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I just smoked my Melzi electronics!

Posted by Dreadnaught426 
I just smoked my Melzi electronics!
May 28, 2014 02:35AM
First off very new to this stuff, so be easy on me!

I was doing a test print with a Prusa I3 nock off(Pangu I3) and it stoped mid print and I noticed smoke coming from the main board. Shut it off and took a look and saw that the heat bed chip(or something around it) was toast! The queastion I'm asking is weather it was due to poor cooling or trying to heat the bed up to 110 was too much for the board? My buddy put it together for me because he has a maker bot and I figured he knew what he was doing, but when I took a look the cooling fans he had on the case he made for the board they were attached the cooling fan for the head. I tuned the cooling fan off to keep the bed from cooling(stupid me in retrospect). I wanna say it was due to poor cooling, but I figured I'd also ask.

open | download - heatbed2.jpg (246.5 KB)
open | download - board2.jpg (325.1 KB)
Re: I just smoked my Melzi electronics!
May 28, 2014 11:38AM
An FDD 8580 is rated for 20V, 35A per [www.fairchildsemi.com]

Were you putting 24V through it?

At 12V & 10A, an Rds of 0.009 ohm would mean 10^2*0.009=0.9W of heat at the MOSFET to dissipate.

Replacing it with a better TO-252 N-channel MOSFET with half the Rds would require only half the cooling at the same load.

Maybe look for something like [www.digikey.com] to replace it with.
Re: I just smoked my Melzi electronics!
May 28, 2014 01:12PM
I'm only putting 12V to the heat bed. As for all that other stuff, I'm no elctricition. Seems like a very steep learning curve here. I'm using the Melzi Ardentissimmo electronics from reprap 3d printer with ATmega1284P @ 16MHz. It's under warranty, so i should be able to get another one. just want to know what went wrong.
Re: I just smoked my Melzi electronics!
May 28, 2014 03:42PM
Yes, take the warranty of you can get it.

On the other hand, if you are stuck with the smoked board, and the other functions seem to work, replacing the MOSFET might be worthwhile. If it doesn't do anything at all, you might try unsoldering the burned up MOSFET and maybe the capacitor and see if the rest of it works.

From the pic it looks like the MOSFET burned up, which it can do if there is too much energy going through it.

[reprap.org] says:


The MOSFET transistors used will literally let out magic blue smoke if used with a standard Mk2b PCB heated bed wired for 12V. With 12V input voltage, and the Mk2b bed wired for 24V, the bed does not get hot enough to be useful. And when wired for 24V and given 24V input voltage, it does work, but only if an external heatsink is added to the MOSFET. This is very dangerous, and at the very least, the specifications of voltage and current should be disclosed to the user so they can add appropriate external MOSFET or solid-state relay. Obviously, as mentioned before, there are no specs anywhere on what sort of current/power the heated bed circuit can handle. The Geeetech boards do not use the RFP30N06LE's like the original Melzi board spec'd out, so I am unsure of exactly how much current and power it is capable of. Please use caution when hooking up a heated bed to this board.

In your pic, it looks like your MOSFETs are not rated for 24V.

One possible wiring/operator error that would cause too much energy to go through it would be if the output was shorted to ground somehow, bypassing the 1.1 ohm resistance of the heated bed. I'd measure the resistance of the bed as seen at the wires to be connected to the terminals. But if that is a good 1.0+ ohm, one should expect a set of electronics with a heatbed connection to drive a 10A into a heated bed without problems.
Re: I just smoked my Melzi electronics!
May 29, 2014 01:36PM
My bed is reading 1.7 to 1.8 ohms. Is that too much or too litttle resistance?
open | download - ohms.jpg (381.8 KB)
Re: I just smoked my Melzi electronics!
May 29, 2014 03:13PM
With 12V, by Ohm's Law, I=V/R, that would mean 12V/1.7ohm=7 amps through the circuit you are measuring.

That is a good picture of the measurement. From the earlier picture I can see that you've got a dual power board, and from this picture, it looks like this half of the board is 1.8Ohms. If terminals 2&3 aren't shorted together, as would be normal for regular 12V use per the table printed on the board, then the terminal 3-to-1 resistance is likely 1.8ohm as well, and powering the whole board would take 7+7=14Amps. 14A seems high per [reprap.org] By my uncalibrated eye, the 14 amps seems high compared to the size of the screw terminals and probably the copper traces on the Melzi. If this will be the load, I might use a separate solid state relay, like 25A DC-DC SSR, externally to control the load.

If 2&3 are shorted together and the resistance between 1-to-2&3 is 1.7, then the heated bed might be ~40% underpowered compared to most boards.
Re: I just smoked my Melzi electronics!
June 04, 2014 09:52AM
All sorted out, must have been a defect in the board. New one works fine! Thanks for all the imput though, it will definetily come in handy!
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