Choosing the fuses (Mechanical) for my Ramps

I wanted to build my own RAMPS Board (I don't have one for reference )
I need to choose the fuse (Mechanical) for the Board, I wanted to use 24V in the Ramps board, so I need to choose the fuses for 24V or greater... the confusing part for me is whether the Heater, Heated bed, and Motors requires the same current while using 12V ?

My opinion is the current should be approximately same as of 12V for heater and heated bed because I have to use different values of resistance (slightly higher) for both heating elements when shifting to 24V. Is it correct ?

Guys am I going in correct path ? Please Advice

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Enjoy life

It's not the current that stays the same, it's the power output, and that's proportional to the power supply voltage.

If you choose to use 24V, you want to use different hot end and heated bed resistances.

For my heated bed, I use a 24V kapton heater that was made for a Mendel Max, so it's got a known power output (200W). From this and the voltage, I can work out all the other details using the power law ( P=V*I ).

So for my heated bed, it supposedly draws 8.333A. You need to allow some overhead (tolerances, etc), so a 10A fuse is what I could use for that side.

For the hot end, you want between 30W and 40W of power (the cartridge heaters are around the 40W mark, and a standard 6.8 ohm resistor heated block setup is about 22W, which is really a little low).

Using the power law and Ohms law (V=I*R) you can figure out the current it will draw, and the resistance you want. For 40W @ 24V, you want ~14.4 Ohms (~1.667A) - I'd choose 15 Ohms which is a nice round value. For 30W, you want 19.2 Ohms (~1.25A) - I'd chose 18 Ohms, which actually gives you about 32W (from memory).

Note: Since the hot end, the fan and the motors are all on the same input, you probably want to use at LEAST a 5A fuse for them all, though 7.5A is probably a safer bet. If you're using the second output and running 2 extruders, remember to take the extruder current into account.

PS: In my case I actually use a 15A fuse for the heated bed, and a 7.5A fuse for the hot end/motors. The fuses simply blow if the current is over these values, so a short will definitely cause them to blow, and that's the most common fault. The extra current shouldn't be much of an issue, as long as the PSU can provide it.