I am using a 7805 and a LM3940 to supply 5V and 3V to my embedded system, which draws only 100mA.
The 7805 is so hot (after just 1 min or so) until I cant touch it (burning hot) and have to shut down to avoid damage. The DC input is 12V from a power adapter (which rated 1A). I know 7805 is dissipating (12-5)x.1A= 0.7W, but that should not be that hot right?
Attached is my schematic. Is there any problem with my power supply circuit.
I know I should add a diode between 7805 and LM3940 to prevent current flowback to 7805, but is that the reason of why 7805 is so hot? Also, the output cap of LM2940 is tantalum to reduce oscillation.