Can I Use an Isolated DC/DC Converter to Step Down a Switching Dynamic Signal?
Both 24V and 36V inputs are the same output in this system, while a 0V input is still a 0V output. So you just have to clamp the output to 24V. Here are two possible clamping approaches.

The top circuit will clamp at the Vz and all its variation with current.

The bottom circuit will clamp at 24V + Vd.

The resistor size is not critical. It is simply sized to limit the current going through the diode and the current that the output must source. Use the maximum expected input voltage in this calculation, 72V.

In the top circuit, with a 72V input, 48V will appear across the resistor. You can use this to calculate the current.

In the bottom circuit, with a 72V input, 47.3V will appear across the resistor (because the diode forward voltage drop takes up some of the slack).

Note that the bottom does not clamp if the system is not powered. This could fry your input depending on how your system works and is expected to be used. However, the 2K input impedance and R2 may still save your system in this case since it forms a voltage divider. With the shown value of 10K, it steps down 72V to 12V when unpowered.

If you are worried about resistors failing and frying your 24V output then you can use multiple resistors in series to achieve your desired value. If one fails, the others are still there to limit current. Diodes normally fail short which is fine diode failures probably won't fry your input.







simulate this circuit &ndash; Schematic created using CircuitLab