Understanding Active and Passive Sensor Signals

Part One 

If automation instrument technicians do not understand passive signals, active signals, passive contacts (dry contacts) and active contacts (wet contacts), they will be confused.

(1) Passive and active signals (analog)

(1) Definitions of passive and active signals

For current signals, if the device has its own power supply line, its output (for example 4-20mA) is an active signal; if the device has no independent power, its output is a passive signal. Three-wire and four-wire transmitters output active signals; two-wire transmitters output passive signals.

(2) How to collect passive and active signals

① Collecting active signals is straightforward: power the signal source device (typically three- or four-wire), then connect the acquisition device input positive and negative terminals correspondingly.

② For passive signal collection, a DC24V supply is usually required because two-wire transmitters lack independent power. The DC24V may come from a dedicated supply or from the acquisition device. The DC24V positive connects to the transmitter 4-20mA positive output; transmitter negative connects to the acquisition device 4-20mA input positive; acquisition device input negative connects to DC24V negative, forming a complete loop.

(2) Active and passive contacts (digital)

(1) Definition of passive contacts

Passive contacts, also called dry contacts, are electrical switches with closed and open states; the two contacts have no polarity and are interchangeable.

(2) Common passive contact signals

① Various switches: limit, travel, rotary, liquid level, temperature switches;、push buttons;

② Outputs of various sensors used in environmental monitoring such as water leak, fire alarm, glass break, vibration and condensation sensors; ③ outputs from relays and reed switches.

(3) Definition of active contacts

Active contacts, also called wet contacts, have powered and unpowered states and polarity between the two contacts and therefore cannot be reversed.

(4) Common active contact signals

① Powering a dry contact and using the other pole as output creates an active contact signal. Common industrial wet contact voltages are DC0-30V (standard DC24V); secondary circuits may use AC110-380V.

② TTL outputs can be used as wet contacts if properly buffered; ③ Darlington transistor collector outputs with VCC; ④ NPN transistor collector outputs with VCC; ⑤ Infrared reflection and through-beam sensor outputs.

(5) Advantages of passive contacts

Because passive (dry) contacts have no polarity, they are more numerous and offer advantages:

① Easy to connect, lowering engineering cost and skill requirements; ② Uniform interfaces; ③ Even prolonged short circuits on dry contact wiring will not damage local or remote equipment; ④ Suitable for handling large numbers of digital inputs.

Part Two

1. Definition of passive nodes: passive switches with closed and open states; no polarity between two nodes; common passive signals include limit, travel, foot, rotary, temperature and level switches; push buttons; sensor outputs such as water leak, fire alarm, glass break, vibration, smoke and condensation sensors; relay and reed switch outputs.

2. Definition of active nodes: active switches with powered and unpowered states; two contacts have polarity and cannot be reversed. Two-wire loop signals like 4-20mA can be active or passive depending on whether the transmitter provides power. Two-wire loops typically operate below 25mA with a fixed DC24V supply; higher-power transmitters require separate power (four-wire).

Active means the transmitter provides a power loop (four-wire); passive means it does not and requires an external supply (two-wire).