How do you create a 4-20mA source?

To create a 4-20mA current source, you can use an operational amplifier circuit configured as a current source. Start with a precision resistor (usually around 250 ohms) connected to a power supply. The op-amp regulates the current through the resistor to maintain a current flow between 4 mA and 20 mA based on the input voltage or signal, which is often derived from a sensor or a control signal. Additionally, a reference voltage can be set to ensure the output current starts at 4 mA and scales up to 20 mA according to the desired input range.