Dead 18v7 Simple Motor Controllers

Support Motor controllers/drivers and motors
1

Hello Pololu Community,

We purchased two Simple Motor Controllers (18v7, Fully assembled) to use with the Dagu Rover 5 tracked Chassis. We wired up the controllers and put them in RC mode with our Traxxas RC receiver. Initially we connected the controllers Vin to a 12v 5amp bench top power supply. All worked well so we then moved to the AA battery (6 AA cell) pack supplied with the Dagu rover and everything worked great then too. So, as a test, we then decided to swap out the AA battery pack for a 12v sealed led acid 12ah battery (This will be our power source during competition). Immediately upon connecting the battery the rover started to misbehave. One track went on full speed while the other track stopped moving all together. Clearly something was wrong. So we did the following next:

  1. Connected the controllers over USB to a PC to see if we could still communicate with them via the Simple Motor Controller software. - Software had no problem connecting to controllers and reading config and RC signal was received as expected.
  2. Measured the voltage across OutA and OutB pins. Voltage on controller 1 was fixed 12v regardless of input signal coming from RC controller. Voltage on controller 2 was 0v regardless of input signal coming from RC controller. (I’m starting to think something is wrong with Hbridge section now)
  3. The little green dot on the four square grid moves accordingly to RC input.
  4. No errors where displayed in the Simple Motor Controller software.
  5. Visual inspection. We noticed that some of the transistors in the H bridge section were missing the little black covering on them. One was still on the transistor but partially pulled off. We found the others in the base of the Dagu. (See attached photos)
  6. First thought we had was the Dagu motors drew too much current exceeding the design specifications of the 18v7 drivers. So, we connected the Dagu to a battery through a true RMS (Fluke 33) meter to try and capture current peaks. Readings resulted in 2.5A peak during startup. Seems like this is well within the design specifications of the 18v7.

So, did we do something wrong or are the 18v7 units not operating as expected?

Thanks,
Eric

Hbridge1.jpg2448×2254 1.5 MB


hbridge2.jpg2448×2230 1.49 MB

2

Hello, Eric.

I am sorry you are having problems with your Simple Motor Controllers. We have not seen MOSFETs break like that before; that looks pretty extreme. (Thank you for posting clear pictures.) I suspect that something was inadvertently shorted when the 12V battery was connected.

In your second image, I noticed what looks like a white plastic standoff between the boards with only a single screw head showing above it on the top of the top board. I cannot tell how long that screw is, but if it is long enough to contact VIN on the bottom board, it would make it easier to accidentally short various parts of the top board to the head of the screw.

By the way, it is difficult to measure peak currents using a multimeter like that (large current draws like that can happen quicker than the multimeter can register and represent accurately in its display). We mention on the product page for the Dagu Rover 5 that the motors have a 2.4A stall current at 7.2V, so I expect the motors to draw about 4A at stall when supplied with 12V. That should still be well within the capabilities of the SMC 18v7.

I noticed that you also sent us an email about the issue. I have responded to it, and we can continue handling this over email.

-Jon

3

Jon,

Thanks for your response and email. The screw in the stand-off only goes about half way into the stand-off so there is no chance it’s shorting agains the VIN header on the other board. I will look for email now and respond.

Thanks again,

Eric