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in a electronic controller why we are using 250 ohm
resistance in input signal?is i/p is necessary in control
valve when we are using electronic controller give details?
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Answer Posted / sam
The hardware electronic controller you are using has a 1-5
Volt input but the transmitter you use in the field, is a 4
to 20 mA transmitter. So in order for the hardware
controller to accept this input, you install a 250 Ohm
resistor on the controller's input which will convert it to
a 4 to 20mA input.
Weather you need a I/P converter on the control valve
depends on the positioner you will use. The hardware
electronic controller will give a 4 to 20mA signal to the
control valve, so if you want to use a full pneumatic
positioner on the valve, you need to install a I/P converter
and calibrate it for a 4 to 20mA input and a 20 to 100 Kpa
pressure output. You then need to calibrate the pneumatic
positioner for a 20 to 100 Kpa = 0 to 100% stroke.
If you want to use any other type of positioner a I/P will
not be necessary, since in electromechanical positioners the
I/P is already part of the positioner and obviously in SMART
positioners, the input to the positioner is 4 to 20 mA.
Good luck
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