In industrial control systems, the terms “active signal,” “passive signal,” “dry contact,” and “wet contact” are frequently used. However, these concepts are often confused due to overlapping characteristics and similar application scenarios. This article aims to clarify these concepts, explain their differences, and provide examples to help engineers and technicians make more informed decisions during instrumentation design and wiring.
1. What is an Active Signal?
An active signal refers to a signal that has its own power source and is capable of actively transmitting voltage or current. These signals are usually generated by powered devices such as transmitters, sensors with built-in amplifiers, or controllers. Active signals can amplify or modulate output and are widely used in analog and digital communication.
Key Characteristics:
Requires external or built-in power supply
Outputs voltage or current signals (e.g., 4–20 mA, 0–10 V)
Typical in powered sensors and transmitters
Example: A Coriolis mass flow meter outputs a 4–20 mA signal proportional to flow rate. This signal is generated internally and sent to the control system. Since it can drive a load on its own, it is an active signal.
2. What is a Passive Signal?
A passive signal, by contrast, does not generate power by itself. Instead, it reflects the change in a physical property, such as resistance, inductance, or capacitance. To collect a passive signal, the system typically provides a loop power supply (usually 24V DC), which energizes the sensor loop, and the sensor changes the signal according to the measured parameter.
Key Characteristics:
No power output from the signal source itself
Requires an external loop power to function
Often seen in RTDs, thermocouples, and dry contacts
Example: An RTD (resistance temperature detector) changes resistance as temperature varies. It does not output any voltage or current on its own, and the measurement device must provide excitation current.
3. What is a Dry Contact?
A dry contact is a passive mechanical switch that doesn’t carry any voltage or current. It simply opens or closes a circuit, allowing external voltage or current to pass through depending on the state of the contact. Dry contacts are typically found in relays, mechanical limit switches, or some field instruments for signaling.
Key Characteristics:
Purely mechanical or relay-based
No internal voltage or current source
Signals are determined by physical open/close status
No polarity requirement
Example: A mechanical valve limit switch provides a dry contact to indicate whether the valve is fully open or closed. The contact itself does not carry voltage; it needs external wiring to form a circuit.
4. What is a Wet Contact?
A wet contact is a powered contact, meaning it has voltage or current present at the contact terminals. When the contact closes, it transmits this signal to the receiving system. Wet contacts are often found in solid-state sensors such as proximity switches or NAMUR sensors.
Key Characteristics:
Carries its own voltage or current (e.g., 24V DC)
Output depends on transistor logic (NPN/PNP)
Has polarity – connections are not reversible
Often used in PLC DI modules with voltage detection
Example: An inductive proximity switch with PNP output provides 24V when an object is detected. It sources current to the PLC DI input, making it a wet contact.
5. Typical Examples and Use Cases
| Device Type | Signal Type | Contact Type | Characteristics |
|---|---|---|---|
| Mechanical limit switch | Passive signal | Dry contact | No internal voltage, relay-style contact |
| Inductive proximity sensor | Active signal | Wet contact | Internal 24V supply, PNP/NPN logic |
| RTD | Passive signal | None (resistive) | Requires external excitation to measure resistance |
| 4–20 mA transmitter (2-wire) | Active signal | None | Self-powered via loop, sends current actively |
| NAMUR sensor | Active signal | Wet contact | Requires isolator or amplifier, used in hazardous areas |
6. Additional Notes on 4–20 mA Signals
The 4–20 mA analog signal is a common form of transmitting analog data in process control. It can be either:
Active 4–20 mA: The transmitter powers the loop and outputs the signal directly.
Passive 4–20 mA: The transmitter does not power the loop and requires a separate power source (typically from the receiver/controller).
This distinction is essential when wiring transmitters with PLC input cards, as incorrect assumptions can lead to measurement failures.
Conclusion
Understanding whether a signal or contact is active, passive, dry, or wet is crucial for proper selection, installation, and troubleshooting in industrial systems. The wrong assumption may cause signal loss, control errors, or even equipment damage. Always verify the specifications and wiring requirements of both the sensor and the receiving device to ensure compatibility.