[GUIDE] Vref adjustment – practical guide

There are plenty of ways to start talking about motor drivers and their adjustments (Vref adjustment).

We want to be traditional and answer the essential questions:

What is Vref? What is it for?

The Driver is the electronic device that drives the stepper motor (step by step) through the translation of signals that are given by a processor, in electrical current.

Stepper motors move differently from normal electric motors, in fact they do not run very fast. They move slowly (they can even move a few degrees) but they are more powerful and very precise.

The calibration that we are going to perform on the drivers of these motors is the Vref, this term means “Voltage reference”, and it is the amount of energy that they allow the motor to release. The more precise this value is, the more optimized the movement of the motor will be, without having waste of energy and overheating typical of a value too high or loss of steps typical of a value too low.

The canonical method for this fine adjustment is:

—>   Vref = Amax x 8 x Res  <—

Where Amax = Max working Ampere for step motor

Res= is the value of the resistance on the SENSE 1 or 2 pin, in ohms. This value can commonly be o,o5 or 0,1.

To do a practical test we can try to calcorale the Vref of a typical Nema 17 engine connected to a Driver A4988, on my MKS card.

these are the calculations:

Vref= Amax (1,75A) x 8 x Res (0,05) = 0,7V

0.7V is the ideal setting value for our driver. Having said that, we can slightly “pimpare” this value for a greater cue. My axis drivers are set to 0.8 without giving any sign of suffering or heating.

Now, let’s move on to practice.

For this part we recommend the use of a ceramic screwdriver with a cut of about 1.5 mm.

We are sure that no amateur/neophyte will have this kind of tool in their workshop because it is quite technical. We will then explain how to make the same identical adjustment using a normal iron screwdriver.

Vref Regulation: Tools:

  1. Small screwdriver – Best if ceramic
  2. Electrician tester

First we have to go and analyze our Drivers and we identify the adjustment screw, it has exactly the appearance of a star screw.

Vref adjustment

The motherboard of some 3D printers is equipped with removable drivers, others have the drivers directly above the board and are less recognizable, the help always comes from the adjustment, which always has the shape of the head of a star-shaped screw.

We switch on our tester and bring it on the 2V scale, in direct current (DC ), at this point we put the negative (black) on the negative of our power supply or on the negative of our Arduino board (on the arrival of the black wire that carries current to our electronic board).

Vref adjustment

We put the red tip in contact with the screw we have identified before and read the value. Since we are using an iron screwdriver, we turn off everything, at the risk of burning the driver, and we increase/decrease the power by making a quarter turn at a time.

NB: in order to increase the power of the drivers, you must turn the screw counterclockwise, going against experience and habit.

Now all you have to do is “tinker”, gradually decreasing the rotation, until you find the right voltage.

Many times, however, this approach to the regulation of the drivers, for the fact of keeping us very “safe” does not give the desired results. In fact, sometimes the extruder still loses a few steps. (We are only talking about the extruder because the loss of steps in the motors of axes movement, having already made this adjustment, is a symptom of some mechanical hitch).

Vref adjustment

Rough method

If the extruder still loses a few more steps, it can be adjusted brutally. Follow the same procedure as before (switch off each time before placing the iron screwdriver on the driver’s screw!!!). Set the tester to Ampere and by turning the screw an eighth of a turn, always counterclockwise to increase the power, increase the power until the complete disappearance of the loss of steps.
From experience we can say that already at 1.2A will disappear all losses of steps and you will be on a voltage / amplitude well supported by the motor. However, make sure you have a fan for the drivers/cards as the extra power corresponds to the extra heat.

We recommend to monitor the temperatures of drivers and motors for at least the first 2 to 3 hours of printing. The working temperature of the drivers should not exceed 60°, as well as the temperature should not exceed 50°. You can easily monitor the temperature at regular intervals with a digital infrared thermometer, and if necessary add heat sinks on the drivers, heat sinks on the motors and fans.

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