I have been asked by a workmate if I could design a circuit which dynamically switches to another voltage source if the voltage of the primary source has dropped. He wants to install Solarpanels and would like to have a circuit which primarly uses the batteries being loaded by solar panels, for powering some lightening, but when the batteries are low, the power should be taken from the energy network. So I did some google research and some thinking and finally got the following circuit:
The red bordered area represents the batteries (the capacitor on the left) and the solar panels (the 12 Volt voltage source on the right and the switch on the top, simulating the sun shining on the solar panels). The 12 Volt voltage source left next to the red bordered area represents the power from the energy network after it has been transformed down to 12 Volt and having passed a rectifier. When the batteries are loaded, the P-Channel MOSFET is turned off, so the power is taken from the batteries. The diode which is passed by the current from the batteries, prevents the current to flow from the energy net to the batteries, so the batteries only will be loaded by the solar panels.
The part on the left (consisting of the bipolar transistor, the base resistance, the TVS diode and the consumer resistance) is a simple voltage regulator circuit as it can be found on wikipedia. The TVS diode of the circuit limits the voltage of the consumer to a fixed value. So if you choose a TVS diode with a breakthrough voltage of 12 Volt, the consumer will get a maximum voltage of 12 Volt. The bipolar transistor regulates the current flow in a way that it will be constant for the consumer.
When the voltage of the batteries drop, the P-Channel MOSFET will turn on, so the reduce in voltage and current flow will be compensated by the energy network. The lower the battery get, the more power will be taken from the energy network. In my simulation, the voltage of the batteries dropped from 12 Volt to ~9 Volt, then no more power was taken from the batteries and all power flowed from the energy network.