Have you ever wanted to experiment with the LM317 IC in EveryCircuit? I propose a solution. You can make a decent approximation using just a single opamp and a 1.25V source.
The LM317 is a linear regulator integrated circuit that is pretty popular, powerful, and easy to use. It has three pins: input, output, and adjust. In my diagram, the input pin is on the left, the output pin is on the right, and the adjust pin is at the bottom. The unconnected switches are there to indicate the boundaries of the chip and are not necessary. You may notice the input pin is disconnected also. I put it there for completeness only, and it is not necessary. The real power input adjustments can be made in the opamps' internal Max Output setting. (Mine is currently set at 5V, which would correspond to an input voltage of about 8V accounting for a 3V overhead).
The external resistors and potentiometer are connected to the LM317 chip to complete the voltage regulator circuit. Turn the potentiometer to change the output from 1.26V to 5V. The output voltage is the green trace, and the blue trace is the output current. Change the load resistor (rightmost one) and notice the voltage will remain constant, but the current will change. This is the mark of a good regulator.
The general formula to find the output voltage is:
Vout = 1.25 × (1 + (R2 / R1))
R2 is the upper resistor, and R1 is the bottom resistor of the resistive divider. The "1.25" comes from the fact that the LM317 tries to maintain a 1.25V difference between the output and adjust terminals. Also, there is no variable for the load because the output voltage is independent of the load.
In this specific case, the equation becomes:
Vout = 1.25 × (1 + 10x / 3.3)
Where "x" is the potentiometer position.
LM317 Series:
Constant Current Source: https://everycircuit.com/circuit/5424946364547072
Adjustable Current Source: https://everycircuit.com/circuit/4902539157569536
Soft Power On: https://everycircuit.com/circuit/6070007032971264
Simple Battery Charger: https://everycircuit.com/circuit/4594156412076032
CCCV Li-ion Battery Charger: https://everycircuit.com/circuit/5336266329292800
Linear Power Supply: https://everycircuit.com/circuit/6413550326906880
