Heatbed stopped working  

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magnus.e
(@magnus-e)
Active Member

Hi.

I upgraded my i3 mk2s to mk2.5s a few weeks ago. The otheer night i started a ABS print with heatbed at 100c. After a about an hour the print stopped with heatbead thermal runaway. After that my heatbed wont heat up.

What i have done to check whats going on.

1. Checked the 15 amp fuse on the rambo = OK

2. Checked that i get 12v from the power supply on the heatbed cable = OK

3. Disconnected the heatbed from the rambo, and checked that the circuit in the heatbead was 'continious' = OK

4. Disconnected the heatbed from the rambo, and checked that there was no short circuit  on the rambo = OK

5. Started heating heatbed and cheecked that there was 12v on the heatbed terminals = FAIL, no current

6. checked that termistor works = OK

 

When i start heating the bead there is one single blink on the red led, but not possible to measure any current on the terminals. About one year ago the connector for the heatbed on the rambo got fried, so the heatbed is soddered to the rambo, but this has been working without issues for all that time. The connector from the powersupply to the rambo looks ok. I have tried 'wiggeling' it to see if i get a better connection, but it does not seem to be the problem.

I'm not that skilled in electronics, but i guess there is a mosfet for the heatbed? On the rambo is there a mosfet i could replace? It does not look like there is anything easly replacable???

 

Hopefully there is a way to fix this, preferably something easy that i missed 🙂

Thanks in advance, Magnus_e

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Posted : 27/01/2020 11:07 am
magnus.e
(@magnus-e)
Active Member

Hi.

So now i have made a few more tests.

1. I disconnected both the heatbed wires from the heatbed and mesured the cable to make sure its not broken = OK

2. I checked the rest of the fuses, just to make sure = OK

3. With the printer powered on and the heatbed disconnected, I mesured the circuit for the heatbed = OK, mesured 0.5v

4. -------------------------------||------------------------------------, I set the heatbed to a higher than ambient temp and mesured = FAIL, 3.7v

 

Looks to me like the heatbed is only geting the logiclevel high '3.7v', without any 12v amplification?

Is this a MOSFET 3.7v to 12v boost converter that has failed?

 

I just upgraded this printer from mk2s to mk2.5s two weeks ago as i need to do a lot of prototyping before springime.

I do not see the RAMBO or any MOSFET in the prusa3d store, and i'm not sure what MOSFET i should by to replace the broken one, (If it's accually the MOSFET thats faulty).

 

I really need some help here.

Thanks, slightly desperate,,, Magnus_e

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Posted : 01/02/2020 12:53 pm
Hyta;
 hyta
(@hyta)
New Member

Hi. I have the same issue like you. I decided order new RAMBO. So I hope it solve this issue.

Posted : 16/02/2020 10:49 am
ian.s3
(@ian-s3)
Eminent Member

Info on mini rambo is at https://reprap.org/wiki/MiniRambo which has schematics.  Heater circuits are on schematic page 4.  Looks like the heated bed is a simple MOSFET pulling one side of the heated bed to 0V by Q5 which is a PSMN7R0-60YS which is apparently a single N-Channel 60V 356A 117W 9.3mOhms MOSFET, which looks to be near teh 5A fuse which is near teh 15A fuse.  At least, that's true of the version shown on the website.  There's no boost circuit, just a simple MSOFET switch switching the 12V from the PSU.

I'd try and eliminate lots of other stuff before assuming it's that though - I'm not sure what would make a MOSFET minding its own business blow, and desoldering and replacing it will be fiddly and as likely to damage something else as fix it.

I don't understand some of the diagnostics you report - you say "Checked that i get 12v from the power supply on the heatbed cable = OK" but then "checked that there was 12v on the heatbed terminals = FAIL, no current", but were you checking voltage or current?  With the board powered and everything connected  but bed off both heatbed terminals should be at 12V, and when it switches on one should get pulled down to 0V.

All with the heavy caveat - assuming that web site accurately reflects your hardware.

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Posted : 16/02/2020 10:29 pm
magnus.e
(@magnus-e)
Active Member

@ian-s3

"Checked that i get 12v from the power supply on the heatbed cable = OK"  Means that the cable coming from the psu to the rambo has 12v

"checked that there was 12v on the heatbed terminals = FAIL, no current" Means that there was not 12v on the heatbed itself.

As I sad, my skillset is not optimal for debugging this. So I went to 'build night' of my local makerspace, and got a lot of help. We debugged the mosfet, and there was nothing wrong with it. If it had failed there would likely be a short between signal and drain, but there was not. However the MOLEX plug from the PSU to the Rambo was faulty, so I was simply not getting 12v to the heatbed circuit of the rambo. When I measured that I got 12v from the psu at home, I measured crudely at the tip on the psu 'female' side, but the inside of that connector was toast, and did not pass on the current to the rambo.

 

Now I have replaced all the molex plugs for screw terminals, and all is good. Printing a 3dHubs Marvin as we speak to verify, and no issues 🙂

 

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Posted : 18/02/2020 7:16 pm
ian.s3
(@ian-s3)
Eminent Member
Posted by: @magnus-e

Printing a 3dHubs Marvin as we speak to verify, and no issues 🙂

 

Glad to hear it!

Yes when a MOSFET fails it normally fails to something shorted (you blow holes in the internal insulating structures and let electrons through), but it's not universally the case.  That MOSFET looks like it's well within specc, which was one of my reasons for doubting it as the cause. 

With screw terminals passing high current, it's not a good idea to have the clamped wires soldered.  You mentioned that you'd previously had wires soldered (but I'm not sure which).  If the wires under the clamping screw are soldered it would be wise to check they remain tight - it's not something you need to check every day, or probably even every month, but the wires can relax and you can get a bad connection that starts to get hot, and that can lead to a fire.  If you don't have soldered wires under the screw clamp (or can re-arrange your wiring to avoid it) that's more reliable, long-term.

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Posted : 19/02/2020 2:46 pm
magnus.e
(@magnus-e)
Active Member

@ian-s3

I cut the wires with the sodder back to the copper, so they are clean. 

After printing the marvin, I desided to redoo xyz calibration, as I have been moving the printer arround. Now I get a calibration failure on the 3'rd point. I see that others have experienced it on the mk2.5s. 

It is However totally unrelated to the original issue, so I will read up on others experience 🙂 

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Posted : 19/02/2020 4:02 pm
john.m48
(@john-m48)
Active Member

@magnus-e

Hi Magnus

You need to sign in to your Prussa account to see many of the Prussa parts.  Rambo Boards were shown their yesterday.

I think this is the link to sign-in page https://auth.prusaprinters.org/login/?next=/

This post was modified 5 months ago by john.m48
Posted : 10/04/2020 11:35 am
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