Fuses

[Andrew Leigh]

Hi,

How does one calculate a fuse / overload value?

For example I have a 10A 250V rated overload in a circuit producing 9V d.c. and less (have duplicated an existing circuit). The resistance of the load changes marginally through time as it is a saline solution, but this should not be an issue. The voltage drops from 9V down to as low as 3V depending on the the position of the control pot. Currently the circuit is drawing just over 10A causing the overload to keep dropping out.

I need to increase the salinity of the solution to the point where the unit will draw up to 18A. I don't need necessarily need an overload (it is more convenient) and can use a fuse instead. Surely the value of the fuse can't be a function of the wattage?

i.e. 18A at 3V = 54W. That would be a standard 5A 12V rated fuse ..... that is going to pop in a matter on minutes.

What am I missing?

Thanks
Andrew

 

[ericgibbs]

hi,
This pdf may explain.

Google also for fuse current graphs

 

[Andrew Leigh]

Hi Eric,

Gotcha, done a bit of gooling and reading.

It would appear as if the voltage rating has little to do with the matter providing the voltage of the fuse you select exceeds the voltage you require AND that the current rating is the same, no problems. It would appear that as a fuse generally blows quickly that it is more a function of time vs. current.

In my case a 20A 12V or 220V for that matter should do the trick. The secondary winding has a wire diameter of ~ 3.0mm that could take about 32A @ 4.65/mm2 so that should more than protect it.

Any further thoughts.

Thanks
Andrew

 

[Willbe]

There are two asymptotes on a current vs. time curve for a fuse. The current that will not blow the fuse no matter how long it is a applied, and the I squared T curve.
The I^2T for the fuse should be less than the I^2T for the equipment you're trying to protect. Good luck figuring out these values!!!