A new problem I have is to create an automatic switch between two sources, only one of the sources should be active at a time. There is a "backup" source (5V battery), as well as another source. Once this source drops below a 3V threshold it should stop (and go back to the battery).
We wish to use a power (N- type)mosfets (with a \$V_{gs-th}\$ of 2V) to do the switching. The best circuitry that goes into the direction I've come up to is as following:
simulate this circuit – Schematic created using CircuitLab
(Notice the diode above the mosfet is to emulate a power mosfet)
Now the gate voltage is simply the \$\frac{2}{3} \cdot \left(V_{bat} – V_{var}\right)\$. (PS: I know this 2/3rd is not correct, it is currently included in the schematic so I have a point where I can edit the exact voltage difference)
Now this works for the battery: it is disabled as long as the variable source is above the threshold. However this isn't the case for when the variable source becomes too low. Then the variable source should be "disconnected" (so it can charge up again).
But I can't seem to add such a feature without breaking the current way.
Best Answer
Well I finished it, basically I had to switch the source instead of the drain using comparators to prevent the transistors of acting like resistors and draining power. For those interested here falstad simulation the actual circuitry what we choose/created is placed.
It basically switches either one of them (by means of a comparator, op amp in circuit, - I simply forgot active items when placing this question). It switches based on some reference values. Experience in practical comparators dictated now that the source for the comparator always should have a higher voltage than both inputs - thus the inputs should simply be halved. Also from experience we had to add a "pull up" resistor. But other than that the circuit worked.
The digital schematics are there to provide hysteresis as the solar cell array can only provide power for half the time (values of current source & capacitor bank are adapted to be able to simulate without waiting hours).