Problem reading Vref for current control of a4988 driver

I am attempting to connect a stepper motor rated at 600 mA to a Pololu a4988 driver, and thus need to limit the current coming out of the driver. With a 0.05 Ohm sensing resistor, I am assuming that I need Vref to be 240 mV or lower. However, when I try to measure Vref from one of the points of the board, I am only seeing the logic source voltage (around 5 V). When I checked it yesterday it was reading around 760 mV, but I did not have time to work with the potentiometer at that time. Any help in explaining or overcoming this problem would be very appreciated.


I recommend setting the current limiting by adjusting the pot while measuring the coil current directly, because it is much easier to do. You should not be getting 5V on Vref. Between what two points are you measuring 5V?

- Ryan

Hi Ryan,

I have been attempting to measure the Vref from the pass-through pin and the top of the potentiometer in comparison to the logic ground pin. This should read the Vref if I was understanding the device page correctly, but I may be wrong. The positions are marked in red on the following image.

If I were to measure the current directly off of one phase, would I need to have a load of some sort on the loops to have the proper comparative current across them? Can I simply leave the phase not being measured open or does it also require a load?

For reference, the motor I am attempting to control is also a Pololu product:

Thanks for the assistance.

Those measurement points should be okay. I would recommend measuring it from the via and from the bottom side of the board if you can. That way there is less chance of possibly damaging the pot or shorting the probe to another component. Are you still measuring 5V on Vref?

Yes, if you want to measure the current directly you need to have a load applied. My recommendation is to connect all the motor leads except for one where you connect it in series with a multimeter set up to measure the current.

- Ryan

Yes, I was able to read a Vref within the preferred range by using the bottom via. I will hook up the motor to make sure the amperage is at the correct values. However, in the process of measuring Vref I had some trouble with getting a good connection between the driver and the wire connectors that came with it. Is there a recommended way (other than soldering) to ensure that the driver is connecting firmly to a breadboard? Thanks once again.

Not really that I know of. I recommend soldering it to the header pins. A lot of people make sockets with female headers so they can swap the drivers.

- Ryan