# Calculating Fuse Size

A time lapse camera rig I'm, working on needs an appropriate fuse. The camera will be powered by a 12 volt deep cycle marine battery. The instructables that I'm using to build this rig is found here.

The camera I'm using is the same as in the instrutables: 8 V, with an unknown wattage.

I bought a dumby battery that had a 120 V AC to 7.5 V DC converter inline. I swapped the AC to DC converter out for a Drok 12V DC to 7.5V DC/ 3A Max converter rated at 22.5 watts.

In the inscrutables, they call for a 20 A fuse. Without knowing how to do the mathematics, this number feels way to high and would likely not blow before it wreck electronics within the camera. I'm I correct in thinking this?

If someone could show me the calculations for an appropriate fuse size, and/or tell me why this rig would need a fuse in the first place, that would be awesome.

• "inscrutables" Only sometimes... Dec 6, 2016 at 21:25
• Fuse is there to stop fire, not to save any device past the fuse from damage. Dec 6, 2016 at 21:43
• Size the fuse to protect the wiring to the unit at the high end and make sure it doesn't blow at the low end. If the fuse blows the electronics has already run down the curtain and joined the choir invisible. Dec 7, 2016 at 3:48

A first-order approximation would be:

${22.5\text{W} \over 0.7} \approx 32\text{W}$

${32\text{W} \over 12\text{V}} \approx 2.67\text{A}$

So a 3A fuse should suffice.

... would likely not blow before it wreck electronics within the camera.

That's not actually what the fuse is for. The fuse breaks the circuit in case something is already wrong with the camera or converter in order to prevent more damage from occurring.

In all honesty you could probably get away with an even lower fuse value, but you would need to use a logging (or at least high-refresh-rate) ammeter in order to find out how much current is actually being used by the converter and camera during normal use.