Quote
waitaki
Quote
ndujar
Hmmmm..my only advice against RAMPS is how it deals with its MOSFETS...it has a flaw in design that makes them enter in resonance with the hotbed and, unless they are quite powerful, they tend to fry:
[*] [www.buildlog.net]
[*] [www.soliforum.com]
[*] [forums.robo3dprinter.com]
I have one myself and got it burnt in the first day...actually is just a matter of luck whether your provider has assembled the correct type of MOSFET or not. So, just for precaution, I would warn you to get some with the specifications of any of the above links, just in case.
However, for a first printer, an even better solution is to avoid ABS and focus in PLA. As PLA doesn't require of a hotbed you will be safely printing for a long time.
Happy printing! :)
What?
Yes, it is all explained here: [spainlabs.com]
For those who don't understand Spanish:
Resulta que las salidas de la RAMPS controladas por mosfet no llevan ninguna diodos de flyback en antiparalelo para absorber los picos de tensión que aparecen al conmutar cargas inductivas. ¿Dónde hay bobinas en la impresora 3D? Pues la cama es bastante inductiva (aunque no exageradamente) por el hecho de ser una pista larga, los propios cables tienen inductancia parástita y la resistencia que calienta al hotend es una resistencia bobinada. Meter pulsos de gran corriente a cargas inductivas sin protección es muy mala idea. Pensaba que la inductancia no sería muy grande y, aunque no deba hacerse, habían dejado los mosfet sin protección para ahorrar costes.
Pues bien, me ha dado por mirar si alguien había medido las inductancias parásitas de los elementos calefactores y efectivamente en reprap hay alguien al que le dio por medirlo e incluso simular los transitorios al conmutar:
[www.reprap.org] ... th_MOSFETs
Resumen: se provocan picos de tensión en el MOSFET de cerca de 70V. Más de 400 veces por segundo. [...]
means:
It happens to be that the outputs of the RAMPS controlled by MOSFET have no flyback diodes in antiparallel to absorb the tension peaks that appear when inductive loads are commuted. And where do you have coils in the 3d printer? Well, the hotbed is pretty inductive (although not dramatically) just by the fact of being a long track; wires have some parasite inductance and the resistor of the hotend is a wound up resistance *. Hitting inductive loads with high current, and doing it without protection is a very bad idea. I thought that this inductance would not be very big and, although it shouldn't be done, by design they left the MOSFET unprotected to save costs. But then, I though of having a look in case someone else had it measured, and there it was:
[www.reprap.org] ... th_MOSFETs
Summarizing, we are creating tension peaks in the MOSFET around 70V high. Over 400 times per second. [...]
To me seems like a reasonable explanation...I didn't have the opportunity to check the proposed fix in that page but certainly I burnt three MOSFETs of the hotbed in that RAMPS of mine...