How (24Vdc) feed & control signal(4-20mA) are transferred on same 2 wires in 2 wires transmitters (circuits)?

Question

how current varying from 4 to 20 mA although there is a constant voltage 24Vdc on the 2 wires...

Answer 1

You can combine constant voltages and frequencies on the same wires, if the wires can withstand the combined maximum voltaghes, and are rated for the frequencies involved. A simple low-frequency filter will separate the constant voltage from the control signals.

Answer 2

This is not at problem at all. You can have a constant DC voltage of 24 V across the wire that comes from the instrument controller (indicator/recorder) and supplies the sensor. The sensor transmits its measured signal as a current back to the instrument controller. This is a current ranging from 4mA (zero value signal) to 20mA (full scale signal). This is done by the electronics in the sensor that controls a current proportional to the measured value (0 to 16 mA) while the first 4mA are used for the sensor electronic power supply.

This is all on the same two wires on DC - there is no AC or frequency spectrum involved. Ohm's Law allows for variable current at constant voltage by means of a variable resistor.

Answer 3

This is not quite as you think.

This is called loop power. The instrument introduces a resistance into the circuit to control the current. A voltage is dropped across this resistance and is what is used to power the instrument. It may not be the full 24v and will even vary as the signal changes.