Passive Vs Active Concept


Thread Starter


Hi guys,

What is concept of passive and active puls output in flowmeters? Is it same like a dry and wet contact output?

I'm going to quote ABB's "Digital Outputs" app note because it is so well written, and it applies to almost all flow meters:

Passive Outputs
All WaterMaster digital outputs are passive Open Collector designs.

(missing) Fig. 1

This requires a voltage be applied to the Open Collector and the output signal driven to ground from the Emitter through the input of the device receiving the signal.

It's important to understand that there will be no voltage measurable from this output if none is applied. The Outputs employ Galvanic Isolation, which simply mean that there is no current passed from the controlling circuit to the output circuit. This is possible because the Output transistors are photo transistors, that is to say the base drive of the transistor is driven by light, not a direct connection. This offers a certain level of safety with respect to the outputs.

The missing Fig 1 image is on the first page, left column of the document "Digital Outputs" here:

One note on active and passive analog or discrete (frequency or pulse) outputs. Each manufacturer does it their own way.

Some manufacturers require specifying the type of output when the flow meter is ordered and that's what is, there's no changing it. Cast in stone.

Other manufacturers use configuration and/or jumpers to select active/passive.

Active outputs are always powered by the flowmeter, which means the output occurs whether there's anything attached to it or not.

Passive outputs require DC power from the field side.
A passive output means the flowmeter is externally powered. An active output means the flowmeter is internally powered such as 4 wire device. It appears you are asking about a pulse output. An analog such as 4-20mA signal would be the same passive = external and active = internal power.