Difference Between 2-4 Wire Current transmitter

[bodoo23]

I have read a lot about this. One of what I saw is 4 wire is used when the voltage drop is high because of wire and transmitter itself but I don't understand this.

http://www.plctalk.net/qanda/attachment.php?attachmentid=50220&stc=1&d=1549150324

http://www.plctalk.net/qanda/attachment.php?attachmentid=50221&stc=1&d=1549150385

If 2 wire configuration and 4 wire configuration is supplied with (4 wire is from external one) , Cable and transmitter itself won't make same voltage drop ? It may be because of my lack of knowledge but it seems to me 4 wire's advantages is that we can supply transmitter with a different source with a different level of voltage and nothing more. Could anyone explain this voltage drop issue for 4 wire with mathematically or with a scheme like below one.

http://www.plctalk.net/qanda/attachment.php?attachmentid=50222&stc=1&d=1549151028

I am not sure if I could explain well my thinkings because of my english. I hope you will understand.Thanks

 

[Mickey]

I have read a lot about this

More to read..

See link..

https://www.google.com/search?q=Dif...=chrome..69i57j69i61&sourceid=chrome&ie=UTF-8

 

[bodoo23]

More to read..

See link..

https://www.google.com/search?q=Dif...=chrome..69i57j69i61&sourceid=chrome&ie=UTF-8

I saw them and I couldn't find what I am looking for. This is why I asked here.

 

[rupej]

Since a 4-wire system is not loop powered, you are correct in that it can be theoretically powered by any voltage source for the power side of things.

I guess that could allow it to boost the voltage on the loop side to help account for voltage drop, instead of relying on the loop's 24VDC source.

 

[Peter Laing]

A 2 wire transmitter is 'loop powered'. It is usually preferred for low power transmitters.
- The power available to the transmitter is limited by the minimum loop current (4 ma minimum) and the voltage drop.
- The maximum voltage drop across the transmitter will be: Supply Voltage (24v) - input module (receiver) voltage (at 20mA). A typical input may be 250 ohms, thus the maximum transmitter voltage drop would be 24v - (0.020A * 250ohms) = 19V.
- So, in this case, the maximum power available to the transmitter is 19V * 0.004A = 0.076 Watts.

A 4 wire transmitter does not have this power limitation. It is typically used when a transmitter has a higher power requirement than what can be supplied by the loop.