# Do I need a higher amperage power supply for more stepper motors

I have a very simple question:

I am trying to power five 28BYJ-48 stepper motors attached to some ULN2003 driving boards but I am confused about the amperage I need. Each of the stepper motors needs 5V and 250mA to run and my driving boards take up to 500mA.

I'm wondering if I need to buy a power supply that's 5V/250mA or 5V/1.25A? If its the latter I'm worried that my driving boards may fry under the high amperage, what should I do about that?

Many thanks to anyone that can help.

• Are you aware that ULN4xxx rises about 1V when turned on, so current is only 100mA per phase x 2 = 200mA per motor thus max no load Revs/s increases from 1s to about 1.2s if full or (double?) step so 1A total but why not use a Molex output from a spare PC PSU for now. If you wanted more speed and torque use 10V with the ULN400x or use better switches with low Vce(sat) – Tony Stewart EE75 May 3 at 19:01
• @TonyStewartEE75, Tony I have not clue what this means man. I do not know any of the technical words you just said – KidWithComputer May 3 at 22:54

If each stepper requires 250 mA then your power supply current rating should be ≥ 5 × 0.25 A = 1.25 A in order to run all the motors simultaneously. The combination of motor and driving board will draw the required current and no more.

• So even if I supply 1.25A to a single board/motor they will be okay? – KidWithComputer May 3 at 22:50
• Thought experiment: A 12 V car has a battery capable of supplying 100 A. What happens if you connect a 12 V, 6 W (0.5 A) bulb to the battery? How much current will the battery supply? – Transistor May 4 at 7:54

Adafruit/Digikey specifies at DC R=42 Ohms per winding, but since it is a unipolar with a common tap, I believe that is per half winding and only 1 leg of each winding can be powered on so if using 5V across both coils it would be 5V/42ohms x2= 238mA per motor x5 =1.2A

But the ULN2003 Darlington has a Vce(sat)= 0.9 V typ. 1.1 V max. @ Ii =250μA and Ic =100mA so the motor winding only gets 4.1V typ or about 100mA/winding or 1A total.

But you should always use a supply rated for much more current than you need like 25% min. but if you have a spare PC PSU you can provide up to 10A from the Molex plug for HDD’s and use a decoupling cap near the load drivers.

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