I am looking to see if anyone has a circuit to detect if a 24vdc power wire falls below lets say, 22 volts. The circuit must run on the same 24 volts, but a regulator can be used to bring the voltage down if needed. It also needs to be pretty effecient since it will be running off of a super capacitor. The output of this circuit should be sinking.
A way to minimize current would be to use N-Chanel MOSFETS. They have a high input impedance so you can use resistors in the megohm range for bias.
For example, you could connect a 1 megohm pot (pot wiper connected to CCW end of pot) from the transistor gate to common (transistor source to common). Also connect a 1 megohm resistor from the 24V to the gate. Connect the transistor drain to the plc input.
Adjust the pot to give the desired 22V threshold.
The current draw from the 24V would be less than 20uA.
The output of this circuit is low (sinking) when the voltage is above 22V and high otherwise. Hopefully, you can adjust the signal requirements for the plc to accept this. If not, then you would need to add another transistor to provide the inversion. Just add a 1 megohm resistor from the drain of the first transistor to the 24V. Connect the drain to the gate of the second transistor.
The circuit does not have high stability with temperature but may be sufficient for your needs.