Do I need resistors to pull a pin low? (A4988 Driver)

Other Hardware Motors, Mechanics, Power and CNC
1

Hi, I'm currently working on a project involving A4988 Stepper Drivers.
I'm using a knockoff PCB Board, like this one: botland.de

I am using 4 of these with modified 28bj-48 Stepper motors, but somehow they only work sporadically. I've connected them like this:
VDD -> 3.3V
VMOT -> 8V
Pins A1-B2 -> corresponding Motor pins
GND -> 0V
EN -> not connected, but seems to be pulled low using a 100k resistor
MS1, MS2 and MS3 are tied together, I only need full steps. They are left unconnected, but seem to be pulled low by a 90k resistor-
SLEEP and RST -> connected together
STEP and DIR -> connected to my microcontroller

When sending step signals, the motor sometimes works, but other times doesn't. When I connect EN to GND using a Wire, it works. But shouldn't it work without this wire? It is pulled low using a 100k ohms resistor on the board.
I'm wondering if it's safe to just connected it straight to GND without a resistor?
I appreciate every answer

2

Similar boards are not reliable. It must be exactly the board You use.
Know that a PCB is not useful when fault finding. Schematics are needed.

Modified how, to what?

There's a fear the stepper and the driver are not compatible.

Please read and use this link: How to get the best out of this forum - Using Arduino / IDE 1.x - Arduino Forum

3

Okay, I will add some info. I bought the drivers from a eBay Vendor, so no Datatsheets or Schematics., they are at least visually the same as the ones in the link, same pinout and component placement. I modified the 28bj-48 Stepper to be bipolar, by cutting the common +5V PCB trace, according to this tutorial: https://www.youtube.com/watch?v=lLF9_rc9G3I

I am using a ESP32 Dev board. I hope this info helps

4

That's not at all good enough. This is not about make up, it's about electronics and precision is a must.

That's a mistake buying such things.

Sorry but I don't spend time watching tutorials, often made by ignorant people.

The 28bj-48 stepper is a 50 Ohm stepper and it's not compatible with the A4988 driver. That driver works with low Ohm steppers that need current controlled. The 28bj-48 is not that type.

5

"That's a mistake buying such things." I'm a student and this is for a hobby project, I didn't want to spend too much money, so I ordered from eBay. I guess I will have to order them from a different seller. But the Drivers seem to be working well some times, so I don't think that the driver is broken.

I don't understand this. Sure, the normal, unmodified (unipolar) version isn't compatible, but when removing the common 5V, it's just a normal bipolar stepper. And the current is controlled by the current limiting driver. (The current is set by the potentiometer). So the coil resistance shouldn't matter, right?

6

No. It's resistance is too high.

That's given You this situation.

No, it'a wrong. Several other members have tried this, and failed. You're the next.

7

Small Update, all 4 Motors and drivers work now. There was a short between EN and 3.3V. I fixed it, and it seems to work flawlessly. Of course I will do some further testing, but not today anymore. Thanks for your help

8

Not much of help but telling about a problem is magical, and the troubled person often finds the mistake.
All well now.
Thanks for telling!

9

What voltage is Vmot.
I assume you modified the steppers for torque at higher speed.
Shouldn't you use a 24volt supply for that.
Leo..

10

VMot is 8v, because I drive the motors with a 2S Lipo battery(8.4V when full and ~7V when empty). I'm using a boost-buck converter to keep the voltage at 8V. I mainly modified the steppers because I didn't think it would work without the modification, right? The A4988 driver can't drive unipolar steppers. That's why I modified the 28bj-48. My application requires very little torque and speed, so not that important. But I might try out 12V or 24V at some point.

11

Then why didn't you use the ULN board that came with the motor.

8volt isn't good enough for that A4988 setup, which is the absolute minimum for that driver. I would crank up the supply to at least 12volt.
Leo..

12

I didn't use the ULN2003 boards, because they are way to big and require to many pins for my application. In my tests, the A4988 worked well until ~7.4V. I'm planning to use a 3S lipo(11.1V nominal voltage) in the future, that will ensure reliability

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