Friday, May 8, 2015

Prusa i3 Rework- MOSFET Repair, Part 1

When I first got the RAMPS 1.4 board, I wondered why the power MOSFETs for the hotends and the heated bed had no heatsinks. However, after connecting the RAMPS board to the printer, they seemed to work just fine. I soon forgot about the problem entirely... but that was the wrong move.

Fast-forward to four months in the future. Something strange was happening- the Full Graphic Smart LCD Controller that I set up would dim every time I started to turn on the heated bed.

Very quickly, I had a much larger problem on my hands- now the heated bed wouldn't heat at all.

There was no voltage across the heated bed terminals, and the bed MOSFET refused to show any signs of life or warm up. I believed that I had found my problem- the MOSFET had fried itself to death.

I came up with a plan. Armed with a small perfboard, some TO-220 heatsinks, and a couple spare MOSFETs, I would build a breakout for these transistors that would allow room for them to cool. 0.1 inch headers have the same hole spacing as the TO-220 package, which is what the MOSFETs on the board are. So I would connect male headers to the RAMPS board, and female headers to the breakout board. A cable harness would connect the two boards.

Here is a picture of the RAMPS board in its original state.

The very first step was to remove the RAMPS board in order to desolder the original MOSFETS. This meant unplugging some of the cables and detaching it from the Arduino Uno.

Here you can see the detachment of the RAMPS 1.4 board from the Arduino Uno. I made sure to pull it straight out... I didn't want bent pins on the RAMPS board.

And here is the RAMPS board, ready for desoldering action.

I highly recommend using solder braid, otherwise known as desoldering wire, for desoldering components. You can buy it cheaply at pretty much any brick-and-mortar or online electronics store.

The principle is that the hot copper lattice, which is being pressed on by the soldering iron, will wick up the solder from the joint. Sometimes a little flux helps as well.

Another shot of the desoldering process.

Here, one of the MOSFETs has had all of the pads completely cleaned of solder. The other two have been removed from the board, but the solder in the pads still needs to be wicked up.

Almost there... nearly all of the pads are free from solder.

The brown marks on the board are from solder flux melting from the joints. We can remove them later, once the headers are soldered on.

Here I am setting up a male header to be soldered in. I put a piece of tape on the other side of the RAMPS board to hold the header in.

A side view of the header before it is soldered in.

Here is what the header looked like before it was soldered.

And here is the header, now soldered in.

Here is a view from the other side.

It's easy to keep a header in place if you can solder in one pin. Then you can take the tape off.

Now all three headers are soldered in. However, there is still a huge amount of messy burnt flux on the board.

By dipping a Q-Tip in flux remover and scrubbing the board, the flux comes right off.

Not only does cleaning off the flux look good, but it also makes it easier to see whether there is a bridge between solder joints.

All clean. It looks as good as new.

And here's a quick top view- looking good!

After I finished with this part, I began with the breakout board. More information on that next week!