Connecting +Vs to GND damaged SMC 18v7 - overheating problem

Hi there,

I was wondering if anybody could help me with my Simple Motor Controller 18v7 (Assembled version) - I received it last night and powered the low current side from USB, but in my rush to get the show on the road, stupidly managed to connect the high/positive side of a fully charged 7.2V 3700mAh NiMH battery to the (high current/motor side) GND terminal and the low/negative side to the to VIN terminal. After a flash of sparks and a puff of expensive blue smoke billowing away from the large power capacitor, the temperature rapidly spiked and I pulled off the power.

Powering it back up today off the USB is fine and all the low voltage side appears to work fine, but connecting the battery correctly now sends the temperature spiralling upwards, both with and without a motor connected. (EDIT: Without a load, the temperature does not rapidly increase) I’ve taken it to about 60 deg. Celsius and then removed the power for fear of damaging it further, especially seeing as parts of the component wherever the thermistor is are probably nearing 80 deg. Celsius at that point. Correction: Having left the controller for longer, it appears to now work fully. I am confused: What was the blue smoke and if something is broken, why does the controller still work?

Correction 2: I’ve done some further testing, and appear to have a grasp on the behaviour of the controller. I’ve tested at approximately 8 and 16 volts, measuring the increase in temperature during the test - it appears to be abnormally high, despite functioning in an otherwise normal way.

(All temperatures in degrees C)
Test 1 - 15.9V: 29.3 -> 37.3 in 5 seconds with motor at 100% - Safe-Start Failed at this point, ending the test.
Test 2 - 15.8V: 33.4 -> 40.6 in under 2 seconds with motor at 100% - Again, safe-start cutout ended this test.

I assume that the safe-start function is triggering because the motor driver chip is registering extreme temperatures, as this is listed in the manual as one way which safe-start failure can trigger? Is there anything I can do to fix this problem?

Thanks in advance,

Hello, James.

I’m sorry to hear about your trouble. Unfortunately, the SMC does not have reverse voltage protection, so the damage to the board is potentially extensive, to the point where repairing it is likely not a viable option. However, if you email us and reference this thread, we can probably get you a discount on a replacement board.

- Ben

Hi Ben,

Thanks for your prompt reply. It’s a shame to hear that, but alas, these things happen. Before I resign my current board to the ‘dead hardware’ shelf, do you think it is worth me replacing the large capacitor, as it appeared to be the source of the smoke and would save me a tidy sum over replacing the board? And thanks for the kind offer, I’m sure I shall be in touch to discuss a replacement board.


I suggest you desolder/cut off the big power capacitor and then, with no motor connected, see if the controller gets hot or reports motor driver faults while you command the drivers to do something. If there is no indication of a problem with the capacitor removed, then you should take it one step further and try replacing it and connecting a motor. If the board still inexplicably gets hot with the cap removed, then there is a damaged component shorting something out on the board.

- Ben