Re: Transmitter issue


"Steve & Lizzie" <steveandlizzie@xxxxxxxxxxxxxxxx> wrote in message news:IEluj.120985$3m6.14170@xxxxxxxxxxxxxxxxxxxxxxxxxxxx
Dear All

An instrumentation issue -

A machine is utilising three 4 to 20ma pressure transmitters for control
purposes. They are all on different circuits which have their own supplies.

For certain reasons it has become desirable to have just one transmitter
perform the function of all three (same pressure being measured) - how can
this be achieved? A splitter has been tried and whilst this did indeed
generate three signals from one instrument (the original and two additional
outputs) it could not be connected into the existing installation - for each
circuit a loop was required and it's own voltage was present, whereas the
splitter generated its own power etc.

Is there a method or device that can do this?

Any advice is greatly appreciated.

Thanks in advance

Steve

If I understand correctly you want to lose two transmitters and just use the sgnal from one at three different places? Maybe one transmitter is used for pressure control and the other two are just for indicating displays somewhere. The first thing I would do is check all three units that receive the signal and see if any of them can be configured to accept "isolated inputs". By that I mean the 4-20 is powered from an external source and the units do not connect the signal to ground inside the instrument. If the input resistance is low enough on the receiving units it's possible that the power supply that's powering the loop has enough balls to drive two additional units. If that's the case wire the 4-20 in series with all three units receiving the signal. But that will only work if the display units can be configured for isolated inputs.

I once had an AB PLC that was controlling tank level. A Bristol-Bab*** PLC needed to see the same signal. Both PLCs had powered analog input modules. I used a Weidmuller signal conditioner that would accept power on both loops. The only caveat was that the conditioner needed an additional 24 volt power supply. The conditioner I use is on page 19 of the pdf below:

http://www.weidmuller.com/downloads/pdfs/datasheets/LIT0310_AnalogSignal.pdf

The input resistance on these conditioners is only 50 ohms, which is pretty low. It's possible that you could insert two of these in series in the main control loop. That would give you the two additional 4-20 outputs you're looking for. You will need an additional power supply *just* for the conditioners however.

What are the three signals used for? What are the input resistance/voltage drops of all three receiving devices? What are the ratings of all three DC power supplies?

.

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