Fire and Sadness with A4988

Hi all, hoping that I can find some help with a rather perplexing (and costly) problem.

I ordered several pololu A4988 stepper motor driver carriers, and was attempting to run a “sanity check” test to see that I could power a motor coil. I used an Arduino connected by usb to my laptop for 5v, and an AGT 24v power supply for the motor power. There is one strange bit there–I wired the mains power coming into the power supply through one light switch, of the sort that is typically put into walls to control overhead lighting. The 24V coming from the power supply goes through another light switch before reaching the breadboard. I’m not sure if that might cause a problem, but I thought I’d mention it. Also, I apparently may have reversed polarity on the power supply hookup, but I’m not sure how to test that. I had the black wire coming from the power cord connected to “L”, if that helps.

Following the minimal circuit described on the product page, I tied the grounds of the two power supplies together and to the two GND terminals on the board. VDD got 5V from the arduino, VMOT got 24V from the power supply, the coils were connected as per the diagrams in the FAQ, RESET and SLEEP were tied together, and the rest were not connected. Powering this up caused the motor to jump. Powering off and back on led to fire. The previous setup without the motor also led to fire, although curiously not on the first time the power was applied.

I then discovered the excellent tutorial by decrDude, A4983 and A4988 Getting Started Guide. Following the instructions exactly, I managed to get as far as tuning the amperage of the motor to 1amp, and then powered off. The next time I powered on, a tiny hole blew out of the board and a spark shot up into the air.

So. I’m out of luck, and out of boards. What could be the problem? I am eager to provide any information that might be helpful.

For the benefit of any other readers, mossy173 and I have been discussing this issue via email. At the moment, I suspect something strange might be going on with his power supply (such as a brief voltage spike when power is first applied), since the A4988 should easily be able to handle 24 V, but we have not come to a definite conclusion yet.

- Kevin