The DC motor used on the MK5 Extruder head seems unusually prone to sudden death, either by mechanical failure or something electrical. A stalled or shorted DC motor becomes a low resistance that destroys the A3977 H-bridge driver chip on the Extruder Controller board.
Makerbot recommends inserting a 10 Ω power resistor in series with the motor, so as to limit the maximum current. Other folks have build a diode decoded relay driver that’s certainly more durable than the A3977.
The power resistor reduces the voltage available to the motor, which draw something like 40 mA when unloaded and up to maybe 250 mA at full load. I don’t know what load the extruder puts on it, but at 100 mA the resistor drops 1 V, which seems excessive.
The relays seem like a nice solution, but they go clickety-clack and require actually building something, of which I’ve had quite enough lately, thank you very much.
While I was mooching those lugs, my buddy Eks suggested simply putting a low-wattage 12 V incandescent lamp in series with the motor. The cold filament has a very low resistance, but limits the current
when if the motor shorts out.
A bit of rummaging in the Lamp Box produced an old automotive #89 lamp that allows 560 mA into a dead short, which works out to 7 W.
If the motor draws 100 mA, it drop only 100 mV: good enough!
Not finding a suitable socket in the heap, I wired it in by soldering the wires directly to the brass shell and central solder tip and taping up the mess. Next time I get near the local AutoZone I’ll pick up a socket.
The Anderson Powerpoles may look like overkill, but they make life a lot easier when you’re fiddling with the machinery all the time.
Now, the lamp won’t prevent inductive transients from blowing away those puny signal-level Zener diodes that should protect the A3977 chip, but it’s exactly what you need for long-term overload prevention.
[Update: An experiment that shows why a lamp works better than a fuse.]
As an added benefit, when the bulb lights up you know the Extruder Motor has died!