Experiment 1 - Using Vref method

I started out using the Pololu method.  You change the potentiometer that changes a voltage reading that then is used to calculate the maximum current.  The equation is:

Imax = Vref / 8 / Rcs

Rcs is the resistance of resistor on the A4988.  The clones seem to have either 0.05, 0.1 or 0.2 ohms.  Mine uses 0.1 ohm version, so my equation becomes

Imax = Vref / 0.8

For my test:

1. I got a fresh A4988 out of a package
2. I did not touch the potentiometer. 
3. Using an ESP8266 WeMos since it has both 3.3V and 5V output pins.
4. The 5V pin, reads 4.96V.
5. Connect to the A4988 VDD pin
6. Ground to ground
7. Took a Vref reading of 0.80V
8. Using the equation of Imax = VREF / 0.8
9. Imax is calculated as 1.0A

Same test, but...

1. Using 3.3V pin, reads 3.29V.
2. Took a Vref reading of 0.54V
3. Imax is calculated as 0.678A

Note:  How this is dead-on a ratio of 0.678*5/3.3 = 1.0A

Although the Pololu site mentions that the pot calibration must be redone when changing the VDD voltage, it does not mention that we can get the full potential of the A4988 while using a 3.3V on the VDD input.

Experiment 2 - Measuring Coil Current

I now hooked it up in-series with one of the coils as Bill has recommended in his video.  

When using a VDD power of 5V, we get a coil current of 0.61A.  When using a VDD power of 3.3V, we get a coil current of only 0.38A. 

Three things come out of this experiment.

1. The output current to the stepper is dependent on VDD voltage.
2. The equation Imax = Vref / 8 / Rcs is not correct.  It needs to also be a function of VDD.
3. That the equation is still not predicting the coil current.  We were supposed to get 1.0A with VDD of 5V.  We're only getting 0.61A.

Experiment 3 - Adjusting potentiometer for maximum Vref.

1. Using a VDD input of 3.3V, I adjusted the Vref for its maximum value.
2. This gave a reading of 1.08V.
3. Imax(3.3V) = 1.08/0.8 = 1.35A
4. Using the same setting, but with 5V on VDD, we get a reading of 1.614V
5. Imax(5V) = 1.614/0.8 = 2A

Experiment 4 - Measuring Coil Current at this "max" setting.

1. Using VDD=3.3V, read 0.85A.
2. Using VDD=5V, read 1.3A

So far, I have not found any reference to this behavior of dependency on the VDD input voltage.  

https://amz.run/5z7Y

  OK, that makes a bit more sense ... you are using the transistor as a level shifter, though it will probably be open-drain or open-collector. which is an inverter circuit, so the software will need changing.

But if the A4988 is running on a 5V supply, why would you need a battery?

I too think the packaged level converter would be more convenient than this ...

  and fixing the resistor on the A4988 board better still ... providing you have the stuff to solder a surface mount resistor.

Best wishes my friend, Dave

What I was suggesting was to use the 3.3V signal from the ESP to drive a transistor to pass 5V from the battery to the A4988 for both the Step and Dir pins. The A4988 pins are meant for 5V and there is no chance of the voltage being passed back across from the A4988 to the ESP.

I prefer the level converter because it has 2-way conversion and is ready-made so no calculations or prep is required, just bolt it on and solder the wires.

re:Just use a transistor to drive it from your battery.

Sorry, perhaps I don't understand what you are suggesting, but powering the A4899 at a higher voltage than 3.3V means the I/O pins that connect to the controlling microcontroller will expect the microcontroller to be running at 5V

Connecting to a 3.3V processor like ESP8266 or 32 to a A4899 running at 5V is risks damage to to the microcontroller.

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Using a level shifter circuit, either a board/chip designated 'level shifter' or one of the discrete equivalent circuits is a more plausible solution.

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Although personally, I would start by trying to change the resistor on the A4899 board.

Best wishes, Dave

re: Are you saying I should be able to get the Vref than 0.88V or are you saying it is understandable that I only got 0.88V? 

I don't have a resistance value for the potentiometer R9 - as you can see, it was omitted on the schematic I showed above ... and I don't have a board, so I can't measure it ... so I can't directly estimate the voltage ... but I can tie the information you have tried provided to what I know.

I am saying:

• You need to be able to get Vref up to about 1.33V to see 2A current ... 0.88V will not be enough
• With VDD of 3.3V, the maximum voltage will only be about 66% of that available when VDD is 5V
• If the resistor and potentiometer were chosen for a maximum voltage of 1.33V when VDD is 5V, then when VDD is 3.3V, the maximum voltage would be 66% of 1.33V = 0.88V ... which matches your experience! (Although I am not clear how you managed 1.6V with VDD of 5V)

So, although the chip is probably fine with 3.3V VDD and microcontroller logic interface, the board is not due to the choice of the resistor value for R5, shown as 20K in the schematic above.

If the value of that resistor in your board is 20k, then reducing it to about 10k would increase the maximum voltage to a little over 1.33V, when VDD is 3.3V ... a lower value would be required for 1.6V.

Hence, if you wanted to build a system using the A4899 board, with a 3.3V microcontroller, such as ESP8266 or 32,  then it should be possible by changing the resistor.

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BTW, I note https://www.pololu.com/product/1182 says

(The A4988) ... can deliver up to approximately 1 A per phase without a heat sink or forced air flow (it is rated for 2 A per coil with sufficient additional cooling)

.. suggesting additional heatsink may be required for 2A operation

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I just saw this photo of A4899 board at

https://www.pololu.com/product/1182#lightbox-picture0J10063;main-pictures

which showed a 30k resistor in series with the potentiometer in the corner of the board.

If your board is like this one, then this is the resistor to change ... but the value required for VDD of 3.3V  might be a bit higher ... say around 15k.

Changing the resistor takes a couple of minutes careful soldering, but is otherwise a cheap fix!!

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A quick look at the TMC2209 chip data sheet ...

https://www.trinamic.com/support/eval-kits/details/tmc2209-bob/

it shows a power supply pin that may be driven to 3.3V or 5V that is only used for the I/O pins, so assuming the board you have has that connection point to your 3.3V supply, then I would hope it would be fine.

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Best wishes,

Dave

Since you'll probably be using a lot of them, it may be easier to use level shifters.

Try looking up "3.3V 5V level shifter" on your Amazon site.

EDIT: or "level converter"

@inq 

I thought it should work, you're just using the 3.3V to drive a 5V signal through a transistor.

I agree that it sounds stupid to me too, but I thought hey - IF it works ...

It's all black plastic magic to me... that setting the logic side to 3.3V limits my high-voltage 24V, high-current side to only 1.1 amps... yet changing it to 5V logic lets me go up to 2 amps... seems well ... like the typhoon in China causing the butterfly wings to flap in Florida.  I'm all for throwing 24, 5 and 3.3V at it if it gets me my 2.5 amps.

VBR,

Inq

I thought it should work, you're just using the 3.3V to drive a 5V signal through a transistor.

I agree that it sounds stupid to me too, but I thought hey - IF it works ... My main concern is whether it would be fast enough to drive the wheels at speed.

Just use a transistor to drive it from your battery.

 🤣  At first I thought you were pulling my leg!  Drive a stepper motor with just a transistor.  Remember, I'm hardware challenged... then it just hit me.

You mean drive the driver board logic side with 5V from the battery.  I wonder if that would work... will it let me trigger the step and direction from 3.3V  ESP8266 pins, yet power it with 5V.

I'll wait for someone less challenged to say if it might work... then I'll see if the A4988 lets the smoke out.

VBR,

Inq