Temp Fluctuating when connecting and disconnecting the power to stepper motors on Arduino Uno CNC Shield

Projects General Guidance
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Basic Information:

Printer Model: Custom
MCU / Printerboard: Arduino Uno
Host / SBC: Raspberry Pi
Firmware: Klipper

I have connected the wires for a thermistor to my Arduino Uno at Analog Pins 3 and 4. Whenever I connect the power supply, the temperature reading jumps up about 20 degrees, and when I disconnect, it jumps back down. It occurs both when the thermistors are connected and when they are disconnected. My temp circuit has no joined connection to the heater. The power supply for the stepper motors is 24V 2.5A. I need help to figure out the cause of this bizarre behaviour. Thank you

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Please supply a circuit diagram.

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Request for Detailed Information Hardware :

I’m not sure I can help without seeing exactly what you have in front of you. To assist you effectively, please provide the following:

  1. Annotated Schematic: Post a detailed schematic of your setup exactly as you have it wired, showing all connections, including power, ground, and power supplies. This was requested by @DrDiettrich
  2. Technical Information Links: For all hardware devices in your circuit, provide links to technical information. Be aware that many items in the Arduino/Pi world may have the same name but function differently. Links to sales pages like Amazon often lack the necessary technical details, so ensure the links you provide contain the correct specifications and data sheets.
  3. Be Thorough: It’s your problem, and the more precise and detailed information you provide, the faster we can help you troubleshoot. This saves everyone time and helps us give you the best possible assistance.
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Hi! Sry for not providing the wiring diagram. I am using genuine Arduino uno with Cnc Shield V3.

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This is the diagram for the connections to the 24 V. I have tried to disconnect the probe, but the temp still fluctuates. Both the thermistor temps show the same fluctuation. Only other thing powered by the 24V are the stepper motors. Nothing else is connected to the circuit. I googled how to connect a thermistor to Arduino, and it said to connect a 100k thermistor to a 100k resistor. Sry for the bad drawing. Thank you

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Stepper motors consume most power when standing still. All details (soft/hard) of the motor drivers are important.

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I have local stepper motors that are rated for 1.5 amp. I am using A4988 stepper motor driver. HANPOSE 17HS4401S Nema 17 Stepper Motor - Mechblock This is one of the motor that I am using. Thank you

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Ok I solved it. Thank you everyone

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It will draw 9A on each active coil, or whatever limited by driver or power supply overcurrent protection.

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A DRV8825 would have been a better choice for those 1.5Amp motors.
The A4988 is only rated for 1Amp.

The ground link between 24volt and thermistor ground should not be there.

The 100k pull up resistors are likely too high.
The value should be about the same as the thermistor resistance at the measuring temperature. (Google for 100k thermistor resistance tables).
About 10k is more suitable for a heatbed.
Leo..

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