Is it possible to add two independent power supplies together to achieve a steady 3.3V powers supply? One power supply will be running from a 9V battery and the other is a 12V source. They both don't have to necessarily run together i.e. the if the 9V battery is just connected, it should still supply 3.3V. I have a buck converter that can output a steady source of 3.3V (LM2576-3.3V) and can handle a wide range of voltages (up to 40V). Is there some kind of passive voltage adder circuit that I can use that has some power protection diodes?
If you don't care about efficiency, use two diodes, like below:
- Ideal/smart bypass diodes (such as SM74611) instead of D1 and D2.
- Some ORing FET controller such as LM5050 + nMOSFETs.
- Some some autoswiching power mux.
If, instead, you must have 2 separate 3.3V regulators, then use something like a TPS2113 (2.8 to 5V autoswitching power mux).
Use an electromechanical relay whose coil is powered from the 12 volt supply.
When the 12 volts is not present, the relay isn't powered and the 9 volts routes through the normally closed contact to the relay's contact switch pole. When 12 volts is there, the relay switches and disconnects the NC contact from the pole and connects the normally open (NO) contact to the pole. You have 12 volts also connected to the NO contact.
Or you use diodes as mentioned in another answer but diodes drop a small voltage that you may (or may not) be able to live with.