Hotend heating problems

Hi. I wanted to make a ramps 1.4 to test my soldering skills and I also wanted to build my own 3d printer. So I downloaded the open source schematic and soldered all the components accordingly. Though I left out some parts as I knew I wouldn't use them and by the time I needed them I would actually buy one. Then I connected the board with Arduino and to my computer and to my 12v 5a power supply. I was using x,y,z and e0 motor pins and e0 thermister and d10 for my only hotend. My Arduino was already uploaded with the Marlin firmware without any changes except to the part "cold extrusion prevented". I uncommented it as I needed to see if all the motors worked. So I started a print,all going well when suddenly my hotend starts smoking. So I stopped the print and let it cool. But since then the hotend doesn't heat up. Can anybody help me please. I have been trying to fix this for the past week. Thanks in advance.

You might have damaged the temperature sensor or even the heating cartridge. Check if the thermistor still has about 100k. The heating cartridge should have a single digit ohm value.

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check resistance of the hot end heater. for 40 watt 12v is 3.6 ohms.

If this is ok, check your thermistor is working and reading room temp. If it is plug hot end heater directly into 12v power. Watch temp on thermistor, It should start to heat (uncontrolled) so do not leave it un monitored.

Did it heat? now make sure you turned it off.

Next test your mosfet. Load up a blink sketch, change the pin to the mosfet pin you want to test. stick a multi meter into both the mosfets plug pins (you have to use the gnd pin on the mosfet plug) and check voltage. It should blink 12v, gnd, repeating.

If mosfet doesn't wok, check gate pin with respect to gnd on power supply. is it blinking 5v, gnd?

If gate pin is blinking, but no mosfet output, its a dead mosfet.

if gate pin isnt changing remove ramps board and test pin again. If it starts to work, mosfet is probably at fault. if it doesn't, its a dead io pin on mega.

Yeah it's a dead pin on mega. Any way to fix the mega or still use it for this purpose? Maybe assign pin d9 for it? But how do I do that? Thanks.

If you can do surface mount soldering you can replace the main chip.... but it really isn't worth it.

you simply update the RAMPS pin.h file swapping pins 8 and 9.

Thanks. But how did it happen? Need to know so that it doesn't happen again.