QuadrapCoreXY added heated bed
Alirubber FTW!
finally found time to install the heater, it’s a custom made silicon heater from AliExpress. 330x300mm, 350W, 24V including thermistor and adhesive tape for 40$ (including shipping). It works great but takes quite long to heat up due to heat dissipation and the mass of of my build plate (350x350x6mm). I guess this is normal for a bed of this size. 350W might be a little bit to low, it reaches 90°C in about 10min, max is 100°C (thermistor reading). On the top surface I get about 90° (measured with cheap temp. measuring gun).
Much better than a cold bed, on the cold bed, ABS sticks really hard on the PEI sheet, some parts broke when I tried to remove them. With the heated bed they can be removed easily after they cooled down.
Electronics: I modified my ramps, replaced the heat bed FET (NR8) with this one: http://www.ebay.com/itm/271394098512?_trksid=p2059210.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AIT
It allows up to 80A. Of coarse I also had to replace the Fuse, I put a 25A micro automotive fuse to protect the electronics and the PSU.
The STP140NF75 is a very poor choice as a switching tranistor that is going to be driven by logic-level voltages (5V) from the Arduino. The Mosfet will drop at least 0,5V at the 15A your bed is rated for, thus producing about 8W of heat (which is a lot without active cooling) and will reduce your bed’s output power by around 5%.
Why did you switch it out in the first place? The stock STP55NF06L is already a good transistor for this application and is much more resistant against voltage fluctuations on the logic side.
@Thomas_Sanladerer ok, I see that’s why it gets so hot, so I think it’s better to switch back to the original. For some reason I thought the max current ID was not big enough but even @ 100°C it’s 39A (according to the data sheet) which is more than enough for 350W with 24V. Thanks for the input.