[amacal1]

I just got a new MMBe module in, and it finally worked! However, I destroyed it in short order. I can't quite figure out why and I was wondering if anyone can shed some light on it. I have one more module and I don't want to make the same mistake twice.

I had just gotten it to work with this configuration: I used the Vin (9v from the board, before the 5v regulator) for the Vmotor of the module. I used a small 9-18v hobby motor from radio shack. There were really no other specs for the motor so I have no idea how much current it used. Anyway, this all worked fine. The motor couldn't have used too much current since the board never had any problems supplying it. But, I wasn't happy with the performance of the motor at 9v so I decided to try a little higher voltage (after all the MMBe can handle, i think, up to 36v and the motor supposedly could use up to 18v).

Okay, so I used a 13.5v DC power supply (probably from a drill or something) that said it could provide .8A (I think). This seemed sensible since all of this is well within the range of the MMBe and the motor, right? I connected the neg. terminal from the power supply to the ground of the board, and I connected the +13.5v terminal from the power supply to the Vmotor pin. When I powered it up the fault indicator light on the MMBe came on and in short time the module was smoking. Dead.

The module still comes on and blinks to confirm that it's receiving commands, but there's a short somewhere on the Vmotor circuit because the entire board goes berzerk if I try to send any power to the Vmotor pin. What did I do wrong? Does anything that I did sound bad? I just want to know so that I don't make the same mistake on my only remaining MMBe module. Any insight would be much appreciated. Thanks.

[Lon]

amacal1,

Sorry to hear about the smoking MMBe. There are two pieces of information that are missing from your post.

The first is the peak current your motor draws. If you don't know that you should place a current limit resistor between the power supply and the VMOTOR pin. You really can't assume that you're not drawing too much current if you don't know the specification. Some small motors, like those used in RC cars, will easily draw 20A at startup.

You should also measure the voltage on your power supply to be sure that it is at 13.5V. If it's unregulated it could be higher. That probably isn't an issue, but transient voltage spikes stemming from abrupt motor changes could caused high voltages on the motor pins.

If you're in the US and mail the unit back I can probably repair it for you. As long as no traces are burnt open I can probably repair it for you (call for an RMA number).

Lon

[amacal1]

Hello, I decided not to send the broken module back since I had another one, just to let you know. But, I'm having the same problem again with my other module. I was content to run my motor off of 9v for a while, but now I need more.


This time around I am using a power supply that I am much more confident is providing a consistent 13.5 volts. I wire everything up and then power on the ucontroller and the MMBe. The MMBe gives me the "all clear" green blink at startup and then its "on" light remains green. When I turn on the motor's power supply, though, the module's fault indicator turns on. Now, I immediately remove the power supply to prevent burning this one up like the last module I had.

I should note that the motor draws 1.5A continuous and probably close to 6A at startup. But, I've tried to alleviate that problem by using a motor start capacitor and a high power resistor in series. I should also note that the MMBe gives me the fault indicator whether or not the motor is even connected. This leads me to believe that the motor may not be the problem and that the power supply is causing the fault. If voltage spikes from the power supply are causing the problem, is there any way to help that?

Any help would be much appreciated. Thank you.

[Lon]

It sounds like you were able to get your system working at 9V. You should revert back to the 9V system to verify that the fault indicator does not come on. Doing that just assures that the MMBe is not damaged. There's no need to waste time troubleshooting if it is damaged.

Assuming it works at 9V, there are a number of things that could be happening. The new 13.5V supply could be dropping out when too much current is drawn. Or the power resistor you have in series could be dropping a lot of voltage across it. For example, if you are drawing 6A through a 2 ohm resistor, then the voltage on one side of the resistor is 13.5V and on the MMBe side it would be 13.5 - I*R = 1.5V.

What size resistor are you using?

If this is the case you could try ramping motor speed changes up and down in order to eliminate current spikes. If you think the problem is voltage spikes you could try a TVS (transient voltage surpressor) or even place a linear regulator (12V) between the supply and MMBe.

