Hi guys,
I have got a problem with my 3 wire transmitter. It is continuously getting damaged due to some inflow of voltage to its output terminals (4-20)mA. So i am continuously having trouble with transmitter.. I don't know from where this voltage is coming. The output of the transmitter is connected to a honeywell controller and AB compactlogix plc via switch.. so each time one will be active.. is there any one who encountered this sort of a situation earlier..

 

Hi,
This does happen sometimes in the field if the control cables are running close to Power cables (especially 1 phase). I would suggest you to check the path thoroughly & put a repeater/isolator. This may mitigate the problem.

Niraj

 

When did the problem start? Has this configuration--running a single transmitter through a "switch"--ever worked properly?

I'm not familiar with a "switch" for three-wire transmitters. Can you provide a manufacturer's name and model/part number? How many wires are connected to the switch from each of the devices which can receive the signal from the "switch" (in other words, how many wires from the Honeywell controller and how many wires from the AB are connected to this "switch")?

Where does the normal 24 VDC power come from for the transmitter? When the "switch" is in "Honeywell" mode, is the Honeywell providing the 24 VDC, as well as receiving the DC feedback through the other two wires connected to the transmitter?

Again, not understanding where the 24 VDC for the transmitter comes from, are you certain that both of the devices receiving the signal are wired for receiving a two-wire, DC signal from a "self-powered" transmitter? It's possible to conceive of a situation where one or the other device (the Honeywell and/or the AB) is wired so as to provide 24 VDC on one wire and the signal on the other wire of the input, which would be for a two-wire, loop-powered device, which is not what you stated is being used.

Again, we just don't have enough information about the configuration. Please provide more information if you want more assistance.

 

What you have to do is bring the transmitter to the panel where the switch is located. In that panel convert the transmitter signal to a voltage, so you can select how you want the signal routed, you also need to provide a contact to the plc so it knows if the other controller is selected, then you take the signal voltage to the plc (assuming it is in the same panel as the switch), and send the voltage to the analog controller. If the analog controller is remotely mounted you need to use a signal isolator.

It is not unusual for the controller to provide transmitter power, so I would be interested to see however you are powering and avoiding voltage conflicts, and addition to how you are sorting out the dc commons and signal grounding.

 

Check L1 and L2 on your transformer as well as any incoming power distribution the EMF produced by these lines could induce inductance on other parts of your circuit causing fluctuations in current. if isolating these lines is an issue, try adjusting your wire management so all of the power circuit's lines cross your control and communications components/lines at a 90 degree angle. this should reduce the area of the magnetic field your circuit is exposed to as it expands and collapses reducing current spikes IF this is the issue.

Good Luck.