Electrical analog current signals are used to transmit measurement values in control logic, so that to avoid noise in transmission line. Because analog current signals are not effected by noise. 4 - 20 mA is the common range of signals used in instrumentation.
The signal is the current in the loop. This kind of signal can be transmitted for long distances over wire without regard to voltage drops in the wire.If we transmit the signals as voltage, the voltage drops caused by supply currents in the wires usually appear as an error in the signal measurement.
A 4 mA represents the 0% working & 20mA means 100% working. For example 4mA for a valve means it is fully closed, and 20% means fully opened.
A current loop works under simple ohm’s law. V = I * R.
Main components of a control loop:
Sensors : Sensors are device which measures process variable, such as temperature, pressure, flow, level.
Transmitter : What ever sensor measured will be converted into current signal with in range 4-20mA.
Power Source : We need a current source for the loop, usually 9, 12, 24 volt DC current. Choosing the power supply is consider on the power supply voltage must be at least 10% greater than the total voltage drop of the attached components
Loop : There should be a loop connecting the receiver to the sensor with 4 - 20mA signal.
Receiver : Device which is the current loop, that can interpret the current signal in the loop. This current signal is transferred to operators.
Voltage to current converters are used for the conversion of voltage from transmitter to the current loop.
4 - 20mA current loops are also used for the control actions. Command from controller are feed to final controlling element are given through current loop in 4 -20mA range.
Advantages of current loop:
Only two wires are required for each remotely mounted field transmitter. So cost saving is one of the advantage
Current loop is not affected by changes in lead wire resistance caused by temperature changes
Current loop is not affected by electrical noise.