high voltage. So you can't use a 12V car fuse in a mains appliance, but the other way around should be safe.
Automatic fuses also react on current only. They have a coil which releases a latch if the number of coil-windings exceeds the rated value. Also here it's independent of voltage.