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QT PY io pin current output
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QT PY io pin current output

by SmallThings on Mon Feb 08, 2021 11:24 am

I am trying to control a micro stepper motor from a very small microcontroller. This works fine directly from an arduino nano with no dedicated stepper controller, so I can safely assume that my little motor is drawing somewhere between 1 and 40mA. I purchased a seeeduino xiao to run the motor, but the io pins cannot output enough current. I really need the form factor of the xiao, so naturally I gravitated towards the qt py. I cannot seem to find any information on single io pin current output though. Any information on the max output current for QT PY io pins would be greatly appreciated :)

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Re: QT PY io pin current output

by dastels on Mon Feb 08, 2021 1:27 pm

You will definitely need a driver of some sort. I've seen SAMD21 pins rated at 7mA.

Always assume you'll need a driver for anything more demanding than a small LED.

Dave

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Re: QT PY io pin current output

by SmallThings on Mon Feb 08, 2021 1:45 pm

dastels wrote:You will definitely need a driver of some sort. I've seen SAMD21 pins rated at 7mA.

Always assume you'll need a driver for anything more demanding than a small LED.

Dave


Dang, I was hoping that wouldn't be the case. A dedicated driver seems like such a waste of space in my project to run something this small Image

I'll search around to see if there are any small form-factor stepper drivers floating around

Thank you for the information!

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Re: QT PY io pin current output

by adafruit_support_bill on Mon Feb 08, 2021 2:06 pm

Have you measured the resistance of the coils? You can calculate the actual current requirements from that.

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Re: QT PY io pin current output

by dastels on Mon Feb 08, 2021 2:08 pm

That's cute! What's the current draw?

The Xiao uses the same MCU as the QTPY (SAMD21) so it'll be they same functionally.

I'm wondering if you could make something like a shield the same size/shape as the Xiao or QtPy. A surface mount ULN2803 should fit ok.

Dave

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Re: QT PY io pin current output

by SmallThings on Mon Feb 08, 2021 2:22 pm

adafruit_support_bill wrote:Have you measured the resistance of the coils? You can calculate the actual current requirements from that.


Oh that can be done? I was wondering why all the manufacturers included that information lol My apologies, I'm very new to electronics

It's around 30ohm!

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Re: QT PY io pin current output

by SmallThings on Mon Feb 08, 2021 2:29 pm

dastels wrote:That's cute! What's the current draw?

The Xiao uses the same MCU as the QTPY (SAMD21) so it'll be they same functionally.

I'm wondering if you could make something like a shield the same size/shape as the Xiao or QtPy. A surface mount ULN2803 should fit ok.

Dave


I think I was able to find something similar to what you've described! Not quite the same size as the qtpy, but very close https://www.aliexpress.com/item/4001321316224.html?src=google&albch=shopping&acnt=708-803-3821&isdl=y&slnk=&plac=&mtctp=&albbt=Google_7_shopping&aff_platform=google&aff_short_key=UneMJZVf&&albagn=888888&isSmbAutoCall=false&needSmbHouyi=false&albcp=9824643245&albag=103112554314&trgt=896505316494&crea=en4001321316224&netw=u&device=c&albpg=896505316494&albpd=en4001321316224&gclid=Cj0KCQiA34OBBhCcARIsAG32uvP3XfAIOIVynutcebYxNyOXhLd7LBt7qzVTlArpBUfioc-0RVhBxssaAp3tEALw_wcB&gclsrc=aw.ds

still trying to work out the current draw based on coil resistance

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Re: QT PY io pin current output

by SmallThings on Mon Feb 08, 2021 2:46 pm

If I'm doing ohm's law correctly, then my tiny stepper actually draws around 100mA, so there's a decent chance I was pushing my nano a bit too far by connecting it straight to the pins. I'll definitely need a dedicated stepper for the qtpy

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Re: QT PY io pin current output

by adafruit_support_bill on Mon Feb 08, 2021 3:19 pm

If you are driving it with 3.3v, the worst-case current draw will be 3.3v/30ohms = 110mA per phase. The actual draw will decrease with increased step-rate due to the effects of inductance and back-EMF on the impedance. But unless you have active current limiting, it is best to design for the worst-case.
https://learn.adafruit.com/all-about-st ... he-stepper

The smallest stepper driver we have is the DRV8833.
https://www.adafruit.com/product/3297

You can check Pololu, they have a pretty wide variety of stepper driver boards.
https://www.pololu.com/category/120/ste ... or-drivers

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Re: QT PY io pin current output

by SmallThings on Mon Feb 08, 2021 10:24 pm

adafruit_support_bill wrote:If you are driving it with 3.3v, the worst-case current draw will be 3.3v/30ohms = 110mA per phase. The actual draw will decrease with increased step-rate due to the effects of inductance and back-EMF on the impedance. But unless you have active current limiting, it is best to design for the worst-case.
https://learn.adafruit.com/all-about-st ... he-stepper

The smallest stepper driver we have is the DRV8833.
https://www.adafruit.com/product/3297

You can check Pololu, they have a pretty wide variety of stepper driver boards.
https://www.pololu.com/category/120/ste ... or-drivers


thank you very much! this gives me a great starting point to further develop my project :)

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