Analog Signal Mystery!!!!

[mellis]

A dead short is zero ohms. Open is infinity. You need to think backwards when looking at this spec.

Take a 500 Ohm LOAD. Connect another 500 Ohm LOAD in parallel. Now you have a 250 Ohm LOAD. Connect 2 more in parallel, and you will now have a 125 Ohm LOAD.

It's basic Ohms law.

I agree, it's basic Ohm's Law. I think you're just not looking at the circuit from the right point of view.

Basically, what the maximum load specification for a transmitter is saying is: What is the maximum resistance I can shove 20ma through?

Remember, the transmitter controls the current. It can operate with little or no load. That is, it can send a 4-20 ma signal down the wire. It can not operate with an open circuit at all. For a given power supply voltage, there is a maximum load resistance a transmitter can drive. For a 24vdc power supply, the total loop resistance at 20ma will be 1200 Ohms. For a typical transmitter, the load can be up to 600 Ohms. The transmitter makes up the remaining resistance. At 4 ma, the total loop resistance is 6000 Ohms. If the load is a typical 250 Ohm input card, then the transmitter changes from 950 Ohms at 20ma to 5750 Ohms at 4ma.

The example you quoted would apply to a voltage signal. In that case you would have a minimum load resistance. A good example is an audio amp. The output may be specified as 4 Ohm, it means the minimum connected resistance is 4 Ohms. If you connect more speakers in parallel you will lower the connected resistance and exceed the available current.

Also, loads in a 4-20ma loop are in series. So more loads always increases the total connected resistance.

Hope this clarifies things...

edit: Sheesh. Mickey beat me by 5 minutes. Must type faster...

 

[timothywhitson]

Gents:

I haven't done the ground loop test yet but notice the discussion about whether the Grundfos is 2wire or 4wire.

I believe it is a 2 wire system. The 4 outputs only have 2 terminals dedicated to each output. Which to me makes it a 2 wire system with the unit providing the power for the loop.

or am I wrong?

Thanks,

Tim

 

[Mickey]

edit: Sheesh. Mickey beat me by 5 minutes. Must type faster...

With one finger tied behind my back and the other on the keyboard.

 

[keithkyll]

When I connect the pH/Chlorine meter to the PLC the reading on the SCADA screen and the LCD on the unit do not jive. ... However, when I wire it back up to the PLC with a meter in series there appears to be a distortion/or loss (16.85mA when on 20mA). It does not appear to be linear.

Also, note that the register in the PLC (0-4095) jives with what is being see with the meter (hence the scaling working)...
Tim

Here's a quote from the first post. I read that the Grundfos is connected directly to the PLC. I was thinking 2 wire. With 2 wire thinking, everything he says fits.
I understand now the way everyone else is viewing the specs, and I certainly could be wrong.
Several have suggested that the unit be connected directly to the PLC, and that's my main point. It appears it was already done, and it didn't work.

 

[Mickey]

Gents:

I haven't done the ground loop test yet but notice the discussion about whether the Grundfos is 2wire or 4wire.

I believe it is a 2 wire system. The 4 outputs only have 2 terminals dedicated to each output. Which to me makes it a 2 wire system with the unit providing the power for the loop.

or am I wrong?

Thanks,

Tim

See pic below. Which one do you have? My guess is the bottom one with three additional outputs. Post the manual if you have it.

 

[timothywhitson]

I didn't wire the unit but I would say you are correct that I have situation. The unit is plugged in to a wall recpetacle.

Tim

 

[Mickey]

Would it be possible for you to provide a drawing of how you have this
wired up?

 

[timothywhitson]

Thank you to all that helped.


As promised I wanted to close the loop on this (somewhat).

The problem appears to be a ground loop. I was able to go to the site and work through the suggestions to fix it.

Steps taken:

1. Disconnected all the signals from the card minus one (Grundfos Meter A - Chlorine output). Reading on Grundfos A LCD and SCADA correlate.
2. Wired Grundfos Meter B - Chlorine Output. Reading on Grundfos B LCD and SCADA correlate.
3. Wired NTU Meter A. Readings on NTU LCD and SCADA correlate.
4. Wired NTU Meter B. Readings on NTU LCD and SCADA Correlate.
5. Wired Grundfos Meter A - pH output. Grundfos A readings distorted.
6. Wired Grundfos Meter B - pH output. Grundfos B readings distorted.
7. Disconnect Grundfos A & B - pH output. All remaining readings correlate.

I will do my homework on this, already have started, but I have a few questions:

Questions 1:
Why did I not experience a ground loop problem when I used the exact meters, wired the same way, to the same AI card on a different PLC?

Question 1b:
Is the isolation issue with repect to the meters or the PLC card/rack/chasis?

Question 2:
Do I need isolators only for the remaining signals that I left unwired or all the signals coming from the meters?

Question 3:
Is there a good test to do to identify if a ground loop is going to be a problem before hand so I can perpare ahead of time?

Question 4:

Do you have any suggestions on isolators? I imagine some like Phoenix Contact would carry something like that?

Thanks again for all the help!!!!

 

[Tom Jenkins]

The difference is due to how the current flows, if the transmitter is loop powerd or self powered, and how the signal is generated inside the transmitrer. Many three or four wire analog signals are "source" - the transmitter increases its voltage until it either reaches max voltage or the signal matches the process variable. Many two wire transmitters are "sinking" in that they increase the internal resistance to the externally applied voltage until the signal matches the process variable. The three and four wire type in particular may have the internal "-" side tied to ground, and if the analog input on the PLC also has the commons tied together you have a number of alternate paths for the current signal, which will take the path of least resistance. Re-draw the diagrams Mickey supplied above and include the internal analog card and transmitter wiring, drawing all of your signals on one diagram. You can probably identify the ground loop from that.

AutomationDirect.com has some very cost effective signal isolators. I have also had good luck with Action Instruments, M-Systems, and APi.