Active and passive devices


Depending on the source of current for the loop, devices may be classified as active (supplying or “sourcing” power) or passive (relying on or “sinking” loop power).

For example, a chart recorder may provide loop power to a pressure transmitter. The pressure transmitter modulates the current on the loop to send the signal to the strip chart recorder, but does not in itself supply power to the loop and so is passive. Another loop may contain two passive chart recorders, a passive pressure transmitter, and a 24 V battery. (The battery is the active device). Note that a 4-wire instrument has a power supply input separate from the current loop.

Panel mount displays and chart recorders are commonly termed ‘indicator devices’ or ‘process monitors’. Several passive indicator devices may be connected in series, but a loop must have only one transmitter device and only one power source (active device).