We have two wired device. 24v DC i/p is given from PLC. If we connect the i/p terminals to the device then voltage drops to 5V DC & current o/p is 1.5 ma.

Device has a RI converter only & a potentiometer is connected to this RI converter. As potentiometer value changes, current o/p changes. RI converter & pot both are healthy. What could be the cause of the voltage drop?

 

> 24v DC i/p is given from PLC

I'm confused. An I/P should be connected to a PLC analog output that is compatible with the I/P's input; typically a 4-20mA output.

If you are connecting 24Vdc to the I/P input, then remove the power supply. An I/P is not a 4 wire device designed to operate from a DC supply; it's a 2 wire device that responds to a regulated current signal, typically 4-20mA, which is modulated by a 4-20mA output.

If you are connecting an I/P to a 4-20mA DC current output, and the source current drops at all, then either
- the I/P is loading the circuit. The internal coil could be shorting if the coil has been overheated and the wire insulation damaged; or
- the analog output's power supply is insufficient to drive the I/P's resistance load, which is a published spec for any I/P.

A common brand 3-15psi I/P uses a 180 ohm coil for a 4-20mA input signal with the circuit shown here:
http://i39.tinypic.com/347wo04.jpg