4-20mA output driving 2 valves

Hi,

I am having a debate I need an answer to, I have a single (1) 4-20mA output driving 2 separate valves simultaneously

I checked the output rating of the card and the impedance of the actuators and I have plenty of capacity to accomplish this.

I say to wire this all you have to do is double tap the output and send the signal to each valve.(2 wires wired to the + 2 wires to the - each going to the appropriate valve)

My Co-work thinks you have to wire it up in some loop configuration or you will see the signal drop off at the valve.

It depends on what type of analog output you are using.
If it's a current output, you better wire your two valves in series (loop configuration). If it's a voltage output, you will need to wire the valves in parallel.

it sounds like you're planning to wire the two valves in parallel ...

think about this:

suppose that your output module is happily sending out 20 mA (full signal) ... how much current is likely to be flowing through EACH of your valves? ...

even if the valves are PERFECTLY matched (in resistance) then each valve will see only ONE-HALF (10 mA) of the current flow ...

this probably isn't the result that you had in mind ...

(or then again, maybe I'm just misunderstanding what you're planning to do) ...

PS Edit ... I see that jimbo types faster than I do ...

also keep the following in mind ... if you wire the two valves in series, their resistances will ADD ... that MIGHT NOT be an issue – but check the specs of the output module to make sure that it can push a full 20 mA through the resulting resistance ... some modules are only rated for a maximum circuit resistance in the neighborhood of 250 ohms (500 ohms is more common) ... whatever, if your total (series) resistance is too high, then your output module might not be able to fully drive the valves ...

As others have said you wire a current loop in series. See below for another solution. Preferred IMHO.

http://www.laurels.com/tls.htm

kirk said:

Hi,

I am having a debate I need an answer to, I have a single (1) 4-20mA output driving 2 separate valves simultaneously

I checked the output rating of the card and the impedance of the actuators and I have plenty of capacity to accomplish this.

I say to wire this all you have to do is double tap the output and send the signal to each valve.(2 wires wired to the + 2 wires to the - each going to the appropriate valve)

My Co-work thinks you have to wire it up in some loop configuration or you will see the signal drop off at the valve.

Click to expand...

You're co-worker is right...For 4-20ma, should be in a loop.

I hate being wrong!

Mickey, I have used the splitters in the past, I was just trying to eliminate a part and expense.

kirk said:

I hate being wrong!

Mickey, I have used the splitters in the past, I was just trying to eliminate a part and expense.

Click to expand...

"I hate being wrong, but I love it when I'm set straight." -
-- Harlan Ellison

My immediate reaction may be wrong, but why not have 1 output per valve?
Why do I say this?
Often 2 valves are used in Split Range configuration (- typically for temperature control) and it is better to have each valve independantly driven so that you can control the split/overlap in the PLC.

FrancisL "My immediate reaction may be wrong, but why not have 1 output per valve?"


This application we have limited Analog Outputs

Do not spoil the ship for a ha'porth of tar!

4 - 20 mA Series Circuit

Just wire the valves in series. No additional parts required. I have done this many times.