4 to 20mA current loop system|| Test method || தமிழில்

Video explains fundamentals of 4~20mA is using in sensor circuit. Also different method of test procedure explained with animation.

Reason for 4~ 20 mA

1 (a). Live Zero i.e 4 mA. It is easy to detect that the signal wire is not broken. If “0” mA, unable to know that it is a signal or wire broken.

(b). The 4 mA is used as “range zero”. If the range zero were truly zero mA, then there would be no current, and therefore no power to drive the field instrument. Field instrument would require 3 or 4 wire devices. Additional wiring costs come into account.

(c). Another reason for choosing 4 mA is when the transmitter was first introduced, it was working on pneumatic pressure signal for transmission. Later, electronics chips were introduced those require a minimum of 3 mA of current to function. So, a margin of 4 mA is taken as reference.

2. Scale and Calculate: with 4-20 mA signal it is easy to scale and calculate the process parameter.
3. Analog to Digital conversion: CPU in PLC/DCS/ Standalone controller need digital signal 1-5 V. PLC/DCS converts analog to digital signal so that the CPU can read it. For conversion of current from 4-20 mA to 1-5 V needs a 250 ohms resistance.
4. Reason for 20 mA: 20 mA is used as maximum because the human Heart can withstand up to 30 mA of current only. So, as a safety point of view, 20 mA is chosen.