Monday, May 20, 2013

RepRapPro Mendel (Part 6)

I haven't had very good luck with the Melzi board. I burned out the drivers and MOSFETs on my first board (I've never been able to determine the cause). Now I have a burned out heat bed connector. I’m primarily extruding ABS which requires a higher temperature for the bed and I’ve been running the printer for very long periods of time. This may be the reason for this burn out. I’m not
entirely confident that the Melzi board is robust enough for 24 hour runs.

Unfortunately, a simple replacement of the connector isn’t going to work in this case. The burn destroyed the PCB tracing and makes it impossible to solder a new component. In order to have a working bed, I rewired the power leads directly to the power supply and added a switch for manual control. Since no harm has come to the thermistor, I can at least monitor the temperature. This seems to work fine, but I’ve lost automatic control.

I hesitate to buy another Melzi board anytime soon, but if I do I may reinforce the bed connection. A seller of a Sanguinololu board I purchased recently has a recommendation to do exactly this.

The Melzi board is not the only area where I’ve had problems. I’ve also burned out 2 power resistors on the hot end. Once again, I think long periods of operation has taken its toll, but I also suspect that possible damage during installation may also be a factor. Even with additional boring of the hot end’s heat sink, it’s very difficult to install the resistor in the cavity without physically forcing it. This scrapes the surface and causes damage.

Top resistor is the original supplied in the kit and the bottom resistor is from Mouser

The resistors are just too big and coupled with a slight curve in its body, it makes for a difficult fit. I've found a different resistor that works better. It’s slightly narrower, but with a couple wraps of Kapton tape, it makes a snug fit. It can be found at Mouser Electronics:

MFG Part No:RS02B2R700FS70
Vishay Wirewound Resistors

The resistor fits snug from end to end with a couple runs of Kapton tape and a little heat transfer compound