Please explain the difference in active and passive supply.
Is there any i/p variations or o/p variations because of this?
With regards to current loops (instrumentation 4-20mA current loops, for example)
- an active output signal regulates the current level, excites the current loop, and excites other loop powered devices wired in the loop (e.g. a loop powered display). For example, the analog output from a Coriolis meter (powered by line AC power) can be connected directly to a the passive analog input card of a PLC or a DCS. The analog output supplies the power to the loop.
- a passive output signal only regulates the loop current. The loop is powered by some external DC power source. For example, a 2 wire, loop powered pressure transmitter is powered by an external DC power supply in order to generate a regulated current output.
Most receiver analog inputs are passive and do not supply loop power.
Some PLC AI's and DCS AI's can supply loop power. If the receiver's AI always sources DC power for the loop, it is referred to as a active analog input.