Below is a picture of my A-Star 32U4 Mini SV that i managed to smoke. You can see the damage on one chip, which I believe is the ISL85415 voltage step down regulator .
I’m not 100% sure how I managed to do this great feat. A simplified circuit diagram is below, the main features being
- The A-star connects to a Pololu A4988 stepper driver carrier black. It’s protected with a 330uF capacitor (Pololu #882) as recommended.
- The A-star connects to a Pololu #2481 SPDT relay carrier that in turn controls a small +24VDC solenoid air pressure switch. I did not use any flyback diode across the solenoid–probably BAD.
- The A-star was powered by a 24VDC unregulated wall plug power supply (32 VDC at no load). This didn’t seem ideal, but I thought it would be safe, as the A-star’s voltage regulated is rated at 5-36V.
- (A small < 3" TFT display was also connected to the A-Star and undamaged, but I don’t think that was related.)
The lack of flyback diode is my best guess of the cause, maybe somehow coupled with the power supply and capacitor? BUT what is curious is the smoke occurred shortly after I had disconnected the solenoid. I had started using the solenoid valve a few times, then shut down the system, disconnected most components from the A-star (including stepper driver, capacitor, relay, and solenoid, but not the TFT), and then connected the DC barrel power jack again. At that time, there seemed some small sparks in the DC barrel jack and then the A-star was smoking quite much. GND and 5V pins on the A-star were found permanently shorted on the A-star. No components were damaged other than the A-star, and the stepper drivers run cool.
I can only explain this in maybe that there could have been damage prior to the smoke, maybe even when the system was turned off and the capacitor/solenoid still energized a short time and then removed (I think the solenoid was on when the power was removed)?? Does that make sense? Things were soldered together so accidental shortage of wires seems less likely.
Unless anyone would like me to post more such pictures, I plan to place a 1N4001 diode across the solenoid. Is there anything else worth doing?