Keep frying my board

Hey there,

I've been using my Prusa for a couple of years now and some time ago it stopped responding. Noticed that the LED isn't lighting up anymore on the shield.
Thought nothing much of it and just bought a new arduino mega 2560 + shield. Once I hooked up the new board and powered on, I instantly noticed the smell of burnt electronics. I disconnected the power supply and felt around the board.
Noticed that around where the USB connector is located it was warmish. I disconnected the shield and tried to do a re-flash of Marlin on the Mega to see if it's still alive. The LED light up and it flashed ok. So I think it survived and something fried on the shield.

Haven't done any big changes to the printer for a long time. One probable culprit could be the switch that I built for one of the fans. I've been using it for quite a while without any problems. Here is the instruction I used to build the switch.
[www.instructables.com]

I tried firing up the board without the switch being connected. This didn't help. I also checked that the power supplies voltage was ok. Reading around 12.5v

Any ideas what could be burning the board. I'd like to get some pointers before I smoke a third one. Can it be a broken transistor in the fan switch? What else might cause this?

As a side note, if anyone has a schematic for a MOSFET fan control I'd appreciate it. I've read the TIP120 transistor that I'm using here isn't ideal because it draws more amps.

Edited 1 time(s). Last edit at 02/04/2018 07:37AM by Backdraft.

If you have an lcd display and always connecting to USB and computer try removing the diode on the ramps board under the x y stepper driver sockets not the one near power input and this puts less stress on Arduino board. The fan shouldn't be using that much current but try a new one. As far as mosfet you could remove it from board and add 3 wires to mount it on a large heat sink and get a higher current/voltage rated mosfet. Another thing is to replace those onboard fuses with glass automotive types

I don't have an lcd display on my printer. The fan I'm using is a small 40mm so the draw should be really small. As far as I know, the transistor I'm using as a switch for the fan is way over-specced for this and should be near impossible to burn.

Didn't even know there are fuses on the board. Well didn't do much good in this instance.
I don't know how the shield works, but I thought the main purpose was to prevent things like this happening. I guess the logic pins don't have any protections?

The purpose of the RAMPS board is to house the stepper motor drivers, and to have convenient locations for things like the 5V rails for the endstops as well as the drivers for the hotend, heated bed etc. It also provides things like pullup resistors for certain logic functions, and the voltage dividers for thermistors for the hotend and bed temperature sensors.

There is almost nothing there to protect the logic pins for the arduino, aside from the fact that incorrect connections to things becomes more difficult.

There are 2 thermal fuses on the shield, but they will still allow more than enough current to completely destroy the arduino. Their purpose is to prevent you from setting your printer on fire if you short it out.

Generally, once you notice the smell of burning electronics, it's too late for those electronics. However, if your connections are correct, you shouldn't burn the electronics. The schematic for the switch looked good, so unless you did something wrong with the wiring (like reversing the pins on your transistor) I can't see it being too much of a problem, but it would still be good to test the switch. Connecting to fused 12V and a fan would be recommended, you should be able to trigger the fan by grounding the trigger pin with a very thin strand of wire.

Also, just because you can load and run the blink sketch on your arduino doesn't mean that it's not damaged. I don't know that I'd trust either board, and for the price of a combination RAMPS and arduino compatable, I'd hardly think it worth it. I'm sure that I've seen MKS boards (Which pretend to be an Arduino 2560+ RAMPS 1.4 as far as firmware is concerned) going for near the $20 mark. It may also be worthwhile considering upgrading to something with better processing and driving power.

In the meantime, though, you need to start troubleshooting where the problem may be. Something that burns like that is probably short circuited somewhere. I would start by testing everything with a multimeter

Thermistors should be near their nominal resistance at room temperature, so those ought to be near 100k

Test the hotend resistance, the heat build platform resistance. These are fairly low, but should be measurable.

Check your end stops that nothing is shorting. Test fan wiring. Check that nothing is shorted to ground. No pin should have an electrical connection to an unrelated wire, aside from ground connections.

Undo wiring bundles, and check the condition of wiring insulation. I had an issue on my I3 where the wiring to the hotend overheated and melted the insulation on the wires to the fan, resulting in an intermittent short. That wasn't fun to find. (It was the reason why my I3 got put away for a year and a half.)

Thanks for the comprehensive reply. I'll check these out.
One thing came to mind. Most, if not all of the connected device should not get power when I simply turn on the power, right?
A faulty fan, hotend or bed should not cause any damage until I actually try to print something.
I never got that far. just turned on the power supply and it fried.

The fan switch should be connected right. I've been using it for a while and haven't touched anything.

One likely culprit is a shorted stepper driver,remove them all and try powering up again, maybe check the shield for any burnt copper tracks first.

Correct. Most of the components get power. While the hotend, heat bed and fan won't get power until the circuit is turned on, the MOSFETs that control these WILL get power as soon as you turn the switch on.

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MBot3D Printer
MakerBot clone Kit from Amazon
Added heated bed.

Leadscrew self-built printer (in progress)
Duet Wifi, Precision Piezo parts

Thank you for the advice. I found the problem.
My own stupidity. I had mounted one of the end stops on the smooth rod. As you know the back of the switch isn't protected at all and it shorted out when contacting the rod.
Seems the board still survived, I got some movement out of it. I'll make a mount for the end stop and see how it goes.