I am building a pen plotter using an Arduino mega 2560 and a ramps 1.4 board (images below). I have a working code that sets up the stepper motors. I ran my code with the Arduino connected to my laptop, the motors ran fine but too fast, so I re-upload the code with slower speeds. This time, the motors did not move. Moments later my laptop would power off completely and would not power back on until I clicked the Lenovo reset switch on the back. This was somehow caused by my ciruit which was connected still to the laptop. When the circuit is plugged into mains but the PSU switch is in the off position the X stepper motor now has a quiet hum. This hum is no longer heard when the switch is turned on. The power supply and ramps board are both fine and working still. Clearly there is some issue with my circuit or how it is wired however I do not have the experience to know where I have gone wrong. Any help towards working out why my stepper motors no longer move would be appreciated?
(Note, there are two 24V to 12V step down converters connected to the circuit between the PSU and the ramps board)
First read the pinned post re 'How to get the best from the Forum'.
Hi, @foxy8750
Welcome to the forum.
https://forum.arduino.cc/t/how-to-get-the-best-out-of-this-forum
Do you have a DMM? Digital MultiMeter?
Can you please post a copy of your circuit, a picture of a hand drawn circuit in jpg, png?
Hand drawn and photographed is perfectly acceptable.
Please include ALL hardware, power supplies, component names and pin labels.
What are your power supplies?
Thanks.. Tom.... 
What is the current rating of the step down converters? You will need over 2A for two stepper motors. Have you adjusted the current on the stepper drivers?
You only need one converter, the +12V and GND pins nearest the middle of the ramps board are the 11A supply used only for the heated bed (D8 connector), the +12V and GND nearest the edge of the board power everything else.
That shows you are drawing way too much current from the USB port. This is tripping the poly fuse. These take time to recover as they are related to heat. They are also too slow to operate to act as protection for a lot of electronic circuits.
Also never change any wiring when a processor is powered up. That is a sure fire way to damage things.
Note that a "Neam 17" stepping motor only describes the size of the mounting flange. It has nothing to do with the motor's requirements or capabilities.
Apologies for taking so long to reply.
Update: I do now have a digital multimeter
Below is a diagram of the circuit. The components used are:
- Plug connected to mains which has 16AWG wires and was taken from a Lenovo laptop charger
- S-350-24 Power Supply
- 16 AWG wires
- 2x Tobsun DC 24V to 12V 5A DC Converters
- 16 AWG wires
- Henschen Rocker Switch ON-OFF DPST 16A 250VAC
- Arduino Mega 2560
- Ramps 1.4 Shield
- 2x A4988 Step stick Stepper Motor Driver Modules
- 2x 42A02C Stepper Motors
- 2x 3D printer end stops
Update: Last night I was trying to test whether the voltage in my circuit was correct at different points. When I connected the multimeter (IN VOLTAGE SETTING) between the red + terminal of the ramps board and one of the black leads of the 24V to 12V step down converter the meter read 12V and then the board started smoking profusely. Could this be over-current?
Can you identify what was producing the smoke?
Have you adjusted the current limit on the A4988 boards?
Not familiar with the DC converter you are using, possibly the output ground is not tied to the input ground?
Yes, I checked the wiring of the limit switches and it follows both my diagram and tutorials I found on how to connect them to the ramps board
I do not know precisely what was producing the smoke because it was coming from the mega board and that was hidden under the ramps shield. I think it was coming from the mega capacitors because they were incredibly hot after, and the mega's 5V regulator looks damaged. There's no visible damage on the ramps board although the big yellow fuses smell burnt
