I found serious design error in MMU2 electronics hardware  

RE: I found serious design error in MMU2 electronics hardware

Posted by: @heine-mau

I am a Bachelor in Electronic Engineering, with 15+ years experience in MCU HW/SW....

I have had a lot of problems with my MMU2. Before i upgraded my I3MK3 was perfect 🙂  but the MMU2 changed that...

The problem I found is that the 5V supply on the MMU PCB is only 4.2Volts - witch is too little for the MCU/stepper drivers.

The problem can be reduced by attaching some USB host to the MMU - only as extra power supply (no need for software on the host). That raises the +5V by a 50-100mV in my case.

As a proof f concept i made a little risky mod on my MMU2 PCB to test my theory: Shorted D1 and added 1000uF from this short to ground. Since i did that i have had a very well working MMU2. With a 4.8Volt on the +5V supply net.

 

Somebody at Prusa should take a look at this issue.

 

...Your welcome 🙂

 

Hello and welcome to the Prusaforum!

Thanks for the information!
Would you provide a small schematic/pictures to your modification (diode, capacitor)?
Thanks!

RE: I found serious design error in MMU2 electronics hardware

Sounds interesting, is this the plain MMU2 or the MMU2S?  I wonder if it was fixed in a later revision?

RE: I found serious design error in MMU2 electronics hardware

Posted by: @heine-mau

I am a Bachelor in Electronic Engineering, with 15+ years experience in MCU HW/SW....

I have had a lot of problems with my MMU2. Before i upgraded my I3MK3 was perfect 🙂  but the MMU2 changed that...

The problem I found is that the 5V supply on the MMU PCB is only 4.2Volts - witch is too little for the MCU/stepper drivers.

The problem can be reduced by attaching some USB host to the MMU - only as extra power supply (no need for software on the host). That raises the +5V by a 50-100mV in my case.

As a proof f concept i made a little risky mod on my MMU2 PCB to test my theory: Shorted D1 and added 1000uF from this short to ground. Since i did that i have had a very well working MMU2. With a 4.8Volt on the +5V supply net.

 

Somebody at Prusa should take a look at this issue.

 

...Your welcome 🙂

 

This are serious fake news!

@heine-mau

Sit down again and check your knowledge (Bachelor? 🤣 ).

Greetings, Holger 🙂 

RE: I found serious design error in MMU2 electronics hardware

@heine-mau

Here is an excerpt from the schematic of the MM Control. The + 5V are generated by the 3 stepper drivers TMC2130 as soon as the + 24V are present. The other IC (MCU...) only need at least 2.7V.

Datasheet TMC2130.

RE: I found serious design error in MMU2 electronics hardware

@holger-2

Please contribute with some serious information.

RE: I found serious design error in MMU2 electronics hardware

@holger-2

No, it could be generating the +5 but it is not. It would even be a logical choice to generate the +5V from their internal regulator.

But if you study the TMC2130 datasheet you will see that this configuration is not generating any +5V.

VSA has to be supplied with >6V to do this - but it is connected to +5V.

RE: I found serious design error in MMU2 electronics hardware

@3delight

I am not sure - but i purchased it directly from Prusa in august 2020.

I think the pcb reads version 0.4.4.

I was only able to find a "rev.0.3" schematic, but it seems to be very close to the same (i checked many details which were exactly the same)

RE: I found serious design error in MMU2 electronics hardware

@karl-herbert

i refer to the this schematic: https://github.com/prusa3d/MM-control-2.0

i do not think people should copy this "proof-of-problem"...

This diode was put in there for a reason, and i do not understand the entire setup (e.g. startup/shutdown sequence of the entire printer)

But it was a quick way of raising the +5V to something close to 5V.

I just wanted to tell that this was a problem on my printer/mmu - hoping for Prusa to create a real fix.

The capacitor is probably not needed C59 seems fine. I just saw a weak power supply, a MCU which resets all the time - this is sometimes created by noise on the MCU supply combined with the BOD. (and i was tiered of messing around with the MMU)

RE: I found serious design error in MMU2 electronics hardware

@heine-mau

Yes, VSA is bridged with VCC, so the internal voltage regulator is not used. The diode D1 could be bridged to raise the + 5V. The MCU (ATmega32u4) also needs 4.5V to work at 16MHz. The current design is therefore not certain. You are right, thank you very much for your hint.

greetings, Holger 🙂 

RE: I found serious design error in MMU2 electronics hardware

Prusa Staff rarely if ever come on this forum.  To report this to them you need to go to:

https://github.com/prusa3d/MM-control-2.0

and click on Issues at the top, the New Issue.

RE: I found serious design error in MMU2 electronics hardware

@heine-mau

How can an electronics layman solve the problem in this case without having to intervene directly on the board?
Thanks!

RE: I found serious design error in MMU2 electronics hardware

@karl-herbert

I would suggest a 5.2V USB charger, which you connect to the usb connector on the MMU by means of a micro usb cable. That will bring you just inside spec's.

Get a  "5.2V usb charger", the Amp rating does not really matter, you just need at least 0.2A.

The only issue with this solution is:

1. You should turn on the printer first, then wait a sec before turning on/connecting the usb charger.

2. You should turn off/disconnect the usb charger before turning of power to the printer.

This procedure above is the safe way, but i doubt that you will fry anything if you forget to follow it.

BR Henrik

RE: I found serious design error in MMU2 electronics hardware

@heine-mau

Thank you for the information 👍 

RE: I found serious design error in MMU2 electronics hardware

@3delight

Thank you for the suggestion.

I just created issue #15 on github.

RE: I found serious design error in MMU2 electronics hardware

@heine-mau thanks, staff will get to see and assess it now...

RE: I found serious design error in MMU2 electronics hardware

@Heine Mau

The devs have noted it and will look into it as and when they can, obviously they are very busy so please don't necessarily expect a 'quick fix', but thanks for alerting them to the issue.

RE: I found serious design error in MMU2 electronics hardware

Posted by: @3delight

@Heine Mau

The devs have noted it and will look into it as and when they can, obviously they are very busy so please don't necessarily expect a 'quick fix', but thanks for alerting them to the issue.

Seriously? Does anyone care? The project has not seen an update since Aug 2018...

RE: I found serious design error in MMU2 electronics hardware

@robin

Yes! I have a MMU2S sitting in the box to be built. I have put off building it, in part, because of the issues that have been reported. Some are between the chair and the keyboard while others appear to me with the hardware. And if someone like @heine-mau is kind enough to share his knowledge, experience, and findings with us, we should be thanking him and not dismissing it. @heine-mau does not have to share anything he finds, but if there is one member that can reliably print multi-colour prints with a MMU just by adding a USB charger, I am sure that that member will be glad that this thread existed. I will most likely start my MMU printing journey with the  added power.

RE: I found serious design error in MMU2 electronics hardware

@heine-mau

Thank you for sharing this with the community. I care when a solution or workaround is provided by members like you, who are willing to spend their own time researching/testing (even to the extend of possibly frying the electronics) to make everyone's 3D printing just a little bit better.

Kenn