Question about Unipolar Stepper Driver

Just for my own education, why is it necessary to have a driver for a unipolar stepper motor?

Aside from having the ability to take power from a different source, what is the driver doing that Arduino cannot?

Supply orders of magnitude more current and has free-wheel diodes to prevent inductive spikes from
destroying the circuitry instantly on switch off.

Typical steppers take 0.5A, 2A, etc, Arduino pin is limited to about 0.025A before the output voltage drops
significantly and the chip risks frying.

Switching an inductive load without a snubber or free-wheel diode is instant death to transistors as inductive
voltage spikes can be 100's or 1000's of volts if not tamed.

Typical driver chips are designed to dissipate lots of power as heat too, not something a microcontroller normally has to worry about.

So in short, connect some transistors and some diodes and you'll have a unipolar stepper motor driver. The transistors deal with the higher current that the motor needs. The diodes deal with the transistor frying voltage peaks that the motor produces.

Got it, thanks a lot.

And for a bipolar stepper, is it the same deal or does the driver have more responsibility?

Almost same deal. The circuit is a bit trickier, when it has to deal with reversing polarity.

Thanks. Appreciate the answers!

Here's a unipolar 5 wire circuit:

As I see it, the L297 just sends signals to A, B, C and D. This part Arduino could do, using 4 pins. But if you want to use only 2 pins of the Arduino, put Arduino to control the L297 instead, not the four darlingtons. Either way, there are the darlingtons and the diodes, which do the higher current and the inductive voltage peak stuff.

Here's a 4 wire bipolar circuit:

Thanks again. That makes it very clear.