Why are my motors not getting enough power?

Question

I am a newbie electrical engineering hobbyist who is trying to get four 28BYJ-48 stepper motors to turn for a split flap display project. I managed to get each motor working perfectly well individually but when I tried all four together the motors did not have enough torque to turn the flaps. I do not have a multimeter available to me so I require some intelligence in figuring out the problem.

My question is: why are my motors not receiving enough power? Each motor requires 12V to run, I don't know the amperage required and it seems this info is hard to get. Rough estimations online seem to suggest about 250 mA is required per motor but nobody seems sure

Here is the schematic below, I'm using an arduino&ULN203 Driver board to run the steppers I apologize if it is terrible but please know that I am trying my hardest:

If an addition knowledge is needed please do not hesitate to ask for it. Many thanks -KWC

^{simulate this circuit – Schematic created using CircuitLab}

Slightly tidied up schematic by @Transistor.

Links: Wallwart 12V Motor+UL2003 Driver [

Answer 1

If the motors are turning individually (or under no load) then it is most likely a power\load problem. This results from resistance\inductance in the wires, large loads cause the voltage to drop in the cable especially in switching applications. This voltage drop leads to a loss in power.

1. Get a better power supply (like a bench supply with current and voltage display so you know how much power your design is using).

2. Don't step all the motors at the same time, stagger the stepping so the load on the power supply is distributed.

3. Use a [power bypass capacitor][1] (or several one for each ULN2003 placed next to the inputs of the ULN2003 on the 12V line and ground.

4. Use better wires, smaller gauge wires have a lot of resistance and jumper wire may not have that great of contact resistance. It may be that some of the connections may have more than 100mΩ resistance (or the total resistance from the supply to the motor might exceed 100mΩ with the setup above). 100mΩ will cause a 0.12V drop, 1Ω will be a 1.2V drop which would be a significant loss in power.
   [1]: https://www.electronicshub.org/bypass-capacitor-tutorial/#:~:text=A%20Bypass%20Capacitor%20is%20usually%20applied%20between%20the,bypasses%20the%20high%20frequency%20components%20of%20power%20supply.