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Stepping motor controller - is this chip suitable?

I am hoping to build a controller board for this motor

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which says is needs 0.125A (12V version), using the A3987 chip

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What I find staggering is that the above chip comes in a miniscule package, yet it is rated at 1.5A and should thus easily cope with this motor - by about a factor of 10.

Admittedly the PCB design rules for the A3987 are pretty specific, to get the heat out of it, but in this application the heat generated (I2R where R is the Rds - about 0.6 ohm max) should be miniscule - of the order of 20mW.

I am really puzzled how such a motor could draw so little current. Is that 0.125A figure meaningful? The coil resistance is 75 ohms so it can't draw that much!

What concerns me is that I may be misunderstanding something basic about the motor spec. But with the coil resistance apparently being 75 ohms, the most the motor could possibly draw (excluding transients caused by the parallel capacitance of the coil) is 12/75 = 0.16A.

The price of the motor is interesting - about US$500!!

This one

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uses external MOSFETs and should be a lot more robust... but is it necessary?

There is a vast difference - about 10x - between the torque of a stepper like the one above and a brushless motor e.g.

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(roughly 10mNm v. 100mNm) which probably explains the different currents involved.

I am new to stepping motors and just haven't come across a motor of that size drawing so little current. But maybe stepping motors are like that...

Reply to
Peter
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Why do you find that staggering? The chip uses switch mode current control and there should be no other losses than the switching and RdsON losses. There are SOT23 FETs rated at 5A.

[...]

Start off by reading the classic stepper tutorial:

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--
Stef    (remove caps, dashes and .invalid from e-mail address to reply by mail)

Too much is not enough.
Reply to
Stef

[snip]

Steppers usually win on grunt, servos on speed. That stepper is

26mNM, versus the brushless DC-servo's 11.8mNM. Maybe you're thinking about the torque of the servo motor /after/ gearing down?

-- Cheers, James Arthur

Reply to
dagmargoodboat

snipped-for-privacy@yahoo.com wrote

Actually, on a closer look, the stepper does about 10mNm and the brushless is similar. I was looking at the stall torque on the brushless one.

Reply to
Peter

Stef wrote

Yes, I have seen them, but would *you* design a product which had to be highly reliable, over a wide temperature range, which used SOT23 FETs carrying 5A?

With a stepper motor driver one might need to adjust things like the current and various pulse timings, and if anything goes slightly out of perfection, a device like that will go up in a puff of smoke in about 1 second.

The one thing I have sussed out when looking at these stepper controllers is that most new ones work in "current mode" and one chooses a stepper motor which is specced at a small fraction of the supply voltage e.g. a 3V motor with a 30V supply. The resulting performance is much better, over simply switching a constant voltage onto the motor coils which is what all the older stepper chips do.

Reply to
Peter

That was not the point, I just mentioned them because of your amazement by the rated current of a certain size chip. Just compare the rated currents.

But to answer your question: Probably not. I'd feel more comforable if there was at least a 2:1 safety margin. But I would have to know all operating conditions and read the entire datasheet for a definitive answer.

But you seem to be on the virge of dicarding a 10:1 overrated chip just because of it's physical size, that does not make sense to me.

Study the specs, and make sure it is overrated by a marging you are comfortable with under all expected conditions. And if the chip still fits the application, just be happy with the small size. :-)

In chips like these (IIRC you were considering the A3987), current is set by a resistor, switch timing is not adjustable and it has built in protections. Adding external components increases the possible points of failure and you must make sure your upper and lower switch times don't cause overlap. And if you use standard mosfets for your external switches, you lose the benefit of thermal shutdown.

These current mode drivers have been around for a while now, so I would not call them 'new'. ;-) But you can still drive your 12V stepper from 12V with one of these. Have you read the Jones tutorial? I think it has a section on the pro's and cons of current drivers (it has been a while since I read it myself).

--
Stef    (remove caps, dashes and .invalid from e-mail address to reply by mail)

Life is like a bowl of soup with hairs floating on it.  You have to
eat it nevertheless.
		-- Flaubert
Reply to
Stef

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