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Comments on How do PLCs normally measure 4-20mA current loops?

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How do PLCs normally measure 4-20mA current loops?

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I'm looking for input from anyone with insight in the PLC world. How do PLCs usually measure 4-20mA current loops?

I would assume it's one of these two alternatives:

  • Through an external "shunt" resistor
  • Through internal current sense electronics

My scenario is an industrial system where I'm looking to generate different fixed current levels in a 4-20mA current loop based on relays. The fixed current will increase depending on how many relays that are active, such as no relays = 4mA, first relay = 8mA, first 2 relays = 12mA and so on.

The circuit will either look like this:

shunt_resistor

That is: current is measured across a "shunt" resistor of typically 250R added externally.


Or alternatively:

current_sense

That is: R10 has very low resistance and some manner of current sense electronics are built-in inside the PLC.


In case the PLC uses the former "shunt" resistor method, then I will have to adapt my resistor values accordingly. But in case they measure over some internal low ohm resistor like in the second picture, then I don't need to consider that.

I'm looking to create a solution that will suit most PLC solutions out there, but I don't know which one of these two circuits that are the most common.

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you want a current source start with a linear regulkator like LM340 and use the relays to switch in different load resistors.

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Sure but that doesn't answer the question (1 comment)
Sure but that doesn't answer the question
Lundin‭ wrote over 2 years ago

Indeed this doesn't really give a proper current loop unless Vdd is a current generator. But I had already delivered the system when the customer changed their mind completely. So we ended up doing a quick & dirty solution just converting voltage to current over a resistor, since the PLC was located close, in the same cabinet as the relays. Now as it happened there was also some external circuits that needed to be connected, so it had to be tweaked further and it all turned into a strange "ad hoc" solution. The customer is always right, they say... and they got it running fine now though, since a few months back.