Potmeter p,r,v are Given, to Find I

joganand,

I assume by "potmeter" you mean a potentiometer (a precision resistor with an adjustable tap). Your potentiometer is giving you an analog signal of 0-24VDC. If you were to add a fixed resistor of 14k ohm in series with it, the potentiometer would give you an analog signal of 0-10VDC, which is fairly standard. Many PLC's can have the input connected for either a voltage or current input (they are built as a voltage input device and the current input terminal has a precision 250 ohm resistor that converts the current input into a voltage drop across the resistor).

So, first, double check the input capability of your PLC--if it can be connected to your circuit as a voltage input, then your abnormal readings will go away. You will still have 2 problems remaining--the zero and maximum of the voltages. A 4-20mA signal, when connected to a 250 ohm resistor is creating a 1-5VDC input to the PLC. Your range starts at 0-24VDC. Many PLC's allow the input to be set as a 0-5, 1-5, or 0-10 VDC input. I haven't seen any with 0-24 VDC. Therefore, if your PLC will accept a voltage input you still need to scale the 0-24 available into the range the PLC will accept.

--JMM