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4-20 mA current transmitter/receiver and current loop transmitter/receiver

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seyyah

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Can we use single receiver circuit for current transmitters (referenced to ground and has own supply) and current loop transmitters (which has no own supply and receiver supplies the loop) ?
The same question is valid for the reverse. Can a trasmitter circuit acts as a current transmitter (with its own supply) and current loop transmitter? thanks.
 

barry

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Basically, the receiver is a 'floating' resistor across the input to a differential amplifier. The resistor generates a voltage proportional to the 4-20mA current. Whether the low side of the resistor is grounded or not makes no difference to the diff-amp (as long as common-mode voltages aren't exceeded) If you're using a ground-referenced current signal from the transmitter, you have to ground the low side of your input (to complete the loop). If it's a two-wire transmitter, don't ground the low side.

My guess is that a transmitter that generates its own current will not work when connected to an external power supply and could possibly be damaged. Basically you would be connecting an output to a power supply. But you need to look at the specs for the specific transmitter.
 
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seyyah

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But when we design a receiver for current loop transmitter then we should supply voltage to one of the lines. And if the other side has a tranmitter with its own supply then the system will fail. So should we seperate each type of receivers and transmitters which i don't remember to be done in products like plcs
 
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barry

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I think that you can use the same receiver for both cases. For the self-powered transmitter(floating outputs) you connect the +/- outputs of the transmitter to the +/- inputs of the receiver. For the remotely-powered transmitter, the positive side of the power supply is connected to one side of the transmitter; the other side of the transmitter goes to the receiver +input. The receiver -input then goes to ground (which is presumably where the negative side of the power supply is connected).

Does that make sense?
 

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which i don't remember to be done in products like plcs
Depends on. Some products have different terminals for connection of loop and self powered transmitters. Others have e.g. configuration jumpers. Barry is right, that the receiver circuit (shunt resistor) is mostly ground referenced, some PLCs have isolated analog inputs however.

At the transmitter side, it's unusual to have configurable (active/passive) current outputs. Mains powered instruments are mostly restricted to active outputs, loop powered have passive outputs anyway.
 

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