It didn’t take long to get the first Decimus 4 board on the bench for a build. This first one will serve as a static testbed for code development and sensor testing. After verifying that the power supplies were good, the LEDs, display, buzzer and pushbuttons were added for a bit of a play. Since that went well, the rest of the components, except the wall sensors were added. I had run out of the MAX4427 gate drivers so I carried on without them. Don’t do that…
The motor driver MOSFETS are full-bridge devices and would normally have their gates drived directly from the MAX4427 gate driver. That means they would always be at one rail or another and either the top or bottom MOSFET will be on while the other one is off. With the gates floating, there is no guarantee about which MOSFET will be on and they can both be partially on. The result is potentially large currents flowing through the motor driver even without the motor connected. And tendrils of smoke gently wafting up from the devices.
Fortunately, the smoke seems to have just been hot flux and the power supply I was using went into current limit at 800mA. Without the current limit, the outcome could have been much worse. Nonetheless, the motor drivers may have sustained some damage so it would be best to replace them.
Without the gate drivers in place, the temporary solution is to short all the driver gates to ground so that I can continue developing code with this board while I wait for some new parts to arrive. That will ensure that one MOSFET in each half-bridge is firmly off.
So – until you are certain everything is correct, run from a current limited power supply and never, never leave the gates floating on a MOSFET half-bridge. But you already knew that. So did I but I rushed it anyway.
While testing, I have screwed the board to a piece of perspex as a precaution against shorts on my normally cluttered desk.