Extruder temperature rises unboundedly high

pmyx

Time to time heating of the extruder becomes uncontrollable. I'm using ramps 1.4 setup with repetier firmware 0.92.
The tasks are generated with Slic3r and the problem appears during printing either via Repetier-host 1.62 or directly from the sd card

is it a hardware issue or should I play with PID control?

Repetier

Image does not work.

At start of print firmware waits for target temperature. During that time you can not control anything until heating is finished, that would  be normal. Depending on your settings you will see a overshoot at first heatup which can be influenced by pid settings by temp. control range and I think the P part would have big influence also higher damping D might work as well.

pmyx

Thank you a lot!
Usually the problem appears when some time has passed. In case the the temperature plot i'm trying to attach isn't available here is a brief story: for some time everything is ok: the temperature stays around target temperature +/-2deg, then it suddenly begins to grow, and pwm bars seems to respond properly by decaying to zero, but the extruder keeps heating up anyways
Have you heard some similar stories?
http://www.picshare.ru/view/7599669/

Repetier

Might be a defect mosfet on your board. From pic I see firmware tells it to not heat but temp. rises smoothly so reading is ok but it heats without being enabled.

So you have either a short in sending pins and an other pin also can heat the mosfet or it is just defect and sometimes decides regardless of input. Try the other mosfet for extruder and see if it also happens there. Be aware the D9 is also fan output so don't enable fan feature or select other pin for fan.

Alternative reason would be an other function uses the pin as well e.g. fan and when fan goes 100% it enables heater instead. But that normally gives problems during heating at the beginning whcih you do not have, so I don't thinkit is the problem.

pmyx

Thank you again! This is very helpful