I have a project that uses a 12V regulated power supply.
It needs 5V for the MCU, so I have an LM7805 to drop the voltage down to 5V. The MCU is normally fed this way, but when programming it, the 12V supply is removed and the MCU is connected to a USB computer port to flash the program update. The USB power connection is tied to the LM7805 Output. I am careful that USB power and 12V power are never connected at the same time.
The 12V input is also used to run some small motors, with 5V relays controlling which motor(s) operate via MCU control. The motor connections are tapped off the input side of the LM7805.
This all operates just fine.
Today when I was flashing a program update (12V power supply physically disconnected, MCU power supplied by USB), I noticed that the motors would still run under program control. Slower, yes, but still running. The only way they could have received power is if the LM7805 supplied current from the 5V Output connection (USB Supplied) back to it's Input connection.
Does this make any sense? It's the only explanation I can come up with.
It needs 5V for the MCU, so I have an LM7805 to drop the voltage down to 5V. The MCU is normally fed this way, but when programming it, the 12V supply is removed and the MCU is connected to a USB computer port to flash the program update. The USB power connection is tied to the LM7805 Output. I am careful that USB power and 12V power are never connected at the same time.
The 12V input is also used to run some small motors, with 5V relays controlling which motor(s) operate via MCU control. The motor connections are tapped off the input side of the LM7805.
This all operates just fine.
Today when I was flashing a program update (12V power supply physically disconnected, MCU power supplied by USB), I noticed that the motors would still run under program control. Slower, yes, but still running. The only way they could have received power is if the LM7805 supplied current from the 5V Output connection (USB Supplied) back to it's Input connection.
Does this make any sense? It's the only explanation I can come up with.