Bad RAMPS board?

When I set the temperature on the heated bed, the PWM led turns on, 15 seconds later it turns off, and won't turn back on unless I pull the power and wait for it to cool.
I assume the MOSFET is overheating.
The heated bed is about 1.4 ohms which seems like it ought to be in the acceptable range.
It's an assembled board from Lulzbot.
I tried a second RAMPS board and it works fine, so I assume it's a bad board, just checking to see if anyone else has seen it and there isn't an easy fix?

If you can repeat the process, even with resetting the RAMPS, a MOSFET overheat is unlikely, as an overheat would destroy the MOSFET.

The story sounds more like a faulty firmware.

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Generation 7 Electronics	Teacup Firmware	RepRap DIY

I don't think so because I just swapped the ramps board, not the arduino.
My assumption was there was some sort of thermal protection tripping, the heatsink on the mosfet is hot (though I'd expect that), as is F2, so maybe the resetable fuse is tripping, though it's supposed to take 22A before it trips and with a resistance of 1.4 Ohms and 12V it's only seeing 8.5A. Maybe it's the the fuse that's bad?

I'm having the same issue. Assembled board from Ebay. Fuse F2 heats up to 80°C!!!

The board and led turn on, but shut off after about 15 seconds. I'm using Pronterface with Slicer 0.7.1.

If I were to start replacing components should I replace Q3 and F2?

The markings on the F2 fuse are...

B
R1100
1180S

EDIT: I will try a high speed small fan tomorrow.

Edited 2 time(s). Last edit at 08/06/2012 09:05PM by iLikeWaffles.

I resolved mine by having a 80mm fan blowing across the fuse and stepper drivers.

iLikeWaffles Wrote:
-------------------------------------------------------
> I'm having the same issue. Assembled board from
> Ebay. Fuse F2 heats up to 80°C!!!
>
> The board and led turn on, but shut off after
> about 15 seconds. I'm using Pronterface with
> Slicer 0.7.1.
>
> If I were to start replacing components should I
> replace Q3 and F2?
>
> The markings on the F2 fuse are...
>
> B
> R1100
> 1180S
>
> EDIT: I will try a high speed small fan tomorrow.


Check resistance on your heated bed maybe is too low or it have some shortcut (scratches) I saw so hot F2 fuse only when I connected over 24V and then ampers consuming was ~20A but F2 and mosfet didn't turn off just temperature rise so high then after 5-10minutes mosfet melted plastic (I bet there was over 200C).

10 amp is the max fore the head bed i think.

michel pet Wrote:
-------------------------------------------------------
> 10 amp is the max fore the head bed i think.

It is up to manufactured board and resistance on it mostly is 8-12A I tested it in our university lab and power supply which showing 'A' consuming this have a lot options (heavy thing almost 20kg) with 20-24V heated bed easily take 20A so if resistance on heated bed is very low (less than 1ohm) then I think heated bed can take around 13-14A at 12V and whole things (mosfet and possibly F2) need additional cooling but that time I concentrate on testing heated bed, PCB and mosfet temperature and I forgot to check how F2 react in such cases. My heated bed is 1.1ohm - this give measured at 12V - 11A at start and later this slowly going down and then is around 9A when heated bed is hot.

My heated bed is 0.75Ω so it pulls about 16A. I installed 2 high velocity fans around the RAMPS and everything is fine. I was able to heat the bed to 110°C for 1 hour continuous with no problems.

My entire machine pulls 21A when everything is turned on. This is fine because I have a 30A medical grade power supply.