Current Standards

[krishtriram]

Dear All,

Why is (4-20)mA is kept as an standard and not any other value?
and similarly what is the standard for Voltage and why is it maintained at that particular value?

 

[stevez]

Like any standard the purpose is to allow others to interface with a control, sensor, etc. If I am not mistaken 4 ma thru a specific resistor is 1 volt and 20 ma is 5 volts - the resistor is 250 ohms. One standard is 1 to 5 volts but it seems that either 1 to 10 or 2 to 10 volts is more common.

Like other standards, they probably were convenient round numbers or were the standards adopted by a popular manufacturer some time ago and everyone else just followed.