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How to tell the difference between 2 wire and 4 wire industrial instruments?

Kart31

Member
How can I tell the if an industrial instrument (temperature transmitter) is a 2-wire or a 4-wire 4-20mA output? Given that I don't have the manual, can't find a manual but do have two wires and know it is a 4-20mA output?
 

Reloadron

Well-Known Member
Most Helpful Member
A 4-20 mA is only a two wire system. Four wire comes into play with voltage with devices. For example a 4 wire kelvin resistance measurement.

Given that I don't have the manual, can't find a manual but do have two wires and know it is a 4-20mA output?
Depends on what the device is but if it's a 4 to 20 mA sensor of sorts yes, that's a safe assumption. Make sure if you use it you observe correct polarity and applied voltage. Below is an example of a thermocouple transmitter which outputs a 4 - 20 mA current loop shown with resistor to provide a voltage input to an ADC.

Inor Temp Trans.gif

Ron
 

BobW

Active Member
There are also 4 wire 4-20mA devices. These are generally instruments that have a lot more internal circuitry than can be powered off the loop. So they would have two extra terminals to accomodate a 24 VDC supply (or in some cases an AC mains supply). The output would still be 4-20mA. Temperature transmitters are simple enough that they should just be two wire. However, the temperature element itself, if separate from the transmitter, could be a thermocouple (2 wire), or an RTD (2, 3 or 4 wire).
 

Reloadron

Well-Known Member
Most Helpful Member
There are also 4 wire 4-20mA devices. These are generally instruments that have a lot more internal circuitry than can be powered off the loop. So they would have two extra terminals to accomodate a 24 VDC supply (or in some cases an AC mains supply). The output would still be 4-20mA. Temperature transmitters are simple enough that they should just be two wire. However, the temperature element itself, if separate from the transmitter, could be a thermocouple (2 wire), or an RTD (2, 3 or 4 wire).
Good point and having used them I never gave much thought to the power being additional wires. Sort of slow on the uptake on my part. If it's not loop powered obviously it needs power. :)

Ron
 

dr pepper

Well-Known Member
Most Helpful Member
Sometimes (not always) 2 wire is referred to as passive, and 4 active, pretty much similar to bobs post above.
 

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