Heated Bed Causes Melzi to Restart

Hello,

Right now I have a problem with my heated bed. Every time I turn on the heated bed, either printing or starting it manually, it always causes my Melzi board to restart. It is really weird because I have been printing for about 2 months and there wasn't any problem. When I removed the heated bed, it went well, but of course I couldn't print because I am printing in ABS that needs heated bed. Later, I loaded the bed terminal on the board with 1K resistor to see if it would cause the board to restart again, but it didn't. The board supplied the resistor with 12V 0.01A just fine.

I suspect that the MOSFET is faulty and it causes the IO pin in the microcontroller to be short circuited each time it is activated. Does this mean that I have to change the bed MOSFET? Should I also replace the microcontroller? Do you guys have any suggestion?

Thanks.

A 1K resistor is way to high to test with for a heated bed. They have resitances in single digit ohms. Figure out the resistance for your bed (might not be able to measure it if your bed is defective) and try a resistor closer to that size. I have replaced the MOSFET on one of my RAMPS.

Hi Appdev,

Thanks for your reply. I think the heated bed is just fine. I measure the resistance between the terminals and it comes up with 1.3 Ohm. I also test the bed directly to the 12V power supply and it can heat just fine. I think I will test again the board with a resistor close that resistance and see what it will do.

I suspect it is an issue with the power supply, or the wires connecting the power supply to the Melzi. Turning the heated bed on is probably causing the 12V power at the Melzi to drop below an acceptable voltage. So tighten up the Melzi power input screw terminals, and check any other relevant connections. If that doesn't help, try a different power supply.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

I've had a hard time with power supplies and my printers. On my first one a pc supply not able to meet its labeled specs troubled me for a long time with brownouts causing several issues. But hey it made me upgrade my Xaxis and do some other improvements before i figured it out. I would have needed those upgrades anyway, but the printer never did print right until I added that 2nd power supply. My latest one I'm powering off too high wattage Xbox 360 power supplies and they seem to be working rather well and are a lot quieter. I picked them up used at Gamestop for 22 bucks a piece.

Edited 3 time(s). Last edit at 07/28/2015 08:18AM by appdev007.

I've actually solved the problem! It really IS caused by the MOSFET. After I changed the MOSFET with more powerful one, SUD50N024, it worked like a charm! I will add some heat sink and fan on the MOSFET to make sure this kind of problem never happen again. Thank you for all your suggestions and help .

Okay, I think I've been too hasty to conclude it fixed the problem because after 3 runs, it broke again. I think I will test again whether the replacement MOSFET is defective or not. If it isn't, I think I will try to replace the microcontroller with the new one. Do you have any suggestion what I can try to fix it before I replace the micro?

Considering that you can get a new off brand microcontroller ramps and drivers for about 34 bucks off ebay I would just go replace the whole shooting match.

Quote
fatalelieberi
Okay, I think I've been too hasty to conclude it fixed the problem because after 3 runs, it broke again. I think I will test again whether the replacement MOSFET is defective or not. If it isn't, I think I will try to replace the microcontroller with the new one. Do you have any suggestion what I can try to fix it before I replace the micro?

I really think the issue is power or the power supply wiring, not the mosfet or the microcontroller. In particular, check the negative wire between the PSU and the Melzi is secure at both ends, and check the soldering of that connector on the Melzi. If you have a bad connection there, it will try to draw the ground current via the USB port, which may cause the symptom you describe.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

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appdev007
Considering that you can get a new off brand microcontroller ramps and drivers for about 34 bucks off ebay I would just go replace the whole shooting match.

Actually, I have a RAMPS microcontroller ordered and it is on its way here. But, I am just curious and maybe I can save this Melzi so I can use it on another Reprap project.

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dc42
I really think the issue is power or the power supply wiring, not the mosfet or the microcontroller. In particular, check the negative wire between the PSU and the Melzi is secure at both ends, and check the soldering of that connector on the Melzi. If you have a bad connection there, it will try to draw the ground current via the USB port, which may cause the symptom you describe.

I've checked the ground connection between the PSU and the Melzi, it is connected securely. However, I haven't checked thoroughly the soldering of the terminal and I will check it later. But, can it be caused by faulty capacitor? Because, you see, there is an electrolytic capacitor that is attached across the 12V rail and also connected to the MOSFET of the bed. So, I am guessing, maybe because it is faulty, when the MOSFET is turned on, it shorts the power or something like that.

FYI, this problem only occurs when trying to turn on the heat bed. If I try running the printer without the heat bed, it runs just fine.

It sounds very much like the power supply voltage at the Melzi is dropping when the heated bed turns on. This could be caused by the power supply itself not being able to take the sudden increase in load, or the power wiring or connections being poor, or a short in the heated bed circuit causing too much current to flow. The electrolytic capacitor you mention is probably intended to help maintain the supply voltage at the moment the bed turns on, so if it has gone open circuit then that may be part of the problem.

Arduino/RAMPS is very cheap but has its own problems that you may need to work around, see [forums.reprap.org].

Edited 1 time(s). Last edit at 08/14/2015 04:05AM by dc42.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

I actually tried to directly turn on the heated bed by connecting to the power supply. It runs just fine. It reaches 100C easily within 10 minutes. I also measure the voltage of the power supply while being connected directly to the bed. It reads 12.34V. The heated bed itself has 0.8-0.9 Ohms. I guess it's not the fault of the power supply? However, I notice that the wire that connects the power supply and the Melzi has burn marks at the end of the power supply side.

You got some wire strippers? Use them to check what gauge that burnt wire is. Check a dripped piece to see which hole it fits in. I'm guessing it's too small for the current. If you draw too much current from the supply it should trip off, so I'm guessing it's the wire. Keep in mind that when the supply is connected directly to the bed the current isn't being controlled, so it could catch on fire..

Actually, I'm using 16 gauge wire (it's printed on the wire). It also have similar thickness with the wire that come up in the kit, so I think it would be sufficient. Oh, the burn marks are not made when I test the bed directly. It looks like it has been there for quite a long time. Maybe it was made when I was still able to print. And when testing the bed directly, the wire that I use for the bed didn't get hot, only somehow warm. But, I will try to look for some short circuit connection in my printer. Maybe that causes problem.

Edited 1 time(s). Last edit at 08/19/2015 12:49AM by fatalelieberi.