# A4988 Vref too low

I’m driving a stepper rated at 5V/1.7A using the A4988-based driver circuit (Pololu product #2985) with a 12V power supply. An Arduino controls the stepping sequence.

According to the datasheet for the A4988, I should be calculating Vref as:

Imax = Vref / (8 * Rs) so,

Vref = I * (8 x Rs) (where Rs is 0.1 ohm on this particular board and confirmed with meter).

So, Vref should be (1.7A x .8) or 1.36V to drive 1.7amps.

The problem is, I cannot adjust the A4988 to get Vref up that high.

With the adjustment pot full swing, the best I can get is .9 to 1V. When I energize and hold the motor and use an amp meter (ammeter), the coil is drawing about .9 amps – which is still less than the 70% of 1.7 Amps I would like (using full-step mode).

Is Vref potential limited by the input voltage of 12 volts? Is there something else I’m missing?

Hello.

We do not expect the A4988 driver you mentioned to provide more than 1A of current without overheating. The maximum VREF voltage is about 1/3 of VDD. What VDD are you using? Can you post pictures that show the board and how everything in your system is connected to it (including any soldered connections you have made)?

-Nathan

Hi, Nathan. I’m not able to send a pic right now but the circuit is the same one that is all over the web for driving a stepper from an Arduino. Very simple; step, dir, short reset/sleep, 12V on mot, 5v from arduino, common ground. It is running on a breadboard now (which I know is wrong for the amperage), but the issues with Vref started long before I ever connected a physical motor.

If we take the actual motors out of the picture for a moment, and focus on Vref, I should be able to set that value without the load being engaged or even connected. And 1/3 of 5v should give me a max Vref of 1.67 – and I’m not even near that…

I’m powering the A4988 from an Arduino, so the Vdd is 5v.

Are you suggesting that the low Vref could be simply related to a poor connection on Vdd or one of the other lines on the breadboard?

By the way, the board is “working” fine – just can’t get the Vref to where I want it to be. – and I will be conscious of the heat, although I hadn’t noticed anything so far, but haven’t crossed .9 amps yet…

Hello.

Clones of our A4988 carrier board are common and another manufacturer might use slightly different potentiometer or resistor values, so we need pictures of the board in order to continue troubleshooting.

-Nathan

They included the Pololu-branded data sheet, but to you point maybe it isn’t really yours… Let me look into this more closely on my side.