Pololu motor controller buyer beware

Has anyone used their Micro Dual Serial Motor Controller with success? http://www.pololu.com/products/elec.html#motocon
They rate it for 1A per channel. I purchased one and sent it back because it kept going into thermal shutdown seconds after the motor would startup. The higher the motor speed command given, the quicker it shuts off.
After I spent more money shipping back the first one for which a proper RMA# was obtained, they sent me a replacement only after I called them and spent considerable amounts of time talking with their technical person named Jan Malasek who started giving me "ideas" about how to solve the problem with proper power supply design, signal ripple content considerations, voltage regulator flucuations, oscilloscopes and all sorts of advanced measuring and engineering concepts. No where on their website or in the datasheet will you read anything like this, not that these things are foreign to me, but if I had wanted to reinvent the motor controller I wouldn't have purchased theirs.
The replacement did the exactly the same thing even with a different small motor and separate dedicated power supply. This time they refused to accept a return claiming "I broke the first one and they generously gave me another" and "We have thousands of these things out there and don't really have any problems". Incidentally when I first spoke with Jan, he said that he believed that maybe their product just doesn't have the amperage capability I'm looking for. Hmm, let me see, how about not even a fraction of the claimed amperage?
Neither of the two that I had were capable of driving a motor anywhere near their claim of 1A per channel or 2A in single motor mode even with a big heatsink and a fan, not surprising since this thing is about the size of a penny but it is their claim, bogus as it is.
So nearly $30 dollars and several wasted hours later I'm back at square one with my next robot design, finding a suitable serially controlled motor controller. Any suggestions on some controllers that work as advertised? Maybe the solutions cubed stuff, or maybe I should just make my own.
Caveat emptor if you're thinking of buying any electronics from these people.
-Dave
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How much current does your motor draw?
"Rylos" <unknown> wrote in message

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It draws 250mA max, more at the startup transient like any but that doesn't last long.
-Dave

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BTW, that 250mA is no load current which is how I've been testing it.
Rylos <unknown> wrote in message

doesn't
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Rylos wrote:

Small hobby motors don't tend to be very efficient, and current changes with voltage. Is this the spec of the motor, or are you testing current draw during operation? Bear in mind that for small hobby motors the current is often rated at 3 volts, as they are designed for toys and such that are powered by two AA batteries.
If the controller indeed uses a 754410 chip then you should definitely be able to run the motor at 250mA.
You might just try making your own. With today's single-chip h-bridge designs there isn't anything tricky about it. The 754410 costs just $1.88 at Digikey in single quantities. You don't get the serial command stuff, but that can come later if you wish.
-- Gordon Author: Robot Builder's Bonanza Budget Robotics: http://www.budgetrobotics.com
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That's what I've decided on. I think I'll just custom program an Atmel AVR to provide the PWM on a serial command from the main processor and use a suitable driver chip and power mosfets that can handle the motor current.
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Rylos wrote:

Have you tested it with a resistor as a load instead of the motor?
Mitch Berkson
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Not with a resistive load, I did test the output with a DC voltmeter and it does range okay between 0V and full scale which for this motor was 5V, the device is rated for voltages 1.8V to 9V.

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Then that's not the max, is it?
That's probably the no-load draw, which is the LEAST current the motor will take. Motors and light bulbs are tricky loads, they both draw large current spikes on startup.
Read your motor's resistance, turning the shaft very slowly unitl you get the lowest reading. Take that number, and the voltage you're feeding to the motor, and that's a reasonable estimate of the locked rotor or startup current surge. (V/R = I)
I would bet that your startup current is larger than the controller's ratings, and unfortunately, transistors can't take much of a joke. If the controller is rated for 1A, then put a 1A fastblow fuse in series with the motor. If the fuse stays intact, and the controller blows, then I'd say you have something to gripe about.
I am planning to buy one of his 100A controllers, for a motor that draws 3A with no load.
--
KC6ETE Dave's Engineering Page, www.dvanhorn.org
Microcontroller Consultant, specializing in Atmel AVR
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It's the max at no load which I should have stated. I don't believe the startup transient is causing it to shutdown and neither does their technical person whom I spoke with. Remember this device never made it under the easiest of conditions (no load) and using a very small motor that falls well within the acceptable limits.
If the device is rated and sold for 1A continuous per channel then it should also be capable of handling the brief transients that motors with this kind of continuous rating can induce on startup. If not, they should be explicit in their datasheets about these transients.

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What's your motor's stall current?
--
KC6ETE Dave's Engineering Page, www.dvanhorn.org
Microcontroller Consultant, specializing in Atmel AVR
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About 2A.

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Twice rated current.. hmm.
--
KC6ETE Dave's Engineering Page, www.dvanhorn.org
Microcontroller Consultant, specializing in Atmel AVR
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As I mentioned the controller was tested at "no load", so it isn't drawing 2A and causing the controller to thermally shut down as I think you're implying.

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On Wed, 15 Sep 2004 14:56:44 -0400, "Rylos" <unknown> wrote:

Put your multimeter on 10A and put it in series with your motor and battery and see just what current the motor is drawing. You can pinch down on the motor shaft while it is running will see the current under load. Making assumptions about the current draw is time wasting when you can easily get the actual values.
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That's precisely what I did. No assumptions were made.

drawing
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Do you know about the "Open Source Motor Controller?"
The OSMC project may be what you're after...
It uses an Atmel AVR on a separate board called a MOB to convert the servo PWM signals to the kind the motor controller needs.
They're on version 3.22, and version 4 is currently under development.
There is a Yahoo group you can join for free to get the full story:
http://groups.yahoo.com/group/osmc /
There is a way to buy a bare PCB, partial or full kits... Even assembled and tested.
Here is the link:
http://www.robot-power.com
So instead of reinventing the wheel...
And these are battle tested, as seen on TV. Used widely in Battle Bot applications...
Thus MY interest...
;-)
Hope this helps!
Jack :)
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Thanks Jack I'll check it out.

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Rylos wrote:

Even if the motor is drawing 2 amps at start up the 754410 chip (if that's what the board is using, and it's possible) is rated for 2 amps peak (for 5 milliseconds, which is usually long enough for most any onrush spike).
Since I doubt it's a matter of the Pololu product being badly designed -- it's just off the shelf parts -- it's possible that A) you happened to get two bad boards, B) somehow you are smoking the boards doing whatever, or C) there's something screwy about the motors. IOW, it has to be *something*.
Mitch's suggestion of a honking resistor to simulate the load of the motor is an excellent one. Since this problem may reoccur with your own design that also uses off-the-shelf parts, it would be helpful to eliminate any possible issues by narrowing the field of suspects. Consider is a form of forensics for electronics.
-- Gordon Author: Robot Builder's Bonanza Budget Robotics: http://www.budgetrobotics.com
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You're thinking in the wrong timescale. Going to twice rated current, for ANY amount of time, may kill it.
--
KC6ETE Dave's Engineering Page, www.dvanhorn.org
Microcontroller Consultant, specializing in Atmel AVR
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