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DRV8825 Current Adjustment is not correct

#1

Hi,
I am using DRV8825s for my 2A holding torque stepper motors.
I have noticed that the current setting via Potentiometer doesn’t correspond to the measured current
through one coil (Multimeter with RMS current measurement) when the motor is turning.
I know the RMS current divided by sqrt(2) lower than the max. current.
I know that the max current is only 70% for full step mode ( I used 1/4 and 1/8 step mode)
I know it is only possible to set these drivers to 2A max current with a sufficient cooling, but the measured current doesn’t correspond to low values either.
The shunt resistor ist 0.1Ohm for both H-Bridges (both coils).

I would expect the max current to be 2A when setting the Potentiometer to 1V. However, I can only measure 1.4A (max. current) / 1.0A (RMS current).

I have tried both RMS measurement (when the motor is turning) and DC current measurement when only one coil is energized (0° electrical angle). Both values correspond to each other but don’t correspond to the adjusted voltage on the Potentiometer.

#2

Hello.

Can you tell me where you got your motor drivers? There are a lot of knockoffs of our stepper motor driver carriers out there. Are you sure they are DRV8825 carriers we made?

Can you post some pictures of your setup? I would also be interested in seeing how you are cooling the board so that it can handle 2 A.

-Patrick

#3

Hi Patrick,

ty for your answer. Don’t take my post as a complaint because I can confirm I haven’t bought the driver from your website or any of your official distributors.
They look similar to your carriers but the shunt resistor package seems to be smaller (maybe 0805 inch / 2012 metrical). I have found out there are 1/2 Watt shunt resistors but it seems like the shunt resistors on your carriers are slightly bigger. Which package do you use?

Cooling…well, I use a 12V fan to cool the tiny heat sinks delivered with the drivers. Better than nothing. However, I don’t think cooling is an issue here because I can measure the unexpected current right after i energize the cold drivers. I am pretty sure heat can only be an issue if the drivers are connected to supply voltage for 30s or more maybe, but not if the drivers drivers are cold.

I can tell you that I would buy genuine DRV8825 from you or your distributors if I can be sure that this problem doesn’t affect your drivers and if you can deliver a technical explanation why the cheap knock offs cant deliver the expected current. Did you do any tests/research with/on these knock offs and if yes, what was the result?

One reason why I doubt the knock offs use genuine TI DRV8825 is that the unit price for an order amout of 4000 ICs is about 1.60$/part. That is almost double the price they take for the rip offs on AliExpress. But who knows … I am not sure.

#4

My experience with Pololu products and Pololu support for those products has been excellent, and the products work as advertised.

I’ve bought some knockoffs from China, too, and many of those end up in the trash can.

You do get what you pay for, and I certainly don’t see the point of asking Pololu engineers why your cheap knockoffs don’t work.

#5

My experience with most cheap chinese products is excellent as well (for the price you pay).
Anyways, I am just curious if I can expect the correct current with genuine drivers or if they behave the same way.
You have to see it like this: If Pololu support can help me with this question, not just I would be encouraged to buy their products instead of the cheap knockoffs. The help of Pololu engineers in this forum is advertisement for their products.
Its a win - win situation.

#6

You have to see it like this

No, I don’t. I would rather see the engineers engaged in productive discussions about their own products.

#7

Our products will behave as described in our documentation for them. It is not worth our time or yours to wonder why that documentation might not apply to someone else’s attempt at copying our product (especially when that copy probably uses counterfeit parts).

-Patrick