Quick question (hopefully!). I'm new to electronics, but getting quite interested. Coming from a programming background, it's always interested me…
Anyway. I have a circuit with two DC Power sources, one is 12V the other is 7.2. I have a regulator which turns that down to 5V. Now, the 12V power source isn't always available. The 7.2V however, is always available. But I would like to build a circuit that will use the 12V if it's there, and 'fall back' to the 7.2 if it isn't.
Once I have this sussed, I need to have a look into re-charging the 7.2V battery from the 12V source when it's available, but that is a problem for another day!
It sounds simple, but I'm guessing it isn't? Please help. 🙂
Best Answer
Yes, this is simple. It can be done passively with two diodes. At these low voltages, they can be Schottky diodes, thereby dropping less voltage and being more efficient.
Put a diode from each source to a internal power node. When 12 V is present, the internal power node will be at just under 12 V. The diode to the 7.2 V source is therefore reverse biased, essentially disconnecting the 7.2 V source. When the 12 V source goes away, the internal power node will be at just under the 7.2 V.
A switching power supply can deal with this variation in source voltage and still produce your final intended 5 V reasonably efficiently regardless of which source is providing the power.