The LM317 IC in EveryCircuit? Yep. By using just a single opamp and a 1.25V source, you can make a workable substitute.
For a more in-depth explanation about the LM317, see my previous circuit ( https://everycircuit.com/circuit/6138847104335872 ). Instead of a voltage source, this time, I attached the external resistors to make a current source. Adjust the series resistor (top one) to change the current flow. Change the load resistor (bottom one) up and down and watch how the current stays constant, but the voltage changes. This is the opposite behavior of a voltage source and is how a current source works.
The general formula to find the output current is:
Iout = 1.25 / Rs
Rs is the series current limiting resistor.
However, you may notice that it is not a perfect current source. If the load resistor is too high, the current will begin to collapse. (A similar thing can happen if the series resistor is too small. The measured current will be lower than calculated). Why? The answer is that the LM317 output voltage can not go high enough to pump the requested current through the given load.
Ohms Law tells us the required voltage to pump a current through a resistance:
V = I × R.
That means Iout × (Rs + RL) < Max Output Voltage
In this specific case, the equations become:
Iout = 1.25 / 62.5 = 20mA
20mA × (62.5Ω + 50Ω) = 2.25V, well within the 5V capacity.
LM317 Series:
Adjustable Voltage Regulator: https://everycircuit.com/circuit/6138847104335872
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
