Instrumentation Current Loop (4-20 ma loops)
Why to use 4-20 ma loops ? What are the advantages of 4-20 ma?
The DC current loop is used extensively in the instrumentation field to transmit process variable to indicators and controllers (PLC,DCS,Standalone controllers etc).It is also used to send control signals to field device to manipulate process variables such as temperature,level and flow. This can be done by sending signals from a controller (PLC,DCS,or other control equipment) to final control element such as control valve,pumps etc.
The standard current range used in these loops is 4 to 20 mA (milli ampere) and this value is normally converted to 1 to 5 VDC by a 250 ohm resistor at the input to controllers and indicator.These instruments are normally high input impedance electronic devices that draw virtually no current from the instrument loop.
Why to use 4-20 ma or What are the advantages of using current loop ?
1.Only two wires are required for each remotely mounted field transmitter. So cost saving is one of the advantage.
2.Current loop is not affected by electrical noise.
3.Current loop is not affected by changes in lead wire resistance caused by temperature changes.