# Power supply

Hi guys.
I plan on using the A4988 at it’s max 35V 2A per phase. (I installed heat sinks and there is a fan good enough to make a vacuum cleaner )
I will use 4 drivers and 4 steppers 333oz-in.

The question I have is: minimum apms the power supply needs to deliver?
I see in the description of the A4988 that you mention 4A max per driver but in the Allegro datashhet table 2 the max combination is 70.71% per phase both active thus 2.82A per driver.
So, do I need 4A x 4 minimum or 2.82A x 4 minimum?
Reason I ask is the price difference between the 2 power supplies (about \$100).

Thank you.

Hello.

You will never see the driver supply both coils with 2 A simultaneously, so the worst case you’ll have to deal with is both coils simultaneously carrying 70% of 2 A (for a total coil current of 2.8 A, as you already calculated). However, note that the coil current is not necessarily equal to the power supply current. The driver is essentially acting like a switching step-down regulator when it limits the current, so the power supply current might be substantially less than the total coil current. I think you’ll be okay with a power supply that can deliver 4x2.8 A = 11.2 A, though you might not want to cut it quite so close. Then again, I’m not convinced that the drivers will actually be able to handle a 2 A current limit in your setup, so you might more realistically have to go with a coil current limit closer to 1.7 or 1.8 A (which would give you a little more wiggle room if you have an 12 A supply).

- Ben