Turning up current - A4988

[jembb5]

How do you turn up current for Z stepper motor

[William]

Hi,you can increase the voltage between VDD and GND on the driver board of GT2560 is 5V, and on the board GTM32 it is 3.3V.The normal range is 0.8V~1.2V.

[TimK]

Hi,

I am having an issue with my torque on the extruder, so found this post.

I have a Geeetech i3 pro B (or something) with N42 motors, the red A4988 and the GT2560 board. Printing PLA between 205 and 235 gives a click on the motor as it jumps back. I have calibrated Z steps, reduced friction as much as possible and have a good thumbwheel preset to my knowledge. I thne looked at upping this current. From my calcs and the A4988 datasheet, using a Rs of 0.2 Ohm, I got the max Vref to be +- 1.8V. Is it really safe to go up to 5 as you stated there William?

Tim

Edit: Noticed too late that I cannot read dates and this post is a year and bit old. Sorry. I also found a little more information on the site that has helped. The issue still persists and I want to know what the maximum Vref I can use is.

[_kaktus_]

Hi.


A good question.

The stpstick module uses two voltage levels Vmot and Vdd.
You said that you read the catalogue note so you know that Vmot is the voltage supplying the executive part (stepper motor), Vdd is the voltage supplying the logical part of the system.

The content of Wiliam's statement shows that depending on the controller's board model, the Vref voltage can be increased to the limit value defined as Vdd. What seems logical not to supply the system with voltage higher than the voltage supplying the logical part.

If we look at the catalogue note we will see interesting data tables.

Absolute Maximum Ratings

Screenshot_20180908_170346.png (73.53 KiB) Viewed 57812 times

.

ELECTRICAL CHARACTERISTICS

Screenshot_20180908_174830.png (203.24 KiB) Viewed 57812 times

While in the first table the manufacturer allows 5V5 as the maximum voltage for this input, one must be aware that it does not have to be the correct value.
The next table presents the value of 4V0 for this input as the maximum recommended.

I managed to get a module scheme where we can see that the actual maximum value (Vdd) is additionally limited by a resistor.

schematic diagram A4988 rev. md09b

schematic_diagram_A4988-md09b.png (399.23 KiB) Viewed 57812 times

One more essential remark:

Current limiting
To achieve high step rates, the motor supply is typically much higher than would be permissible without active current limiting. For instance, a typical stepper motor might have a maximum current rating of 1 A with a 5Ω coil resistance, which would indicate a maximum motor supply of 5 V. Using such a motor with 12 V would allow higher step rates, but the current must actively be limited to under 1 A to prevent damage to the motor.
The A4988 supports such active current limiting, and the trimmer potentiometer on the board can be used to set the current limit. One way to set the current limit is to put the driver into full-step mode and to measure the current running through a single motor coil without clocking the STEP input. The measured current will be 0.7 times the current limit (since both coils are always on and limited to 70% in full-step mode). Please note that the current limit is dependent on the Vdd voltage.
Another way to set the current limit is to measure the voltage on the “ref” pin and to calculate the resulting current limit (the current sense resistors are 0.05Ω). The ref pin voltage is accessible on a via that is circled on the bottom silkscreen of the circuit board. See the A4988 datasheet for more information.

And here it is very important.

Power dissipation considerations
The A4988 driver IC has a maximum current rating of 2 A per coil, but the actual current you can deliver depends on how well you can keep the IC cool. The carrier’s printed circuit board is designed to draw heat out of the IC, but to supply more than approximately 1 A per coil, a heat sink or other cooling method is required.
This product can get hot enough to burn you long before the chip overheats. Take care when handling this product and other components connected to it.
Please note that measuring the current draw at the power supply does not necessarily
provide an accurate measure of the coil current. Since the input voltage to the driver can be significantly higher than the coil voltage, the measured current on the power supply can be quite a bit lower than the coil current (the driver and coil basically act like a switching step-down power supply). Also, if the supply voltage is very high compared to what the motor needs to achieve the set current, the duty cycle will be very low, which also leads to significant differences between average and RMS currents.

So we all learned some interesting facts.
I recommend to everyone to read the technical documentation. At least a little bit.