Lon

[amacal1]

Okay, I did make sure that the faults did not indicate a damaged MMBe Module. I checked, and it still works fine with 9V.

Otherwise, the problem is that I never get the chance to even try any of the motor speeds. The fault indicator turns on as soon as I turn on the motor power supply. I have learned from experience to quickly remove the motor power or risk burning up the module.

I am using an 8-ohm resistor in series with the motor to limit the initial current to around 1.6A. However, I don't even think I have tested the resistor while the motor was connected to the MMBe module. The fault happens whether or not the motor is even connected, so I never got around to testing the motor with the resistor.

Lastly, I just tested the system again. This time I used a 1.4k resistor instead of a motor. I intended to try this and then test the voltage across the resistor to verify that it worked. Again, the fault indicator flashed immediately after the motor power was applied. I didn't get a chance to send a speed command or check the voltage. I think this serves as proof that the motor is not causing the problems, but the power supplied to it may be.

The power supply for the motor should be able to supply up to 15A, so I doubt that the voltage is dropping. Also, I never get a chance to send a command for the motor to run, so ideally it shouldn't be drawing any significant current.

Any ideas?

[Lon]

I guess I'm a little unclear. I'm assuming the resistor is between the supply and the MMBe. Is that the case, or is it in series with the motor?

When you say it worked with 9V, did you control the motor with the MMBe when using the 9V supply?

Did you have the 8ohm resistor in series when you used the 9V supply?

The MMBe H-bridge will work down around 6-7V, so an 8 ohm resistor (between supply and MMBe) would cause a low voltage fault condition at (13.5V - 6V) / 8ohms = 0.94A. For the 9V supply it would do it at 0.375mA.

I can't see any reason for the connection of a supply to system to cause a fault unless the wiring is incorrectly attached or the grounds are not common.

Lon

[amacal1]

The resistor is not b/w the power supply and the MMBe. I intended to place it in series with the motor to limit the current that could flow to the motor before it begins spinning. And, I really shouldn't have mentioned it because I never really used it while testing the MMBe. So, in short, I never used the resistor with the 9v or 13.5v supply while using the MMBe. The resistor should really be forgotten about.

I did control the motor with the MMBe while the MMBe was supplied with a Vmotor of 9v. When supplied with 13.5v, the fault indicator flashes.

The grounds are common. In fact, I change nothing except the power supply, itself, when I try the different voltages. All of the wiring stays intact.

[amacal1]

Well, I think I killed it.

I found a 12v battery (checked with multimeter, voltage was about 12.5v) and decided to give it a try. It worked, no fault. Everything worked fine. The motor was spinning and it was responding to commands for different speeds.

At some point it stopped working, but never indicated a fault. It was a little hot, but not really excessively so. After a little while, and some frustration, I reverted back to the 9v and replaced the motor with a resistor. Dead. No voltage across the resistor. Though, it appears to work fine: Green light when it powers on, no fault indicator, blinks when it receives a command.

Now, I am thoroughly confused. It never gave a fault, and I was watching it VERY carefully. I was checking how hot it was. When the last one died on me it gave the fault indicator (because I think it over heated). This one did nothing.

[Lon]

I'm sorry to hear that.

Could you answer some of the questions I posed earlier?

What do you think the difference was between the 12V battey and your 13.5V supply? If your system worked at 9V, but not at 13.5V, and worked for a little while at 12V, I suspect that the motor was drawing too much current at the higher supply values.

You can still send them back for evaluation and replacement/repair.

Lon

[amacal1]

I'm not sure what the difference could be between the two voltage sources. However, it probably did just draw too much current. Period. I have just decided to stop trying to sneak in just under the requirements for the MMBe and upgrade to a more powerful controller. I will try to send in the broken controllers as soon as possible. Thank you very much for your help.