Ramps 1.4 - Swapping Hotend and Heatbed relays

[hernejj]

I have a separate relay for my heatbed which is wired into D8. D9 and D10 are wired to two separate e3d V6 hot ends. Each Hotend draws 40W/12V=3.3A of current. Considering that D9 & D10 (and everything else except the heat bed) share the same 5A circuit, how come I'm not blowing the Ramps board 5A fuse when I heat both of the hot ends at once? I've even modified my start script to actually heat them both at once from a cold start. Can anyone explain why this works? Anyone think it is a bad idea?

Since it seems like a bad idea to run both hot ends form the same 5A circuit I am thinking of controlling the heatbed relay with D9 and moving one of the hot ends to D8. This would utilize the 11A circuit path for a single hot end, and lump the tiny amperage used to control the heat bed into the normal 5A circuit with a single hot end, motors, electronics, etc. Does anyone have an opinion on this? It should be as easy as swapping the pin assignments in Marlin right?

[uncle_bob]

Hi

Half of the answer is pretty simple:

The fuse is rated to *pass* 5 amps. It is rated to *stop* something higher than 5 amps. Exactly what it will stop depends on the room temperature, and how long you apply the current. There is also another couple of numbers on the fuse in terms of the maximum voltage it will interrupt and the maxim short circuit current it will handle.

Bob