Pololu motor controller buyer beware

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:

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Reply to
Gordon McComb
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You're thinking in the wrong timescale. Going to twice rated current, for ANY amount of time, may kill it.

Reply to
Dave VanHorn

There is one other possibility that I haven't seen anyone else mention. The issue is, the device may be working fine, doing a thermal shutdown when it gets hot, but if you don't have a heat sink, then the chip can't get rid of the heat that it's generating fast enough.

Many packages can generate more heat than they're able to dissipate without a heat sink and since you're using somewhat near the upper limit of its capabilities, it may require a heat sink to work properly. One way to see is to check the specs for the chip and see if they suggest using a heat sink. Sometimes, they'll tell you the maximum heat dissipation for a particular package and you may be exceeding it.

I don't know whether this is the case, I'm just offering a possible explanation.

Cheers, Ed L

Reply to
Ed LeBouthillier

Thanks Ed, but I tried that to. Using both a heatsing and a fan. It's runs a little longer, but still shuts down.

-Dave

Reply to
Rylos

Well I'm not thinking in any "particular" timescale. As mentioned in my other post and as others have also mentioned and as the technical support person has told me given the motor specs, the transient occurs for a very short period of time and is not likely to cause the problem.

I also tried ramping up slowly from low speed to high. It has 127 speeds. Same problem, starts out spinning fine but as speed comes up it shuts off. Two different motors, (not same model) but similiar electrical specs. Tried heatsink and fan, runs a little longer but still shuts off. I'm beginning to think they probably just sent me back the first one that I was having problems with since I agree with Gordon that two bad boards are unlikely. I even hooked up the H bridge output to a scope amd can view the pulses shutting off and on repeatedly. I'm just going to design my own at this point and I already have an AVR programmed and ready to provide the PWM for a suitable driver so if you'd like to buy this one for a small project, let me know.

-Dave

Reply to
Rylos

One other thing that I haven't seen mentioned yet. Do you have noise suppression capacitors on your motor. Normally this is important for proper H-bridge operation. You should have a 0.1 uF cap between the motor leads and a 0.1 uF cap between each of the leads and the motor's case.

BRW

Reply to
Bennet Williams

What voltage and current, precisely, were you using? Looking quickly at your earlier posts, it looks like:

Average Voltage: 5 Volts Current: 250 milliamps Peak Voltage: 5 Volts Current: 2 Amps

First, I would say that Polulu's documentation is lacking because they neither state the maximum power dissipation or the chip they're using (so you could look up the power dissipation).

However, guessing that the chip is an L293-class chip (which includes the SN754410), then the maximum continuous dissipation is 2075 milliwatts. Based on the figures above, this gives:

Average Power = 5 * .250 = 1250 milliwatts Peak Power = 5 * 2 = 10 watts

My view is that Polulu needs to enhance their documentation to state either the maximum power dissipation or the chip they're using. If one went merely off their current specifications, one might think that one could use up to 9 volts at 2 amps (for 18 watts); this would far exceed the specification for these kinds of chips. Maximum power dissipation is a requirement for properly using the chips.

Cheers, Ed L

Reply to
Ed LeBouthillier

One other thing that might be required is back-EMF diodes. L293's (at least the variant whose documentation I examined) don't have back-EMF diodes built into them. If Polulu didn't add them (assuming they used L293's), then back EMF could have caused a problem as well.

Cheers, Ed L

Reply to
Ed LeBouthillier

Hi Ed,

The 5V and 250mA are the precise values. The current was the steady state, no load current draw. I completely agree with you about their documentation being very lacking. The device, at least mine appears to do everything it's supposed to as far as the control and PWM goes, it just can't drive nearly as much amperage as they claim and that's been my whole gripe, maybe I just got a bad one. A motor controller that could handle 1A continuous per channel would be more than adequate for my next robot but my board doesn't come anywhere close to the rated specs. Either de-rate it, or be more explicit in the datasheets about device performance characteristics. Maybe, just maybe this thing could drive a pager motor.

-Dave

Reply to
Rylos

Hi Bennet,

Good advice and yes I did that too but it had no effect on performance.

-Dave

Reply to
Rylos

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