Parallel 4-20ma circuit

We have a flow meter that delivers a 4-20ma signal to a chart recorder. I would like to feed the same 4-20ma signal into an analog input card on a plc (A.D. DL250). Would paralleling this circuit to both inputs work as long as the neg is made common on the plc input as it would be a separate power source?

Place the signal in series not parallel. + from instrument to plus at the first point then - at the first point to plus at the next point and - to - at the same point.
It does work provided the instrument has enough output to feed the total impedance of the complete loop. If this is in doubt, then you require a 2 channel amplifier.
beerchug

One other option you would have is to place a resistor < 100 Ohm in the 4-20 loop. Then place the leads of the chart recorder across the resistor to track the Voltage.

Sometimes placing two things in 4-20 Loop causes the loop to do strange things.

Another thought, send the 4-20ma to the plc then output from the plc to the chart recorder.

The previous posters are correct you series amperage, parallel voltage.
I will agree with Ron, input the 4-20mA signal to the PLC then bring it out to as many points as you wish, as the dollars permit.

Roger

Drawing

4-20ma to 2 imputs

I will first give the series hookup a try to see the results. Also I did not think about sending the input to the plc and then output to the chart recorder (good idea!) Thanks for the feedback.

******WARNING*****

Think before you proceed. What might go wrong?

If chart recorder is removed for repair or calibration, what will happen to the PLC input? Using Series, loss of Signal. or if tied together then Transmitter will need to be re-calibrated/Spaned.

Sending Analog Out to Chart Recorder if Good, but need to buy Analog Output Card,$$$$$$...

You could install a Single Resistor to create the voltage Drop, and wire each input device to it if a short distance inside Panel, and PLC card can be set to read Voltage instead of 4-20 ma.

Everyone's Post seems to be correct, but there is probably a Best Choice among them which you'll have to make.

Good Luck

If your flow transmitter is loop powered, the V+ must go to the transmitter first, then to the other devices (plc card and chart recorder). Loop powered devices need the full voltage at their + terminal. If your flow transmitter powers the loop, then typically the loop can support 600 ohms of series resistance. Check the flow transmitter specs for this.

Another "gotcha" for multiple divices in series on a 4-20 mA loop is that both loads sometimes tie the signal to ground or a power supply common. If that happens only the first device in the loop gets the signal. This is sometimes called a ground loop. Check the manufacturer's info.

A couple of options if the load is higher than the transmitter can handle of if you have the commons problem are to use a dropping resistor and multiple voltage sensing devices in parrallel as described above, or to use a signal conditioner. Most externally powered conditioners have low impedance and isolated commons, so you can re-transmit a current or voltage signal easily. A-PI or M-Sytems or Action Instruments are all sources.

This is turning out to be a most useful thread with lots of ideas and warnings.
I must admit I am quite surprised at the problems that have been raised regarding 4-20ma series loops. If the load is calculated and the transmitting device can handle the total impedance of the loop, there really should be no problems. Even if the chart recorder is removed for repair, a bridge will complete the loop. The only thing that varies in a properly set up loop is the voltage across each device. Each device acts as a voltage divider and the more devices in the loop the lower the voltage across each device. The current remains the same.
I tend to prefer 4-20ma loops and generally power them with the same 24vdc power supply I use to power the PLC inputs. I can honestly say that I have only ever encountered two problems. One was my mistake when the impedance in the loop was too high for the transmitter and I had to split the loop into two paths with an amplifier for the second part of the loop and the other ocassion was when Honeywell decided to cut into our loop, turning it into a voltage loop for themselves and impressing a signal on our part of the loop with the result that our readings were incorrect. Removed Honeywell and all was well once more.
I might add that I have probably used thousands of 4-20ma loops over the years for process control, emergency power generation systems and base load power stations with only those two problems.
I believe that thanks are in order to all contributors for some very good informed information and comment in the thread.
beerchug

What kind of recorder are you using?
We use ABB Commander recorder/controllers that have
a retransmit option that works well.