Limit Current Maintain Voltage PSU output

I want to maintain the output voltage of a power supply even when a circuit wants to pull more current than a defined upper limit (so I don't want to pull the voltage down due to over current). I'd like to supply 20 V to a circuit, while only allowing a maximum current draw of 150 mA. The circuit will try to pull more current, but I want to observe its operation before the circuit's over current protection kicks in. Would this be best done with an active inrush current limiter? Any circuit/module suggestions?

You are asking for contradictory requirements but what I think you want is a constant voltage power supply with an indication when the current has exceeded 150mA.
You will have to start with a supply capable of more current and add a monitoring circuit to it. My first thought is a simple voltage regulator with a small series resistor in it's output connection and (maybe with an amplifier) a comparator to tell you when the voltage voltage dropped across it means more than 150mA is flowing through it. If you need to delay shutting it down, you can use the output of the comparator to trigger a timer circuit.

Brian.

Impossible.

Voltage and current go hand in hand.

To control one you have to modulate the other.

Voltage sources regulate voltage by modulating current.

Current sources regulate current by modulating voltage.

Use PTC+Zener combination. Better to use PolyZen from little fuse

perhaps you need a current monitor and a microcontroller, so that the micro can allow the overcurrent to be drawn for just long enough for you to make your observations.....your power supply will need to be rated for that output voltage and current, though if not having overcurrent for a long time, then you wont need to have the heatsinks etc made to withstand it for long periods.

Mains transformers (50hz) are known for being able to supply current well above their nominal for intervals and not have a problem......when i did a SMPS for a guitar amp they were moaning that mains transformers were great for their useage because they can handle the average power throughput fine and also handle the severe overpower transients that get pulled.

Also, buyers of guitar amps like them to be big and heavy!...they think its a mark of quality for their heavy metal use.

what you seem to want/need is an active resistor to be in parallel with your load
changing the behavior of this (so-called) active resistor may get close to what you want

use a series current regulator set to about 160 mA, probably with a 30 V source,

followed by a voltage regulator in shunt.

the extra 10 mA will provide some overhead to keep the shunt regulator working
when the load draws 150 mA (or a little more)

use all analog components - primarily transistor and resistors