Heated bed hot mosfet

Hi, Im trying to make my heated bed work and having trouble. The mosfet starts to become really hot (within a minute or so) when the printer is switched on.
I heard the mosfets that come with the sanguino kits are the wrong type. From the wiki I found that the mosfets I have, are not the ones mentioned wiki, which leads me to think they were aware of the issue and used higher power mosfets for my kit.

this is the mosfet I currently have installed:
[www.fairchildsemi.com]

What temperature should I expect this thing to run at? To me it seems to be running hot so I switch it off instantly due to my fear of it burning out, but maybe its normal that it feels hot to the touch? I also did the trick mentioned in the wiki about using large wires to carry the higher current on the underside of the board.
Should I get a different mosfet or is there another way of making this thing run cold?

did you attach a heat sink to the mosfet ? My kit came with a heat sink... The reaon I ask is I am about to powerup my Sanguinololu 1.3b & am a little worried after reading the reports of these overheating when attached to the heat bed... but having said this I notice the latest update..

Version 1.3b - Updated April 4, 2012
MOSFET changed to SMT versions capable of 76A drain current!

Edited 1 time(s). Last edit at 06/08/2013 05:13PM by Mickman.

Yeh I installed one I had lying around (took it out of an old power supply) though I don't know how suitable it is (its quite thick with a right angle flap that sticks out the front). I just tested it and it the temperature seems to stabilise at around 120 °C. I realy dont like it being this hot!

What mosfet do you have by the way and what temperature is it at?

From what I heard, the ideal mosfet should have a Rds of below 0.02 ohms. Mine has a resistance of 0.022 which I assume is why its so hot. Its at the limit of what is possible without a fan.

Also can anyone recommend a mosfet that runs colder than the RFP50N06 I'm using?

You need something with a much lower Rds On. Even at 0.02 ohm, which is not much different than what you got, you'll need some pretty beefy heat sinking. Here's one from digikey. The part number is IRLB8743PBF. Rds ON is 0.0032 ohm, which is almost a magnitude smaller than what you have now. The one I listed runs at pretty much ambient temperature with MK2 heatbed, no heat sink. But then you can find ones with even lower Rds ON.

I used the mosfet from UltiMachine found here:
[ultimachine.com]

Was using it in my RAMPS for a bit, then decided to try out the sanguino i had lying around. At first I didnt change the mosfet out that came soldered onto my fully assembled sanguinololu
(i bought this one: [www.ebay.com])

turned my 12v heat bed on (silicon heatbed, looks like a big orange rubber sticker that I stick under my aluminum build plate) to 60C, and before it even got there (from what I remember around 35-40ish C) it started smoking and the solder started bubbling... not a good sign.

Took the mosfets that I got from Ultimachine from my RAMPS board, put them onto my Sanguniololu, and did the heated bed trace upgrade
[create3d.com.au]

Picture and info courtesy of create3d

just to be safe I also mounted an 80mm fan blowing across my electronics (needed them for the Pololu's anyway) and added a screw on heatsink to the heated bed mosfet, everything is working great.