[MX39]

Hi,
I'm new to 3D-printing and bought myself an A20M some weeks ago.
Today I wanted to tune the stepper drivers, but I got a little confused by this thread as my voltage settings should apparently be totally different.
Supply voltage is supposed to be 5 V.
It uses the A4988ET chips, so IMaxDriver = 2.5 A
I calculated the voltage setting by using: V = IMotor/IDriver

The motors used on my model are:
X-Axis: 42SHD0034-18A
Current: 1.0 A
-> Voltage: 0.4 V

Y-Axis: D42HS3418-24B22
Current: 1.68 A
-> Voltage: 0.672 V

Z-Axis: 42SHD0034-20B
Current: 1.0 A
-> Voltage: 0.4 V

Extruder 0: 42SHD0227-24B1
Current: 1.0 A (42SHD0227-24B)
-> Voltage: 0.4 V

Extruder 1: 42SHD0227-24B1
Current: 1.0 A (42SHD0227-24B)
-> Voltage: 0.4 V

Note: I couldn't find any data for a motor type 42SHD0227-24B1 used for the extruders. Hopefully it's the same as 42SHD0227-24B.


Am I doing this right, or did I miss some important thing?

[_kaktus_]

Hi.


The hardware must be taken into account for the calculation.
Different modules have different resistors to limit the motor current.
I made a presentation here.

[Ayrol]

https://www.youtube.com/watch?v=wpUMptJo-xs

Good morning, following the guide above and adapting it to a Geetech prusa I3 pro b I calculated the following:

- step resolution 1/16
- SMD resistor R200 = 0,2 ohm
- 42SHD0034-20B MOTOR = 5V 1A

SO....
IMAX=VREF /(8xrs)
VREF= IMAX(8xRS)
VREF = 1 (AMPER MOTOR) (8 x 0,2 (OHM smd))
VREF = 1,6 .....TO MULTIPLY X 0,098 (0,098 is (9,8% /100) is the current at the step of 1/16 according to what it says A4988 Datasheet)
vref = 0,15 V

But it seems to me a very little value !!
In short, you to how many volts you put those damned of A4988 on the GEETECH prusa I3 pro B ????

[MX39]

Okay guys, I'm back...
I ended up not changing the voltage on the standard A4988's yet, even though some of the steppers got really hot and sometimes I had repeatable step-skipping on SOME prints.
I had to keep the printer close around me, as there were quite a lot of fails - especially with the PLA getting stuck to the nozzle and forming nothing more than a ball of PLA.

Due to those problems and the pretty annoying noice I decided I want to try TMC2208's or TMC2209's.
I'v measured the voltage that came with the pre-installed A4988's just now and found they were all set to 1V.
They are using R200 (0.2Ohm) resistors.

Using the motors mentioned in my previous post and according to _kaktus_ linked formula, the drivers should have been set to
- 1A-drives: 8*1A*0.2Ohm=1.60V
- 1.68A-drive: 8*1.68A*0.2Ohm=2.67V
...which is way more than what they were preset on and even more more than what I calculated at first.
-> is this correct now?


...continuing for the TMC2208's that I want to use in the original Motherboard (GT2560 V3.0).
They use R110-type current sensing resistors (0.11Ohm), which calculates:
- 1A-drives: 8*1A*0.11Ohm=0.88V
- 1.68A-drive: 8*1.68A*0.11Ohm=1.48V
-> is there someone to confirm my thinking?
At least this sounds more normal to me, so probably also the calculations with standard A4988's are finally correct

Same values for the TMC2209's that I plan to use with an SKR1.4 Turbo-Board in UART-mode later on.


EDIT:
Found TeachingTech's Turorial on how to tune a 3D-printer on github.
For TMC2208 it states:
Irms = Imax / 1,41
Vref = (Irms * 2.5) / 1.77
This sums up to Vref = Imax , so the setting for a 1A-drive = 1V , the driver of a 1.68A-drive needs to be set 1.68V?

[AndreaDS]

Hi MX39,

I've the same issues (or doubts) you mentioned. I've found (in other forums and videos on Youtube) that the correct Vref is more or less the 70% of the calculated Vref.

So, for the extruder driver, Vref is 1.87V but I don't know if is too high.

I haven't tried it yet to avoid possible problem (ex. break the driver or motor).

Have you tried? Wich is your actual configuration?

Thanks

[MX39]

Hi Andrea,
right now I'm on

the TMC2208's in the original Motherboard (GT2560 V3.0)
They use R110-type current sensing resistors (0.11Ohm):
- 1A-drives: 8*1A*0.11Ohm=0.88V
- 1.68A-drive: 8*1.68A*0.11Ohm=1.48V

and I don't see issues yet.
But I really only did some test-prints for calibration.
Or at least not that huge prints.


The original A4988s were set to 1V for all the motors, but especially the two extruder motors got incredibly hot after a while.
And by "a while" I don't mean printing for a day, but like maximum 3hours for a print or so..
So I think that was not correct.