[RAMPS] Fuse overheats when doing dual extrusion print

Hi all,

I'm having a prusa-styled printer using RAMPS.
I'm building a dual extruder setup with heated bed and print fan.
Since RAMPS only has 3 heater output, I branched the third output to a switch to select either fan or bed (since printing with heated bed generally don't require print fan and vice versa).

So, the problem is, when I am trying to do dual extrusion printing, my RAMPS fuse seems to overheat that kills the printer.
Here I attach the temperature graph.


Please note that the bed output is not really bed in this case, it was the fan.

The weird thing is, when I'm printing with only either extruder, it prints fine.
I was printing with both some other time, printing some dual colored dice and it's fine.
But when I'm printing this dual colored vase (I haven't tested more with any other model), this happens.


The primary cause of the print abortion from the error message from the printer is this :
"One of the heater seems decoupled, temperature rise too slow. Raised -19 deg C in 13s" or so, I didn't save the text.

Yesterday I suspected that one of the heater open circuited since the temperature drops. I didn't always touch the fuse, but when I realized that it was hot, it was because of the fuse overheats in the first place that disabled all the printer functionality that in turn caused the heater not heating anymore. So, then I suspected that the heater is short-circuited.
Also, none of the FET are hot.

Buut, I said that printing with only either extruder is okay. The test print I made is just 4 cubes positioned on each corner so that the prints span all directions (so that if it was really connection problem, that should do).

I now have no other idea what should be the possible cause.
Any help or opinion would be greatly appreciated.
Thank you.

Edited 1 time(s). Last edit at 10/06/2016 09:13AM by Vashikovich.

Add cooling fans to your RAMPS?



Or replace those fuses with standard blade fuses?

Very nice wiring Veesta! I really must tidy mine up, it's an embarrassment in comparison.

I think I remember reading that once a PTC fuse has activated it doesn't recover completely and so is more likely to activate again. I haven't had any problems running dual extruders through ramps, but I think the first time the PTC fuse trips out I'll replace it with a blade. I already run heatbed power direct from the PSU via an in-line blade fuse holder, which helps prevent the ramps board overheating at the power terminals.

Quote
JamesK
Very nice wiring Veesta! I really must tidy mine up, it's an embarrassment in comparison.

No no, this is not mine. This image is borrowed from Toolson collection of P3steel parts.
[www.thingiverse.com]

I have that same bracket thou. Nice, orange with a spider web of wires around...