I plugged in my 12v silicone bed. 12" x12".
The MOSFET on my Ramps 1.4 board got really hot and fell into the plastic of the power posts. That smelled bad but I unplugged it in time. Based on some searching, it looks like the MOSFET is just not up to the challenge of the bigger bed.
So which MOSFET should I change it out for?
Initial searching suggests FDP8870.
you should use that mosfet to switch a SSR something like this is what I’m using http://www.lightobject.com/40A-Solid-State-Relay-SSR-DC-In-DC-Out-P315.aspx
+1 for SSR.
I got a dc ssr on eBay but it was defective. Automotive relays are still serving me well. 4 bucks for a 12v40a. I’m running 20a silicone heaters off a 600w psu.
Seems like whatever you use should have a heatsink on it.
@Christopher_Gaul Including many SSRs
yes the one I posted has to have a heat sink
@Andrew_Radovich_BadW you don’t need a special transistor you can replace it by two of old one. It’s really better if you have high amp ratings to replace it by two.
MOSFETs for high amps have usually worst data’s compared to a solution with two lower rating MOSFETs.
But this board has two bad design flaws.
One is they do the whole Pwm wrong.
The right way is to measure the flowing amps on a shunt and set youre pwm signal and also you can easily add a maximum current. If it’s reached turn the MOSFET off till next cycle.
And magically nobody can blow his MOSFET by adding a bigger bad.
Next problem is missing heatsink. In my eyes that’s a no go, with no excuse.
The best advice in this case don’t put a new MOSFET in this place. Put the MOSFET on a heatsink and connect it via cable to the board
Connect to an existing solution of such a relay. To solve this problem you can use the road relay on 12 volts and 40 amperes
I had the same exact problem I use heatsink on my arduino(5v regulator changed to lm7805cv), or use jumper cables on input power(that uses only one power input from psu), the one next to usb there’s four cables going into ramps, just use right most input, get short cable and jumper from the rightmost power input cables, to the left, less problems but slower heatbed warmup, there’s a short on your ramp, it’s a temporary solution, make sure psu voltage is at 12v, or less but above 11,5v, permanent solution : change ramp, or trace the short (I traced mine, and it leads to my y endstop, resolder new pin, never had any problems, but my 5v regulator warms up…no biggie with heatsink)
The fet is 60 amps but you need sufficient heat think to run it at that current.
The problem with the original STP55N06L’s is that they were underrated for the original job, and putting any high load on them will cause lots of heat. They also have a very high Rds(on) value. FDP8870 or IRLB8743 or IRLB3034 have really low Rds(on) values (order or two of magnitude less than the nornal MosFET). I use all of these on various RAMPS boards with no issue, though I always add a small heatsink (usually just a strip of metal with a hole in it), just in case.
Also RAMPS is designed around REQUIRING a fan on it for cooling. If you haven’t got a fan on your RAMPS then you need one.
I would not put 2 STP55N06L’s in parallel as @Sebastian_Schmidt suggests, mainly because they’re getting harder and harder to source and two will cost you more than one of the others. Cheaper to buy one of the replacements I listed above. Feel free to buy any of the above solely on price since they are all quite suitable. Prices vary a lot from area to area, and supplier to supplier, so I can’t tell you what is cheap for you.
Also note: There were a bunch of fake STP55N06L’s at one point. They were relabelled devices that had an even higher Rds(on) and worse heat dissipation specs than a true STP55N06L. Made a bad problem worse.
FYI: The added heating from extra current causes a feedback effect that raises the resistance just enough to make the area even hotter if there is enough current. Too much current and it’s possible for the package to melt or even the solder joints.
If you really have lots of current (and given the size of the bed, quite possible), what @Sebastian_Schmidt says Re: FET on a heatsink and cables to the board. BUT, make sure those cables are damn good, as they will be carrying 10-14 Amps of current, and they will get warm if you use too thin a cable. You might look at the Gen7 Heated Bed external MOSFET board, as it has its own power inputs, etc. People have retrofitted this to a RAMPS board in the past no problems. And the FET the board recommends is one of the above.
@Sebastian_Schmidt Re: Method of driving the FETs with PWM: What you say is true but I don’t know of any of the RepRap electronics that do it this way. They all go the cheap route.
FWIW: When RAMPS was designed, heated beds were not expected to draw more than 10 Amps max. The reality now is that a standard 200mm x 200mm heated bed can easily draw 14A, and larger beds can draw much more. The screw connectors on your board are at most rated to 10A, possibly less depending on brand.
Q: Do you really know how much current your bed pulls?
@John_Ridley You do realise that there is no physical difference in size between the MosFETs? They’re all TO220s. The difference is in the internal specs, specifically Rds(on), which leads them to being cooler when passing the same amount of current.
@John_Ridley Heatsink is ALWAYS a good idea, but also as I note above, drawing more than 10A through that FET is outside of the specs RAMPS was made for. Simple answer: Do both.
Thanks everyone. @Joe_Spanier is hooking me up with a SSR and a spare board.
@John_Ridley Yeah most of my machines use 24V for that exact reason. Cable resistance makes less of an impact too, which is always a plus!
RAMPS isn’t quite 24V ready out of the box tho: The 11A PTC fuse is only rated for 16V, and some suppliers cut corners and use caps rated only to 16V. 
@Stuart_Young I’ve found the same. When the heated beds get more than 200 x 200 the RAMPS at 12 volts starts to bake. The other thing that can burn is the 11 amp input screw terminal. I use Rambos and 24 volts for anything larger.
So I now have the heater working with an SSR just fine. In the future, could I use the 24v setup on a 12x12 bed. with the azteeg x3 pro? The pro is rated for 15 amps and the 24v bed is 10 amps. Theoretically I can’t see why it would not work.
That’s how the taz works with the rambo
@John_Ridley That’s assuming full duty cycle for both beds which in practice isn’t typically part of the application. Except perhpas for 12 volt beds that need to be run at high temps. In real world use I’ve found the 24v heater on a larger bed (for example 320 x 260) heats faster than a 200 x 200 at 12 volts. Some of it is likely due to using an aluminum heat spreader on the big bed (with a kapton heater) compared to a PCB heater clipped to glass.
