Is there any way to show indication to show valve opening (as GREEN LED) and closing (as RED LED) from 4-20mA output which is coming from Valve positioner , I mean drive 2 LEDS.
If you don't have /can't use some intelligence to provide digital outputs based on limits you set, you could solder-up your own little op-amp circuit, or you could do probably the best thing, and put proximity sensors at the valve and run the wires back to where you need the indication, driving 2 LEDs. If you want to try the op-amp idea, I can halp via email.
You can use limit alarms to get contact outputs at adjustable signal levels. The following link is one type made by Action Instruments, which is available with dual setpoints (open, closed). It may not be the lowest cost solution, but is off-the-shelf.
I assume that what you want to do is to have the RED LED on when the valve is closed (i.e. when the positioner says it is less than say X% open) and the GREEN LED on when the positioner says the valve is more than Y% open. You could do this using any proprietry two channel trip amplifier of which there are a number on the market.
Well, driving two LED:s should be possible, especially if you don't require more current thru the led than about 2-3mA (some leds will probably be bright enough by this small current) and
accept a voltage drop around 2.5-3V. You will not (by your description) drive more than one led at a time, that is what counts, not how many leds the assembly have.
If you require higher current you have to make a DC-DC converter and accept a higher voltage drop, especially at low loop currents. This would be more tricky since introducing a DC-DC converter
feeding from the loop most surely will indroduce noice you rather be without and in the end it would probably be easier to just feed the circuit externally.
/Johan Bengtsson
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