Pololu Response

Well, then there's the problem. The device can handle up to 800 mW but your customer was putting 1250 mW through it.

Those calculations of mine were simplifications to hit home the concept of power and its relationship to voltage and current. I purposely chose not to break up the two separate current paths.

Let me start by saying that Polulu appears to be a quality provider of products of robotics hobbyists. It is my hope that you continue to do so and that you profit as a product of that.

However, it is merely constructive criticism when I say that knowledge of the total power dissipation of the device is a necessity for using the device properly. If you don't think your customers can do the math, then you might provide them with charts which relate the maximum acceptable values for voltage and current through the device (even if they are only based on ballpark figures for maximum dissipation).

It will only help your ability to serve your customers if you provide some way for them to understand the limitations of the motor controllers you sell.

Thank for your response, Ed LeBouthillier

Did you read the other parts of his post before excising them? I don't know what you mean by "putting 1250 mW through it", but as was explained in the original post, that is not the power that must be dissipated by the chip in this situation.

As the original post also explained, they've tested these chips with a short circuit and they shut down but were not damaged.

Mitch Berkson

Let me first say what I think my interests are in this ongoing discussion. I want the robotics community to grow and more and better robots to be built. As such, my interests as I see them are:

1) To help Rylos learn what he did wrong 2) To ensure that Pololu's good name (that it appears to rightly deserve) is maintained 3) to try to ensure that roboticists, in general, are given the information they need to be better at building robots.

Second, let me admit that I made an error in my earlier post. Yes, I was wrong about the power going through your board being 1250 mW. I was in a hurry to get to work this morning and I realized while driving down the freeway that the power dissipated in the LB1836 is nowhere near the full 1250 mW. In actuality, it is more like 100 mW which is well below its 800 mW limit (at 5 volts and 250 mA).

Given the above, it appears that Rylos was doing something wrong, but we don't know what. I would like to try to help Rylos understand what he did wrong (assuming he has any interest in understanding that, himself). He should probably post a schematic detailing the way that he wired things up.

Additionally, I think that Pololu could better serve its customers by providing maximum dissipation values (however approximate). I've said it before, and I'll say it again, Pololu appears to be a quality vendor of parts and its board probably works precisely as designed.

I think that the robotics community would be better served if Rylos understands what he did wrong and that's what I've been working to establish.

Sincerely, Ed LeBouthillier

Hi Ed,

I have tried everything I know how, everything that Jan suggested and the suggestions of the other kind folks here on this group in earnest. I don't know how much more detailed I can be as to my application, specifications of the devices I'm using etc. Understand that I have invested many more hours troubleshooting the device than I ever intended to and it still isn't working for me and at this point my time is better spent developing my own design. The reason I bought an "off the shelf" controller was because I wanted to focus my attention on other phases and aspects of the design process, perhaps that was my mistake, thinking I might be able to save some time. Anyway thanks for all the suggestions.

-Dave

Dave,

Since you don't know why this device failed for you when many others have used it successfully, what do you think you will change in your new design that will make your design work any better.

I'm trying to say that if you don't know why this driver didn't work, and you ignore the problem, there is a good likleyhood of your new device also failing, and you won't know why that one fails either.

I would like to strongly suggest that you try and understand (With our help of course) what's different about your application of this driver so that, if you need to, you can design your own driver.

Alan,

Mainly using inexpensive power MOSFETS that are capable of driving many times more amps than I actually need using a standard bridge design with snubber diodes. I programmed an AVR to use the built in 8 bit hardware timers, output compare registers and their associated interrupts to set the duty cycle for a 2 channel PWM scheme (2 motors). I've already tested it driving the MOSFET gates directly with the PWM without any trouble but will probably buffer the PWM input further for additional protection, suggestions here would be appreciated, also to select a proper switching frequency as I have many options here using various clock frequencies and prescalers. Currently it's switching around 4 khz. Using a 16 Mhz oscillator, I can go as low as 30 hz or as high as 31 khz.

As far as the Pololu device is concerned, what else do you suggest I try? As far as hooking it up goes, it was tested on a high quality protoboard and the connections are very simple.

Regards,

-Dave

You could try powering the motor supply from batteries or a power supply (not a regulator chip) and seeing what happens with a simple load like a resistor.

Mitch Berkson

If you are pulsing current to the motor windings, wouldn't that have the effect of almost contineously keeping the motor at the starting duty current draw? Does the motor get warmer when it is running under PWM than it does when it is running off of a straight battery dc supply? I'd put a big capacitor across the power supply to the Pololu and an ammeter in series to see the current being supplied to the Pololu. I would think measuring accurate current flow in inductive/reactive loads such as a running dc motor where the current is being pulsed very quickly would take some specialized equipment. Checking the current in the supply power line that has filtering supplying the Pololu might give some useful data.

No, because the motor is an inductor. As long as the PWM frequency is high enough that the reactive impedance is high, the current will barely change from the +ve voltage to the adjacent -ve cycle. Many small motors have little inductance however, so that requires a high frequency.

For what it's worth, maybe you could seek out a mentor. There may even be someone on this newsgroup in your area who could help you. If not, there may be a robotics interest group close to you. Most roboticists I know love the subject and welcome any opportunity to talk about it; you may be able to find help this way.

Cheers, Ed L

Coreless motors in particular have very low inductance. Many manufacturers want to see a minimum of 30 kHz.

Mike

P.S. I didn't catch what type of motor was being used. Anyone know?

Some of the cheap pager-like motors from Radio Shack r/c cars are surprisingly coreless too. One way around the problem with lower frequency PWM would be to add a series inductance with the motor.

Kevin

I don't want to discourage you, but DC motor controller design isn't a first year maths project. If you can't look at the Pololu unit and figure out why it wasn't working with your motor, my humble opinion is that you won't be able to design one from scratch.

Dan