Heatbed not functioning normally (MOSFET overheats)

[jaapschaap]

Hi,

I contacted the seller on aliexpress where i bought a Geeetech Prusa i3 Pro B, he said i needed to ask here for quicker help, from an engineer.

My question to him:

My heatbed is not functioning normal. Didn't use it since the Sanguinololu got way to hot, and with PLA it's not necessary.
But now i wanted to start with ABS, but i can't because the heatbed makes the controller way to hot (even with a 30*40*40mm server fans which has 5 times more airflow then the provided one).
If i search the forum of geeetech i see it should be at least 1.2Ohm:
http://www.geeetech.com/forum/viewtopic ... 418#p24186
I measured mine and i get only 0.2Ohm, so it draws 6 times more current then it suppost to do (i checked for short circuits, but can't find any)! Can u sent me a replacement?
Kind Regards,
Stefan

I can give you my ordernumber, but don't know if i should post that in public...

[zhangxueyou]

Normally, when heating the bed, the temperature of MOSFET will be very high even you have a fan.
How did you test the resistance of the bed? Is it like this:

test hot bed resistance1.png (395.11 KiB) Viewed 8095 times

test hot bed resistance.png (355.08 KiB) Viewed 8095 times

[jaapschaap]

Yes i did it exactly that way, i saw your pics in an other topic.

I read out between 0.2 and 0.25Ohm. I can make a picture if you want, but at the moment i have a 12+ hours print job so i can do it later on...

[zhangxueyou]

If it is the value you get after you unplug the heating wires, this hot bed is abnormal. Please contact the customer service, let them re-send you a good one.
How are other parts? Are they fine?

[jaapschaap]

If it is the value you get after you unplug the heating wires, this hot bed is abnormal. Please contact the customer service, let them re-send you a good one.
How are other parts? Are they fine?

Yes other parts are great, i'm printing a lot and all goes fine...

Wicht department should i mail? And can i refer to this topic?

[zhangxueyou]

Tell the people who sold you the printer. Ofcourse you need to refer to (quote )this topic. Let them know this hotbed is not good indeed.