DRV8871 Not pushing full volts

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bucc5062
 
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DRV8871 Not pushing full volts

Post by bucc5062 »

It has been a learning curve, but I finally got my DRV8871 drive controller to work with my Arduino program that controls a drive motor. As I put the wires together I would test each step to make sure I was getting power along the way. I have the arduino plugged into my desktop via usb. The program is simple, it sends ramp up and down signals to pin 9 and 10 to drive the motor forwards or backwards (the test program shown in the controller specs).

I decided to try and use an outside battery to run the motor using the MOTOR PWR IN pins on the controller while the Arduino sends the controlling signal. As I understood if you use an external power source with this controller you can use more than 5V and I eventually want to drive a 9 volt motor.

I added a image of the basic wiring I have.

So I am able to put a multi-meter on the wires going to the engine and I can see the change in volts as the program runs first for forward then reverse, but where I was expecting to see @ 9 volts going to the motor, I only get 2.5 volts showing on the meter. If I switch leads I get the same. What am I missing. From the documentation for the 8871 the logic board gets it's power from either the arduino or the external power supply (in this case 9 volt battery). Do I also need to run the 5v power wire and ground from the arduino? Is there some limiter I am missing or is it the program is only allowing x volts max when it hits 255 on the analog pin.

It is cool to see this running, but not cool at the low volts.

Happy to provide more info for guidance. Thanks. I attached the .io file with my arduino code if that helps.
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bucc5062
 
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Re: DRV8871 Not pushing full volts

Post by bucc5062 »

I am going to post my on reply because I tried one thing that I did not see in the documentation or wiring diagrams. While it did work with just the external power supply and no power from the Arduino the volts never exceeded 2.5 volts. However, I decided to plug the Arduino power into the controller PWR (+) and (grd) pins and when I did

SHAZAAM!

Now the full voltage of the battery was going to the motor output pins. Problem solved, but I do feel the documentation on the controller is not quite clear that you need both power inputs to make this work.
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Re: DRV8871 Not pushing full volts

Post by adafruit_support_bill »

Now the full voltage of the battery was going to the motor output pins. Problem solved, but I do feel the documentation on the controller is not quite clear that you need both power inputs to make this work.
You don't need two power inputs. In act it is generally a bad idea to have two power inputs. The missing connection was the ground connection. Your control signals are meaningless without a ground reference.

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