Setting current limit on A4988 drivers

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tioJim
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Setting current limit on A4988 drivers

Post by tioJim » Tue Jul 31, 2012 9:15 am

Hello folks

From the Wiki/Googling I'm thinking I need to limit my A4988 drivers to 1.5A by adjusting the pot until I'm reading 0.6V between ground and the pad indicated on the driver.

But is this with motor connected and if so should it be motor idle or motor under load?

Thanks
Jim

edwardrford
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Re: Setting current limit on A4988 drivers

Post by edwardrford » Tue Jul 31, 2012 12:50 pm

There has been some discussion on this in the past. If I remember correctly, you need only apply power to the board before adjusting the vref. Specifically, the motors do not need to be running in order to dial it in. Just energized.

-Edward
Shapeoko 1 #0 - a couple of upgrades.
Shapeoko 2 #0 - a couple of upgrades.
Shapeoko 3 #2 - Stock

DrRob
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Re: Setting current limit on A4988 drivers

Post by DrRob » Thu Aug 02, 2012 11:44 am

Copying an answer from the old Google-groups ShapeOko forum:

On 12 March 2012 20:50, Rob Noble wrote:

Hi,

I emailed Pololu to ask whether the current adjustment should be made with motor power on or off, and motors attached or not, and here is the reply:

On 12 March 2012 20:43, Clint Barnes <clint@pololu.com> wrote:
Hello, Rob.

Thank you for your interest in our products. We usually recommend that you do not use the VREF to set the current limit for the driver as this method is harder to get right compared to adjusting the limiting while measuring the actual coil current with a multimeter or similar tool. When setting the current limiting by measuring the actual coil current, you should have your motor hooked up and powered on. If you have to use VREF to set the current, you do not have to have the motor hooked up, or the output enabled.

Sincerely,
Clint Barnes
(702) 262-6648
www.pololu.com
..............................
Pololu Corporation
920 Pilot Rd.
Las Vegas, NV 89119
USA

I suppose the motor current would be more stable if the motors weren't turning, but if the motors are stopped then grbl turns the power off after a short while.

However, I note that there's only one motor !enable control for all three drivers, so if you set one motor moving a large distance very slowly, then you can set the motor current on the other two motors while they are stationary.

Rob.

On 8 March 2012 16:16, Rob Noble wrote:

If I read the A4988 datasheet correctly, it's a reference voltage generated by the variable resistor between logic supply and ground, that feeds into a comparator that compares it with the voltage across a low-value resistor that the motor current flows through (i.e. a small voltage that is proportional to the motor current). Therefore the reference voltage won't be affected by the motors or motor supply being on or off, so you can adjust it without motor power applied and without motors running. You need the logic supply on (e.g. connected to your PC via USB) or you won't see any ref voltage. If you adjust it with the motors running, there's a risk that the current will jump around a lot, which you'll probably get away with. Maybe. I adjusted mine with the motor supply off.

Rob.

edwardrford
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Re: Setting current limit on A4988 drivers

Post by edwardrford » Thu Aug 02, 2012 1:08 pm

Hi @DrRob! I thought you had disappeared. It's nice to hear from you again.

-Edward
Shapeoko 1 #0 - a couple of upgrades.
Shapeoko 2 #0 - a couple of upgrades.
Shapeoko 3 #2 - Stock

PLHS54
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Re: Setting current limit on A4988 drivers

Post by PLHS54 » Thu Aug 02, 2012 4:23 pm

I had the same question awhile ago too, you might want to read this thread.

http://www.shapeoko.com/forum/viewtopic.php?f=4&t=447

DrRob
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Re: Setting current limit on A4988 drivers

Post by DrRob » Fri Aug 03, 2012 12:04 pm

edwardrford wrote:Hi @DrRob! I thought you had disappeared. It's nice to hear from you again.
I'm still here, but there simply aren't enough hours in the day to fit everything in.

:-(

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