[3D-Print] Geeetech prusa i3 – improvements

Last year i bought a cheap chinese prusa i3 clone for €270 including VAT and shipping which performs very good overall.

After assembling it – which took me about 8 hours – its print results was.. well.. quite okay.
As expected, because reading forums and blogs already explained Z wobble to be a serious problem with this printer.

So the first thing I’ve printed the already posted “Z wobble fix” which came out wobbly of course, but works well :)
I’ve also decoupled the Z axis from the frame by (gently) drilling a bigger hole in the top part of the printer so it can move freely.

antiwobble

anti wobble z axis screw

Then I also added a ball bearing (608Z) “decoupler” to the top end of the thread so it is not moving too much and is guided a bit better.

Aside of that, I replaced the too small Z axis rods with a proper one and the Z axis threaded rod with a TR8 trapezoid one.

After some printing, i wondered why the MOSFETs get so hot. They used P55NF06 with a quite okay R_DS(on) of 0.018 Ohms. But more problematic is the V_GS which is with 5V just barely above the threshold voltage.
This causes an resistance of far more than the 0.018 Ohms which you will only get at a V_GS of 10V. The result – the MOSFETs get hot.

vgs

The P55NF06 is at it’s minimum switching voltage when driving it with 5V on gate

Solution? Buy some IRFB7430 which have only 0.001 Ohms R_DS(on) at 10V and ~0.013 Ohms at 5V.

Hard to see, but the second curve from bottom (4.8V) tells us that when driving a 20A load, the voltage drop at the MOSFET will be ~0.25 Volts. Thats approx. 0.013 Ohms internal resistance

As these parts are a bit pricey, I’ve chosen to take the cheaper IRF2804 for the hotends. They still provide a low resistance with ~0.015 Ohms and are good enough for hotend heaters.
Probably also good enough for the heated bed :)

But..

before I was able to swap them, a really bad thing happened – my printer heated permanently until it smoked and i powered it off.

First let’s see how the GT2560 board looks like:

no8a1591_corr

Now when looking at the heat sink of the MOSFET, one will realize that they have

a) a sharp corner
b) are electrically connected to the MOSFET with a metal screw and without any insulation pad
c) (no, you can’t see that on the photos) there is a PCB trace *exactly* below the sharp corner of the heat sink, prone to get scratched

no8a1591_corr_small

Please ignore the insulation tape I placed there just as a quick hack to get things done.

After a while the PCB was scratched and the heatsink made a short circuit with a trace on PCB, permanently heating the hotend.
Yay. worst case. Even marlin’s “thermal runaway”-protection couldn’t do anything as they are short-circuited on board.
So the result was an overheated hotend with damaged PTFE inliner, died MOSFETs and me, a bit scared of this really bad engineering.

No fire tho, but there was smoke at least.

Okay to be honest, when paying 270€ for a whole box full stuff you can build a printer of, don’t be surprised if that is not at IBM quality level.
But there is a difference if a connector is of cheap quality or a rod is bent, or if a PCB designer doesn’t realize that a sharp-edged aluminum block just micrometers above a PCB trace that might cause malfunction and finally a fire is a frickin’ bad idea.

Solution:
a) now really buy the MOSFETs i already had chosen
b) along with insulation pads and plastic screws
c) and put some {glue, hot glue, epoxy, cyanacrylate, whatever} on the PCB below the heat sinks (i used a special solder mask pen)
d) put a smoke detector above the printer

Recommendation to all users of a GT2560: ALSO FIX THIS ISSUE, it might cause the printer to catch fire.

The wonderful side effect: you don’t need the devilish loud mainboard fan anymore.

After these changes, my printer works like a charm :)
It happened in october ’16 and geeetech sent me a second mainboard just days later so I could fix the other one properly.
(thanks for this really fast response and good service)

 

I hope this warning reaches out to all “cheap chinese printer” users out there.

2 Responses to “[3D-Print] Geeetech prusa i3 – improvements”

  1. [3D-Print] Custom geeetech i3 printhead and some other stuff at g3gg0.de Says:

    […] [more here] […]

  2. Where There’s Smoke, There’s a Knockoff 3D Printer | Hackaday Says:

    […] These days, it’s possible to buy clones of popular 3D printers from China for satisfyingly low prices. As always, you get what you pay for, and while usable, often they require some modification to reach their full potential. [g3ggo] recently laid down €270 for a clone of the Prusa i3 by Geeetech, knowing it would require some modifications for safety and performance. […]

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