Melzi + Cooling fan = failure?

I had just assembled my RepRapPro Huxley last week, which came with a Melzi 2.0 board. I had been unable to get anything to really print decently even after setting the e steps and trying a bunch of different hot end and bed temperatures, so I tried using a cooling fan to see if it helped. I wired a standard 80mm computer fan to the board's fan connector and this is what happened:

The hot end heated up to the printing temp, when for some reason the fan attempted to kick in, which caused to board to momentarily power off, as if I had bumped the power cord. I had pronterface reconnect with the printer, set the hot end temp back up, but the hot end dropped in temperature and now no longer heats up. The multimeter reports that the heater resistor's resistance seems to have increased by about 1.6 million ohms.

I'm not sure what happened here.

Hi HyraxAttax

Try measuring with your DMM how much volts the heater leads output when you turn on the heater. The fan could have been too heavy for the board, but i'm not familiar with the Melzi
It seems like your heater resistor has been blown, because it would normally have a resistance of around 5 Ohms
Unhook the fan, measure the voltage across the heater leads and try a new heater resistor.
Also check if the fan is still in one piece and if possible measure how many amps it takes.

Problem resolved. The resistor probably had an impurity in one of the leads, as it snapped off near the base when I tried to pull it straight out of the block. The new resistor works fine, as well as the fan.

...or not. First time powering up the heating elements in three weeks. I first set the heater to 185 and loaded insome pla filament. Temperature settled to that point. I then set the bed to 70, and as it heated up things suddenly went wrong. The hot end spiked up past 250, so I turned both off through pronterface. The hot end continued going up and went past 400 before I could pull the plug, at which point black smoke was coming from the hot end.

I disconnected the hot end from the board and powered it back up so that the heatsink fan might prevent the PLA parts from melting down. The damage appears to be limited to a charred resistor and a warped heat-shield thing for the tip. The multimeter didn't detect any shorts with the hot end wires and any others in the cable that leads to the hot end, and the resistor now reads out as 7.4 ohms instead of 6.8. I have spare heating resistors, but I get the feeling that isn't going to help at this point.

Between replacing the resistor and leaving home for the break, the printer worked fine with no issues.

If there's a better place to ask Melzi questions, let me know. I posted here because the information on Melzi said it was based of the Sanguinololu.

Hi,
I'm having a bad time as well.
On the fan terminals (FAN+ and FAN-), i get nothing, no matter what.

On the terminals (FAN+ and a common ground, not the fan ground), i measure 0 to around +4.8 volts. (depending on PWM output in Repetier 0.83 or 0.91).
See attached photo.

But when i connect a fan to it, i get less than 1 volt, and no fan spinning....

Any ideas?

Thanks in advance,
Quint

Edited 1 time(s). Last edit at 01/13/2014 02:51AM by qboney.