4-20mA To 0-10V Conversion Question

[cbuysse]

It occurs to me that you will want the resistor to be in series with the current load of the loop, and not across the terminals as a shunt. @cbuysse alluded to it in his explanation, but the application is not as straight forward as the OP.

Just to be clear - which terminals are you referring to?

In fact the resistor will (or should) be wired across the terminals for the voltage input on the PLC. There are several ways to go about making this loop work depending on the specific devices involved and existing configuration. I had assumed the OP had already determined the best correct way to wire it and was more concerned with functionality.

 

[JordanCClark]

Sorry, I see what you mean. My perspective is from the existing current loop. There, the resistor is series within the loop. For the voltage input, then yes, that is across the terminals.

Must be the snow...

 

[JordanCClark]

Sorry, forgot to mention that this was about MikeVT's existing loop.

Must REALLY be the snow...

 

[MikeVT]

A couple of points of clarification: The (level) I referred to is signal level, not measured fluid level.

I am operating a throttling fill valve via an I/P, using a 4 - 20ma signal from a stand alone PID controller. An outboard piece of equipment needs to know the approximate position of that throttling valve. We want to use the 4 - 20ma signal to also indicate (roughly) the fill valve's position.

Do I run the 4-20ma signal through 80' of cable (160' for there and back), or would a voltage conversion like the original poster presented serve me better?

 

[cbuysse]

This isn't an "or" situation. If all you have is 0-10V input then you are doing a voltage conversion. That's the purpose of the 500 ohm resistor. The current flowing through the resistor will create a voltage drop that will then be measured by the PLC input.

To answer your latest question, though requires a bit more information. You have to look at the whole picture. The first consideration is what the extra wiring does to the total loop impedance as I described earlier.

Also, does it make sense to run the wiring this way, or is the valve closer to the PLC? It may make more sense to go from there.

Where does the loop get its power from? Does the PLC input force one side of its connections to the local 0VDC? That may cause issues in the loop.

How difficult will this be to troubleshoot/maintain later when the next guy comes in and tries to figure out what you did?

I know that's not a direct answer to your question. Generally speaking, running a 2-conductor cable like you just described should work and will likely be the simples solution. Just watch out for the other gotchas.