Click PLC Analog Input

[ascot01]

I have an Automation Direct Click PLC (C0-02DD2-D) with a 24 VDC power supply. I want to make a variable input device to supply the analog input with 0-5 vdc. If I understand voltage divider correctly, I need to drop 24-5=19 vdc across R2, leaving me with 5 vdc across R1, using two resistors in series. Since the ratio of the two voltages dropped across resistors should be the same ratio of the two resistance values of the resistors, I figure I can use a 330 ohm and a 1900 ohm resistor. Problem is I want to use a pot instead and I know I can turn the wiper to the correct 'ratio', but that wouldn't give me 0-5 vdc across the full sweep of the pot. Is it not simpler to just use a pot with a certain pre-defined total resistance?

 

[Saffa]

You're pretty much there! remember that a potentiometer does have a fixed resistance (between legs 1 and 3) - the variable resistance is seen across legs 1-2 or 2-3. see the attached circuit diagram for how I'd build the circuit you want (if this is not a commerical project of course).

The zener diode is a recommended option and should be a 5.1V zener, to protect your input incase your 24V power supply or resistors are not exact.

this circuit suffers from the problem of resistor tolerance. If you can get 1% tolerance resistors it should be OK. Another option would be to use a 5V precision voltage regulator IC to feed the 4.7k pot, but then you need to provide a power supply to that IC...

Set it all up and check with a multi-meter that you get the expected voltage at the output of your divider before connecting to the PLC!

 

[ascot01]

Thank You very much