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RAMPS problems: No power, FET overheating

Posted by Roches 
RAMPS problems: No power, FET overheating
July 16, 2015 12:55AM
I got a RAMPS board that was poorly soldered, with sputtering, joints with gaps, and generally dull looking solder. I used it anyway and I have not yet gotten it to work properly. I am able to get the LCD working and I have been able to control the motors sporadically, but not the heaters.

Mostly, I have tried reflowing the solder. Eventually I overdid this, and shorted out 5V and GND, causing the board to fail completely, but I've fixed that now.
I've also replaced the 10 nF caps for the thermistors with polymer film caps, and I've twice replaced diode D2 (the one by the polyfuses). It seems to have failed, because the first diode and the one I replaced it with eventually gave no resistance in either direction and did not register a voltage drop when I diode tested it with a meter.

I suspect there is a short, but I can't find where it might be. As a rather extreme measure, I completely removed the servo block and the two AUX blocks I wouldn't need, leaving only the LCD connectors. The idea was that I would remove these unnecessary components in case the point of failure was somewhere in that part of the circuit.

There was definitely a short at the VCC pin of the 3-pin connector near the reset button. It smoked and glowed after a few seconds.

For external power, I only have a 5 V 2 A and a 12 V 0.7 A power block. The 12 V one seems to help. When I can get the board powered up, the LCD flickers, or at least fluctuates. When I connect anything to D8 D9 D10, the FETs heat up.

My Arduino Mega clone seems fine. I have tried testing the analog readouts and they work. Sometimes the board does heat up in the space between the barrel plug adapter and the USB port -- AFAIK that is a voltage regulator and a thermal fuse.

Where I'm at now is that the LCD flickers a lot. I suspect perhaps I shouldn't try powering the Arduino from the RAMPS board at all -- does that mean removing the Vin connection?

Any ideas on what I can do? At this point I'm very frustrated because buying a replacement board isn't a possibility for the next few days. I've definitely learned a lot about the RAMPS board and improved my soldering significantly. I really would like to fix the board.

Edited 1 time(s). Last edit at 07/16/2015 01:01AM by Roches.
Re: RAMPS problems: No power, FET overheating
July 16, 2015 01:33AM

I only have a 5 V 2 A and a 12 V 0.7 A power block

12V 0.7A is way to small. I hope it´s a typo...
Re: RAMPS problems: No power, FET overheating
July 16, 2015 02:37AM
Check the labelling on the FETs to make sure what they are. Some cheap suppliers replace them with even lower spec devices than the original designs call for (and even those are under spec IMO). Plus, some of the cheap suppliers have such bad QA processes (ie: none) that somehow instead of FETs, you end up with some other device (ie: they weren't paying attention during assembly).

To stop power from the Arduino, either remove D1 (which connects to Vin) or remove the Vin pin from the header that goes to the Arduino. D1 is easy to replace, so it's probably the best solution there.

Note that by removing D1, you need to use a USB cable to run the Arduino (and therefore the LCD). This at least should confirm if it works.

As per Olaf's comment though, 0.7A is way too low for the 12V side.

Looking at that, I suspect the power is the main problem here.

While RAMPS has 2 inputs, they both require 12V.

One requires 12V @ 5A, and the other requires 12V @ 11A. You can run them both off the same supply IF (and only if) you can guarantee that the supply will provide 12V @ 16A reliably.

If you disconnect the heaters and motors, you should find that the Arduino and LCD combination should work off the 12V 0.7A supply you have. You will need to remove the motor driver boards (they will use a small amount of power - enough that the 12V 0.7A supply will not be enough if they're connected - make sure to only remove/insert them with NO POWER applied, and be careful you put them in the correct way around). It at least will help you determine if you've got any shorts, hopefully without lots of issues that extra amps may cause (such as burnt tracks).
I would check the soldering around the FETs as well. If the pins on the FETs are shorted, then you may find that they are either on all the time, or that they could be damaged.
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