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Answer Posted / chiru mudra...
There r 2 reasons for this.....
1) All micro controllers, plc, dcs etc., are operated with 1
to 5 volts supply.( Ex: 1v=low, 5v=high).
At the input & output of these devices we use 250 ohm
resistor for converting this 4-20mA to 1-5volts.
ie; 4mA*250 ohms = 1Volt
20mA*250ohms = 5Volts.......
2) If we use this range from 1mA to some range there is a
problem with that.
which is, in industries we use bunch of cables together from
field to control room and vice verse. Due to this there is
some signal distortion in the other cables due to magnetic
induction (electro magnetic induction effect in the other
cables). ie, 1-2 mA current will generate in the unactivated
cables which may cause the mall fn in the total industry.
Ex: 1mA = 0% valve open
2mA = 10% valve open
3mA = 20% valve open.... etc.
| Is This Answer Correct ? | 27 Yes | 5 No |
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