MK2S MMU: What limits system voltage?
I'm having a hard time maintaining high bed temperatures in an enclosure with a heater and fan in it (targeting enclosure temperature of 55 C, trying to print ABS). Yes, I'm getting the bed runaway temperature error when it droops to 105 C for a while.
Looking at the schematics for the Mini RAMBO, it seems like nothing on it will be sad until about 30 volts. At that point some TVS diodes will start breaking down. And the little fans on the extruder seem rated to 12 volts, but those are easily replaced.
Is there anything else keeping system voltage down? If I replace the fans, could I run at 28-30 volts? Come to think of it, I have no idea what's inside of the multiplexer box that seems like the only bit of electronics that differentiates the MMU from the standard unit.
Re: MK2S MMU: What limits system voltage?
I found the schematic for the stepper switch:
https://github.com/prusa3d/MK2mm_Stepper_switch/blob/master/stepper-switch.pdf
It looks like it only touches the 5 volt regulated rail. It seems pretty safe to just unplug the stock supply and run in a higher supply voltage, provided I replace the fans. But that looks easy.
Sure the resistance of the heat bed is "tuned" for 12 volts, but the FETs on the Mini RAMBO seem plenty big enough to handle the higher pulse currents of running the bed at 24 volts.
Re: MK2S MMU: What limits system voltage?
I just hate to see people destroying decent equipment, so I feel I must interject...
The heated bed is designed for 12 Volt operation, as is the extruder heater, the fans and the stepper motors.
The bed has a nominal resistance of 1 Ohm. That means at 12 V, it will draw 12 Amps and generate about 144 Watts of heat. At 24 Volts, it will draw 24 Amps and generate about 576 Watts, which means it will destroy itself. As will the extruder heater, the fans and most likely the stepper motors, although not necessarily; you will probably blow up the power supply first.
What you need to do is to check your bed heater cable and RAMBo connector. It you want to increase the power of the heater, there is a pot near the outlet on the PSU which can be used to adjust the voltage up to about 13.5 Volts which will not do any damage, but will give a significant increase in bed heater power.
But really, check the cable and connector first; if you burn the connector, you may need to purchase a new RAMBo.
Peter
Please note: I do not have any affiliation with Prusa Research. Any advices given are offered in good faith. It is your responsibility to ensure that by following my advice you do not suffer or cause injury, damage…
Re: MK2S MMU: What limits system voltage?
The bed has a nominal resistance of 1 Ohm. That means at 12 V, it will draw 12 Amps and generate about 144 Watts of heat. At 24 Volts, it will draw 24 Amps and generate about 576 Watts, which means it will destroy itself.
Only if the FET or control loop fails closed. The bed will not fail in an instant. Plus, it doesn't look difficult to replace (e.g., buy the one from the Mk3) in case of cascading failure. I'm certainly not going to leave the stock power supply plugged in while increasing the voltage so high. I think I have something almost the same size and shape that I could bolt on with a different mount. Or alternatively, I could leave the existing power supply in place to be a support bracket while I run all the wires elsewhere.
But really, check the cable and connector first; if you burn the connector, you may need to purchase a new RAMBo.
It seems unlikely that I'd damage the connectors, but those are easy enough to replace without replacing all of the electronics. My garage is a well-stocked electronics lab.
I am, however, quite curious about the steppers. I have very little experience engineering around stepper motors. The drive circuits look like constant current controllers. If anything, I'm more likely to burn the controller chips than anything else. It wouldn't be outrageous for me to supply 12v for those and 24v for everything else.
Re: MK2S MMU: What limits system voltage?
Do a forum search for "burnt connectors"
The connectors on the RAMBo have been a problem since the Mk1. A poor connection creates a high resistance, beat and burning. The best solution to by-pass two mechanical joints is to solder the bed cables to the underside of the RAMBo.
There's no need to mess about swapping PSUs; increasing the voltage slightly is tried and tested.
And yes, the steppers are current-limited; honestly, I have no idea what would happen if they were fed with 24 Volts.
Peter
Please note: I do not have any affiliation with Prusa Research. Any advices given are offered in good faith. It is your responsibility to ensure that by following my advice you do not suffer or cause injury, damage…