active signal vs. passive signal 1

[hassankh]

Hello
I believe this will sound as a no brainer. I wonder what is the difference between active and passive signals?
Thank you in advance

 

[danw2]

An active 4-20mA signal is powered internal to the device.
A passive 4-20mA signal is powered by an external DC power supply; a power supply outside of the device.

2 wire transmitters are passive devices, primarily for the benefits of running only 2 wires to the field, instead of 3 or 4 wires (conductors).

3 and 4 wire transmitters are active outputs, where power is wired to the device.

Analog outputs from controllers tend to be active outputs, powered by the controller.

A re-transmit 4-20mA position signal from a 2 wire positioner is a passive output because a 2 wire positioner does not have enough remaining power to drive the signal.

 

[scenes]

Well, it's a good question maybe there are more people with a similar issue.

When we talk about a passive signal, it means that the signal from the field device needs an external supply to close the loop and the control system can read the signal from the field device.

For instance, when you have a flow meter that is 4 wire, you can have a passive or active signal. It means if you have passive output the 4-20mA needs a power provided by the control system. When you set it up to be active, the field device will provide 4-20mA + 24 Volts, and the input of the control system should be passive without power supply.

A pressure device is 2 wire, and it needs an external power supply to close the loop, and you get the 4-20mA, it means the 2 wire devices are passive. The input of control system can be active and passive as well if the input has power then it is active if it needs an external power supply, then it is passive.

Below, I've found some articles to give your more technical context about 4-20mA, and analog communication.

https://www.talkbass.com/threads/what-is-the-difference-between-active-and-passive-output.369378/

http://blog.wika.com/general/active-passive-sensor/