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Amp limits on Stepper Motor HAT
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Amp limits on Stepper Motor HAT

by wsmoak on Sat Apr 02, 2016 4:27 pm

The docs for the Stepper Motor HAT say "The motor driver chips that come with the kit are designed to provide up to 1.2 A per motor, with 3A peak current."

Does that mean 1.2 A per DC motor? It can drive 4 DC motors or two stepper motors, so is it double for the stepper motors?

I have a stepper motor that the seller says was tested at 24V and drew 1.2A. I'm using a 12V power supply, so I expect it to draw 2.4A.

Is this safe to do with the Motor HAT or not?

(It's working fine... and it only gets used for 3 seconds every 20 minutes, so I don't think heat will be an issue.)

Here's a photo of how I have it connected: https://www.flickr.com/photos/10803470@ ... 623147925/

Maybe seeing the specification for the capacitor that sits next to the driver chip would help, is that available?

Thanks!

wsmoak
 
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Re: Amp limits on Stepper Motor HAT

by adafruit_support_mike on Sun Apr 03, 2016 12:27 am

wsmoak wrote:Does that mean 1.2 A per DC motor?

Yep.

wsmoak wrote:It can drive 4 DC motors or two stepper motors, so is it double for the stepper motors?

The limit is still 1.2A.

The circuits that drive the motors are called 'half bridges'. They can send up to 1.2A through a load when their output is high, and can accept up to 1.2A from a load when their output is low.

With stepper motors, you need a half-bridge at each end of a coil. During one phase, the left side is high and the right side is low. During another phase the left side is low and the right side is high. You'd connect a DC motor the same way if you wanted to make it rotate forward and backward.

A motor powered by two half-bridges like that is called an 'H-bridge'. When you draw the circuit, the pattern of transistor and motor connections looks like a capital H.

Each side of the H-bridge is still limited to 1.2A. It's just that you can choose which direction the 1.2A goes.

wsmoak wrote:I have a stepper motor that the seller says was tested at 24V and drew 1.2A. I'm using a 12V power supply, so I expect it to draw 2.4A.

Actually, you're more likely to get about 600mA. The current-to-voltage ratio in a motor coil roughly follows Ohms Law. Half as much voltage gives you half as much current.

The good news is that it should be well within the Motor Hat's comfort zone.

wsmoak wrote:Maybe seeing the specification for the capacitor that sits next to the driver chip would help, is that available?

We don't have any formal specs for that. It's an optional part you can connect if you want to reduce noise on the incoming motor power rails.

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Re: Amp limits on Stepper Motor HAT

by wsmoak on Sun Apr 03, 2016 10:51 am

Thanks for your help!

Anecdotally... things are not adding up. We tried with a 12V 1.6A power supply and the Motor HAT seems to be pulling more power than it can supply-- every time I step the motor, it moves a little and the indicator light on the HAT goes out.

With a simple battery pack holding 8 AA's, it works fine: https://vimeo.com/160630840#t=20s

I looked back at the code and I am double-stepping the motor -- single-stepping was not powerful enough to turn the augur it is attached to.

https://github.com/wsmoak/cat_feeder/bl ... ker.ex#L93 (it's in Elixir, but based on the Python code Adafruit supplies.)

We're going to try with a 12V 5A power supply and see how that works.

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Re: Amp limits on Stepper Motor HAT

by adafruit_support_bill on Sun Apr 03, 2016 12:47 pm

If you measure the resistance of the windings, you can calculate the worst-case current draw per phase: https://learn.adafruit.com/all-about-st ... he-stepper

Keep in mind that most stepping modes energize both phases at once, so your total current demand will be twice that.

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