F2 fuse burn

Hi all,
i just finished mouting a prusa i3 3D printer. everything was working well (all motor move in the correct direction) untill I want to heat the hotend and the heatbed to began my first print. The hotend was hot but not the heatbed when the F2 component (a fuse according to the wiki) start smoking.
the power supply is 12v 33A.

what did you think make the fuse burn ?
did you think the ramps is still working (if i replace the fuse) ?

should i cool the ramps with a fan ?

thanks for any help

Yes you need a fan on the ramps, and heat sinks on the drivers.. the fuse will reset when it cools down,unless it shows signs of being burnt.

With a multimeter, measure the resistance of the heated bed. It's possible that it's too low or there is a short.

That said, I hate those PTC fuses because they fail in odd ways (including catching fire). I prefer real standard fuses, like automotive blade fuses.You can't mount them on the RAMPS board, but they definitely are an improvement.

Edited 1 time(s). Last edit at 01/18/2014 06:55PM by Cefiar.

Thanks for the answer,
the fuse is clearly burn, it's open on the side and black. I have ordered some 11A PTC fuses to replace it.

The bed resistance is around 1.6 ohms.

Assuming no other losses, and that as it gets hotter that the bed doesn't somehow short out, that should only be about 7.5A.

Beware of shorts from parts of the heated bed PCB if you're using an aluminium bed. If it heats up and bends slightly (as compared to when cold), things could short out which would cause issues. Also tends to be a right pain to figure out, being intermittent or only when hot.