Did I Destroy my Motor Controller? (18v7)

I’ve got a pair of 18v7 simple motor controller that I am installing into garden trains.

After working nicely for about a week, the first one failed, getting pretty darned hot!

Setup: Both are the kit version. I installed all pins and the capacitor, using skills developed as an electronic technician for several years at IBM and other companies (caveat: it was 35 years ago)
Power: 14.4 volt NIMH battery pack, fully charged, reading 15.06V at rest
Motor: Some little brushed train motor. Not huge.
Other attachments: XBEE, with ground attached to controller ground, RX to DOUT, TX to DIN, and (and hit me if I am being VERY bad here) Vcc attached to BEC 3.3 volt rail. Since the XBee only draws a handful of milliamps, I figure that this is not such a bad thing.

So, this evening, I had disconnected the Battery +, reconnected the XBEE, plugged in the USB and… NOTHING. Sad.

The green LED is solid on. This is unusual, of course with the SMC software running, which usually keeps the green LED blinking like mad. Oh, and it’s getting hot. I feel like the fellow with a similar problem at Simple Motor Controller 18v7 no longer powers motor

I soldered up the other controller, swapped it in to the same setup (connecting battery power), and have no problem.

Any suggestions for troubleshooting? Blame-placing? Cautionary Tale for others?

My fear is that connecting the USB with the XBEE connected to the BEC 3.3 pin drew too much power and fried something. That would seem overly delicate, but I am willing to take the blame.


According to the 18v7 guide, the 3.3V BEC output can supply around 150mA. I believe that is generally enough for the standard low power XBees, but it is not enough power for XBee pros. I think the 3.3V regulator supplies the driver’s logic, so if you fried the regulator, you may have fried the chip as well.

Hello, Cayenne.

I’m sorry you having trouble with the Simple Motor Controller. What kind of XBee were you using? The green LED being on is a good sign. Could you try disconnecting everything, including the XBee, and then plug the Simple Motor Controller into USB? If you have stuff soldered directly into the board then that’s OK to not disconnect it, but you should let me know exactly what is connected when you do this test. When you do this test, does the controller show up in your computer’s device manager? If it does, this would mean that the microcontroller is still running and the 3.3 V regulator is probably OK (and some other part of the board is heating up).

Besides the motor power, motor, and XBee, was anything else connected to the controller?


Some more questions: How long are the power lines from the power supply to the controller, and from the controller to the motors? Do you think you might have accidentally touched the battery connector to something while unplugging it?


Hi David. Okay, let’s give it a try!

Answering your questions in reverse order:

Power supply lanes are 4 inches. Controller to motor is 5 inches. It’s POSSIBLE, but unlikely that I touched anything with the power line while reconnecting, as I was very careful, and the intelligent placement of the terminal block at the edge of the card reduces the risk.

It’s very possible, however, that I connected the XBee to the 5V BEC for a short period. This worries me.

The XBee is a standard series 2 (2mw transmit power) programmed in API mode.

I am now sitting with everything unplugged that is not soldered in. So, only the pins and cap you guys supplied. Other than the XBee, power, and motor, nothing else was attached to the smc.

I like the device manager test! Sad that I have failed it. Nope, no port there. And the square chip is very hot.

Nope, it’s dead. Hot, but dead.

Hello, Cayenne.

Unfortunately, it sounds like your motor controller is dead. Please email us directly and we should at least be able to give you a discount for a replacement unit.


Pololu rocks!

My G-gauge train is merrily running around the track, changing speed at my slightest (Arduino-mediated) whim!

Thanks for the support.