hmm. the specsheet for stepper or for the a4988?
the Imax for my nema17 is 700 mA shall I add these together to get the current for VMOT input (for 10 drivers 10 x 700mA = 7 A)
and how do I get the curent for the VDD?
You need the complete motor data to compute the current needed. NEMA17 tells only about the mechnics of the motor. A NEMA17 with a rated current of 0.7A is usually not suited for a A4988 driver. These motor mostly have a rated voltage of 12V. Motors that are suited for a a4988 have a lower rated voltage and a higher rated current. And you must consider that the rated current for a stepper is per coil, not per motor.
For a A4988 driver the VMot voltage must be much higher than the rated voltage of the motor. On the other hand the VMot current is not the sum of the coil currents.
The A4988 works very similar to a buck converter. In this case, Ohm's law is unsuitable for calculating the currents.
I can confirm that it works fine with a4988 providing 12 volts for the driver and calculated the current limit to Vref = 0.448 v with some headroom. I used this formula:
Imax = 0.7
Vref = (Imax * 0.8) * 8 * 0.1 = 0,448
Imax * 0.8 is for lowering it to 80% to handle the heat better.
Hm. Ok, so since it is 2 phases in 17hs4023 I shall calculate the currentlimitation with 1.4A (0.7 * 2) instead of 0.7?
Yes it works in that the motor turns. But it will not provide the torque as possible - especially with higher steprates. The A4988 makes no sense if you use it that way. You could also use a simple H-bridge in that case.
If you want to stay with these steppers you should at least use a higher Voltage, e.g. 24V.
That was not quite correct. The current at the VMot Pin of the A4988 IS the coil current, and flows in BOTH directions. That is why you need a sufficently big buffer capacitor as close as possible to the A4988 pins. This is missing in your fritzing.
The current from the power source is less than the coil currents - if you use a higher voltage. The higer the voltage, the less the current.
No, the current limitation of the A4988 applies to one coil. But it delivers this current for both coils.
But if you have a VMot of 12V with a 12V stepper, this limitation is pretty useless. The A4988 can't deliver more current in this case anyway, because voltage and coil already limit the current.
Edit: well, I had a closer look at the datasheet of the stepper now:
That means the rated Voltage of the coils is only 2.8 Volt. That means this stepper is designed for a current control driver like the A4988. You must not apply the 12V directly to the coils. The low current of the coils results from its size - they are fairly small steppers.
( Again you see, that NEMA17 tells nothing about the electrical data ).
Ok so I did calculate it correct then after all ...
So I just double the voltage up to 24 volt instead of 12. But maybe you say that I can go with 12 volt anyway?
This is what my powersupply looks like whith 1 motor with 24v. it is 0.09 A when the motor is running and 0.07A when it is idle. (dont know if this way of meassure makes any sense)
Anyway meassure the current between the red wire of the motor and the stepper gives me exactly 1.9A. Does that mean that I need 1.9 ampere for every stepperdriver? Seems to be a lot. For ten of them 19 ampere??
I have actually added a capacitor to the 12v powersupply in my setup.
Well, I updated my post after reading the stepper data more thorougly.
Because the A4988 works similar to a buck converter you must compute the necessary current from the source by means of the needed power.
One coil of your motor needs a power of 0.7V²*4Ohm = 1.96W. So both coils need about 4W in full step and with the full rated current. If you look at the current table of the A4988 datasheet ( Page 17 ), you see that the A4988 delivers 70.71% of the adjusted current to the coils in full step. This means 0.49A or 0.98W per coil, about 2W for the motor.
Your power source delivers 24V/0.09A = 2.16W. That's not too bad isn't it.
But of course that is only a very rough measurement and computing. Your adjusted current is shurely not that exact, and there are of course power losses in the chip. But it may be enough to estimate your power needs.
That must be faulty. If you adjust the current at the A4988 to 0.7A, that is much too much - also for the motor coils. I think they would get burned with that current. It's nearly 3 times the rated current.
you need a capacitor at each of the A4988 - as close as possible to the boards.
Regarding the fact, that the rated coil current is in fact only 2.8V this should work too. But as you can see from your power source, the higher the voltage, the less the needed current. But of course you must not exceed the limits of the chip and the capacitors.
The full coil current flows between the buffer capacitor and the A4988 chip. It flows in both directions. In fact, electrical energy oscillates back and forth between the motor coils and the buffer capacitor. This is how the chip controles the current. Only the loss of energy ( thermal loss in the coils, and the mechanical energy of the motor ) has to be provided by the power source. Therefore the current between the buffer capacitor and the power source is much less.
It may seem simple, but in fact a current stepper driver is a very complex system.
haha okej. The thing is it says 18 mA when the motor is running and 313.5 mA when it is not running using 12 volt. Sometimes it shows -313.5 and sometimes 313.5 but I guess that is because it flows in both directions?
My powersupply shows 12v and 0.17 A. I dont fully understand why. I thought it should be the same?
I tried to explain, but it isn't easy to understand. You need to know the dynamic electrical behaviour of coils and capacitors. It's far more than Ohm's Law. As I mentioned, it is a complex system. This is probably one of the reasons why there is so much incorrect information about it on the internet.
Thanks a lot for your help! But to the core of the question. To put it easy; shall I simply go for the meassurment on my powersupply? Feeding the VMOT with 0.17 A and 12 v. And then estimate 10 of them to use: 1.7 A and 12 v?