# Driving 5v 40 ohms per coil tiny stepper motor

Hi to everybody!

I hope this is the right section for my question.

I have an Arduino Mega, a DRV8825 and a tiny mini stepper laying around.

I've tried to connect the motor like a "normal" stepper (something like a Nema17 or similar) to drive it, starting with DRV8825's Vref set to 0 in full step mode.

While the sketch was running I'd tried to turn up the Vref slowly using the potentiometer, searching for its right value.

As soon the motor starts turning, it get very hot (I wasn't able to keep on hand!) and the movement was a sort of vibration, nothing more.

If I use a different motor (little bigger in size, I don't remember the single coil resistance) and the same setup (also for Vref) I got a more smooth movement and the motor little less hot.

I suspect too much current, but if I turn it down the motor doesn't start...

What I'm doing wrong with the tiny motor? Is DRV8825 suitable for this small motor?

What if I invert for example A+ and A- ?

Thanks in advance and sorry for my english...

Stefano

Rather than adjusting the current limit on a suck-it-and see basis I would adjust it to provide a suitable output current. It is a very small potentiometer and it would be very easy to jump from a too-low current to a too-high current. The Pololu DRV8825 web page explains how to adjust the current.

If it is a 5v 40ohm motor then the coil current should probably be 125mA

...R

Thank you!

The missing information to me was just the current at the end!
This evening I'll try to adjust Vref using the Pololu explanation!

bigfefi:
The missing information to me was just the current at the end!

volts = ohms * amps

or

amps = volts / ohms

...R

Yes...

A merely application of Ohm's Law...

What a dumb I am...

I didn't thought about it...

Just an update...

I've followed Robin's suggestion and inverted A+A- motor cabling.
Now the motor is moving, but not so smooth as I expected... so I have some question...

• As per Pololu's site formula [Current Limit = VREF × 2] to obtain 0.125 mA current I need to set 0.0625 V for Vref. But... Am I working too near the lower limit of the driver? Is it worth using an SN754410 instead of the DRV8825?

• I dont't know the step angle value for the motor... How can I define it in precise manner?

• Which is the best way to drive this type of motor to obtain a pushing force of about 10 grams (I need to compress a small spring) ? Does the microstepping setup influence the torque obtainable as soon as I have fixed the current ?

bigfefi:

• As per Pololu's site formula [Current Limit = VREF × 2] to obtain 0.125 mA current I need to set 0.0625 V for Vref. But... Am I working too near the lower limit of the driver? Is it worth using an SN754410 instead of the DRV8825?

You may be too close to the bottom range for the DRV8825.

The Sparkfun Easydriver uses a stepper chip that maxes out at 750mA.

If you are going to use a pair of h-bridges to drive the motor then you will be limited to a 5v power supply because they will have no current limiting capability.

You probably need larger current sense resistor(s) for the lower current.