# Switching 4-20ma signal with relays



## MotoGP1199 (Aug 11, 2014)

I have a situation were I need a drive to run off of a 4-20ma signal that could be from one of two other transmitters. The drive would be an alternate for the other two if they go down. I'm concerned about getting ground loop and interference issues. Will it work or will I have issues, I've never switched or run a 4-20ma signal through relays before. Attached is a simple diagram showing the relay setup on a PLC. PLC would choose which outputs are closed at a single time. Note the that pump 3 has two sets of wires going to it. Only one set would ever have continuity at a time. Not sure if I would need signal splitters or isolation devices. Wires going into the I/O module from the transmitters land on commons, the lines kind of block the c1, c2, c3, c4 labels.


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## splatz (May 23, 2015)

In general I have found 4-20ma to be pretty immune from interference but you can have issues if you wind up with two sensors / transmitters in parallel, shared common, etc. I had a pump controller with a 4-20ma input card and I forget why but they needed to land the signal on two inputs. It was some programming limitation of the pump controller. Turns out you couldn't put the 4-20ma signal on two inputs or loop it through two inputs. 

I used a 4-20ma splitter to handle it, simple and not too expensive. 

I am sure you already thought of this but if you wanted zero issues you could just use two analog inputs for the transmitters, three analog outputs for the drives, and let the PLC logic handle everything, and no worries about isolation. 

With relays and no PLC, I think you could do this with three SPDT relays. One each for drive 1 and drive 2, to switch between online and offline mode, and one for drive three, to switch between backing up drive one and backing up drive two. You could of course do the same thing in the PLC, or you could go with a hybrid controlling the coils of the three relays with the PLC. (I don't know how involved the logic behind the decision is...)


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## dronai (Apr 11, 2011)

splatz said:


> I am sure you already thought of this but if you wanted zero issues you could just use two analog inputs for the transmitters, three analog outputs for the drives, and let the PLC logic handle everything, and no worries about isolation.


This seems like the right way to do this


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## gpop (May 14, 2018)

It can be done using relays as long as you are not using loop power (there will be a reboot time between switching)

Depending on the drive you can also do fancy tricks like repeating a signal using a AO or using multiply AI which would have been acceptable years ago when a plc was expensive. 

Now it just seems easier to use a cheap plc to deal with it and to control the drive's digital inputs.


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## JRaef (Mar 23, 2009)

Use hermetically sealed relays, preferably ones with gold flashed and bifurcated contacts. That's because there is not enough energy in the circuit to burn through contaminates, so a film builds up over time and the resistance gets higher and higher, which changes the mA signal values.


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## paulengr (Oct 8, 2017)

JRaef said:


> Use hermetically sealed relays, preferably ones with gold flashed and bifurcated contacts. That's because there is not enough energy in the circuit to burn through contaminates, so a film builds up over time and the resistance gets higher and higher, which changes the mA signal values.



On voltage loops I agree. Not on 4-20 mA loops. That’s the beauty...no calibration. It will continue to work until the resistance exceeds the transmitters rating then it drifts/fails. But agreed very low power so gold contacts are kind of necessary.


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## SCR (Mar 24, 2019)

Being that you're using a PLC anyway,how about if you just get a PLC with 2 analog/in/out. Then you can scale the signals however you want and send to whichever drive needs it.


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## glen1971 (Oct 10, 2012)

SCR said:


> Being that you're using a PLC anyway,how about if you just get a PLC with 2 analog/in/out. Then you can scale the signals however you want and send to whichever drive needs it.


The two AO are essentially the same as the two transmitters, but they need to go to 3 drives.
I think the OP needs a PLC with at least:
- 2 analog in (transmitters)
- 3 analog out (to vfds)
- 3 DI in (from vfd status)
- 3 DO out (to VFD start/stop control)


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## MotoGP1199 (Aug 11, 2014)

glen1971 said:


> The two AO are essentially the same as the two transmitters, but they need to go to 3 drives.
> I think the OP needs a PLC with at least:
> - 2 analog in (transmitters)
> - 3 analog out (to vfds)
> ...


Thanks for all the responses. I decided to just run the analog signals through the PLC. Its about $200 more to add the analog signals that way. There's other stuff running on the plc as well. I will be using a total of 5 DI in, 16 DO, 2 analog in, 3 analog out. Unfortunately the combo cards for analog only have 2 analog out. So I'm upgrading the CPU to have analog and I have to buy a separate card for an additional analog out.


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## glen1971 (Oct 10, 2012)

MotoGP1199 said:


> Thanks for all the responses. I decided to just run the analog signals through the PLC. Its about $200 more to add the analog signals that way. There's other stuff running on the plc as well. I will be using a total of 5 DI in, 16 DO, 2 analog in, 3 analog out. Unfortunately the combo cards for analog only have 2 analog out. So I'm upgrading the CPU to have analog and I have to buy a separate card for an additional analog out.


Great call! I think you and the client will be happier and have more flexibility. Keep us updated with the success of this one!


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