PLC to act as or simulate 4-20 mA transducer

current measurementplc

I have a PLC Control system that measures a few 4-20 mA pressure sensors. Today we test these with loop calibrators on each channel.

I want to set up a PLC that simulates these pressure sensors so I can program a complete test, or simulation if you will, of the system.

The PLC system has these 4-20 mA modules:

If I understand current loops correctly I need a passive 4-20mA output module (and of course I need zener barriers for the Ex/IS).

Question is that I havent seen any passive 4-20 mA output modules, at least not with Profibus or K-bus, and that makes me think that this might be a flawed idea. Anyone care to help me along?

Best Answer

What I believe you need here is a DAC to give 4-20mA output to feed your 4-20m4 inputs.

The more flexible and expensive solution is a real DAC. Usually 0-10V. Simply drive a current amplifier scaled to give a maximum 20mA. The analogue outputs of PLC can sometimes be configured as current sources as well.

The cheaper way is to use a voltage source and series resistors switched by some relays. Unless there is some compelling reason I would use 3 relays and resistors to give me 4mA, 12mA and 20mA sources. (Bottom, Middle and Top of the analogue range to be tested). This catches most issues I've seen in production. The relays can be driven by any suitable digital output of a PLC.

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