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

Status
Not open for further replies.

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?
 
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.



Ron
 
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
 
Sometimes (not always) 2 wire is referred to as passive, and 4 active, pretty much similar to bobs post above.
 
Status
Not open for further replies.
Cookies are required to use this site. You must accept them to continue using the site. Learn more…