MicroLogix 1100 4-20mA to 0-10VDC issue

Hello,

I've hooked up (2) 1000 ohm resistors twisted together in series across the analog input on my PLC to convert the 4-20 mA signal to 0-10VDC. I've confirmed that the two resistors twisted together get close to 500 ohm, I measured with my meter.

The output on my controller to the PLC is scaled appropriately where 0 ft = 4 mA and 10.7' = 20mA.

With a level of 3.12' in the tank I would expect to get 3.12/10.7 = 29% of 10VDC or 2.9V DC. I measure 4.39 VDC and my PLC reads 444.


With a level of 5.24' in the tank I would expect to get 5.24/10.7 = 49% of 10VDC or 4.9 VDC. I physically measure 5.8VDC and the PLC reads close to that at 611.

It appears the PLC is reading accurately, but there is something wrong with the signal being converted by the resistors. Any thoughts?

Nothing wrong. I think you mean you twisted them together in parallel (2 1000 ohm resistors in parallel to get 500 ohms).

The signal is 4-20mA which means when you put 500 ohms across that you're going to get 2-10 volts, not 0-10. So when 3.12' you should be getting 4.33 volts, and with 5.24' you should get 5.9 volts, roughly. So it's close. It's just 2-10, not 0-10.

Yes, I did mean in parallel, thank you. Now I see the light!

If you want accuracy you can count on use a signal conditioner.
LINK

If you are just playing around then what you are doing is fine.
If it needs to be reliable invest in the correct tools.

You also aren't counting on the parallel loading of the actual input. I doubt that it is 10+Meg as you would expect on a DMM.