How do you make sure you have the right voltage on the trimpots on an A4988 stepper driver?

Question

I've been looking into this, but:

1. I'm not certain how to configure my multimeter;
2. I don't know how to keep the voltage going, and;
3. I don't know how to keep the multimeter connected to the VMOT?

I'm told you're supposed to aim for about 1 A.

Answer 1

Generally speaking voltage on stepstick output should be around 1V.

To imagine more or less what the current and what the voltage is, you can think about it in the same way as about water.

The wire is more or less the same as the pipe. The voltage can be imagined as (sort of) the height from which the water flows but the current can be imagined as an amount of water which flows. To simplify things we assume that all our pipes are closed into circuit and we have pump/battery and we have a motor which is a reverted pump ;) and finally we have our stepstick which is a tap in our model.

So no matter what the height (voltage) is we know that tap (stepstick) will pass some amount (current) of water. We can drive it turning tap or turning a potentiometer on the stepstick PCB.

So we got it. Principles (deadly simplified) are now clear. See here for more details

Getting back to your question. You have to know what is your stepstick reference voltage. To make sure about that you have to check out resistor(s) next to main black element on stepstick board. There should be R100 or R200 which are very common.

Now you should read data from motor label to know what is proper current for your motor and calculate

voltage = motor_current * 8 * resistance_of_resistor

So now you know what is proper voltage for your motor and stepstick.

You measure voltage between potentiometer and GND (see on the picture)

If you set and connect everything and start printouts you should check motor temperature. Use your finger. If you can touch motor and hold your finger not more than half a second then probably the voltage set on potentiometer is too high (motor can reach 80° Celsius and it's fine but more will shorten its life span) and you should reduce it a bit (reduce by 5/100 V). If you notice that motor growls or barks then your voltage is probably too low and you can increase it by 5/100V.

Too high current will also reduce longevity of stepstick so cool them out with fan.

Please be noticed.

Z-axis motors will usually be not too hot as they work less than X and Y but as they are both connected to one stepstick so they need more current - set higher voltage there.

Here is a reprap.org site to get basic knowledge about stepsticks.

Answer 2

I want to add some points and clarifications to the answer that @darthpixel already has given. Most information you need is in there, I want to give some more practical advice, since that is what I understand you're question is asking for. I'll start with some points on the more theoretical side, though:

• notice that the Vref is not a voltage that is passed on to your motor. The described pipe analogy is very good, but the Vref is outside of this analogy. The reference voltage Vref is only used to set the current limit. This seems confusing, but has electronic reasons. One can understand the major (side-)benefit easily: Voltages are very easy to measure externally, because you connect your voltmeter in parallel. If you wouldd want to measure the current, you would need to get your ammeter in series with the circuit.
• The stepsticks work by supplying the needed current for movement of the motor (current, because it works by creating magnetic fields), the voltage the stepstick supplies is 'just' supplied as high as needed to feed the desired current through your motor (determined by its resistivity/impedance). This just as an add-on.

Now to the practical side and the application of darthpixel's answer and the above:

You want to measure the reference voltage to limit the current that produces the torque, but also heats up the motor - let darthpixel's advice be your guide: if you can't touch it because it is too hot, then there is too much current, i.e. Vref is too high). To do so:

1. Set your multimeter to volts, range can be autorange or something bigger than 2V.
2. Connect one lead of your multimeter to the ground of your Prusa i3 controller board's power input (I use the screw that fastens the ground input of the RAMPS). The other lead goes directly to the center of the trimpot on the Stepstick. I took the best of my paint skills to create an image showing the process:
3. Note the value you read (12V power has to be on)
4. Use an insulated screwdriver and turn the trimpot slightly.
5. Get a new reading by repeating the measurement process.
6. Repeat the whole process until you get the desired Vref.

Warning: While I've had no problems turning the trimpot while everything was switched on (with my DRV8825 drivers), you should switch the power supply off when doing so.

The described process allows only for a stepwise and rather slow setting of the Vref, but this is the easiest way I've found. I have read of people that use a crocodile clamp to attach the multimeter to the screwdriver for a readout while turning.

If you don't have any idea whether you need more or less current on the motor at the moment, check your resistors on the board and calculate the Vref you should need (see darthpixel's answer for the formula). I would however just do what darthpixel already suggested: figure out the trimpot position by ears and touch: klicking motor: go to higher Vref. Can't touch the motor for more than some seconds: go to lower Vref. It might be a lengthy process, but in the end you'd need to do it anyway to get the best out of the printer!