DRV8871 Motor Driver Outputting Low Voltage?

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adiromualdo
 
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DRV8871 Motor Driver Outputting Low Voltage?

Post by adiromualdo »

I have a 12V dc motor that I'm trying to control with the Adafruit DRV8871 breakout board. When I plug the motor directly into the 12V source, I measure a current of ~2.16A. When I plug it in to the DRV88771 and run it through an Arduino outputting a 255 PWM, I get a different output. The voltage outputted is 9.2V and the current output is 1.8A. I already tried changing the RLIM resistor value to 15k to see if the current limiting was causing my problem. Does anybody know why this is happening? I need to be able to run the motor all the way up to 12V for my application.

Thanks!

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Re: DRV8871 Motor Driver Outputting Low Voltage?

Post by adafruit_support_bill »

Let's remove the Arduino from the equation:
What happens if you connect IN1 directly to GND and in2 directly to 5v?

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Re: DRV8871 Motor Driver Outputting Low Voltage?

Post by adiromualdo »

adafruit_support_bill wrote:Let's remove the Arduino from the equation:
What happens if you connect IN1 directly to GND and in2 directly to 5v?
I just tried and saw that the output from the driver is still the same. Is there an inherent voltage drop across the driver?

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Re: DRV8871 Motor Driver Outputting Low Voltage?

Post by adafruit_support_bill »

There will be a drop across any driver. But the DRV8871 has fairly efficient FET outputs and at those current levels I'd expect the voltage drop to be less than a volt.

Do you have access to an oscilloscope to look at the output? That would tell us for sure if the current limiting was active.

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Re: DRV8871 Motor Driver Outputting Low Voltage?

Post by adiromualdo »

I do, but I'm not the most competent with it. I hooked the scope up to one of the terminals and to ground. See the linked photo. https://imgur.com/a/PMpSwss

Edit: I zoomed in a bit and this may be a better image for you. https://imgur.com/a/E4SDhNb

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Re: DRV8871 Motor Driver Outputting Low Voltage?

Post by adafruit_support_bill »

Something going on there. But I'm not sure I understand the scope display correctly.

Based on the 50mV/division, it is only about 150mV pp. But it looks like the scope is seeing 737mV rms.

And the frequency does not make sense either. It appears to be saying it is < 10Hz. But based on a horizontal scale of 50uS/division, it looks closer to 1KHz.

If you zoom way out on the time scale, I suspect we are jut looking at a piece of a larger waveform.

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Re: DRV8871 Motor Driver Outputting Low Voltage?

Post by adiromualdo »

Here's another picture of the scope reading that I took using the "auto" button: https://imgur.com/a/Lzpo0GT

What waveform should I expect to see if the driver was functioning normally?

Also, it's worth noting that I have 2 of these drivers hooked up to different motors and experience the same issue with both of them. It makes me think I'm hooking them up incorrectly or the board just drops more voltage than I thought it would.

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Re: DRV8871 Motor Driver Outputting Low Voltage?

Post by adafruit_support_bill »

I think what you are seeing on the scope is probably just brush noise from the motor. The scope can't lock onto any frequency, so it is just saying "<10Hz".

We can try replacing the driver. Please contact [email protected] with a link to this thread.

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