Professionals working in automation control frequently encounter various signal types and contact types, such as passive signals, active signals, passive contacts (dry contacts), and active contacts (wet contacts). This article aims to provide a comprehensive explanation of these concepts to support better understanding and practical applications.
Understanding Passive Signals and Active Signals
1. Definitions
Passive Signals: These signals do not require a power source for operation. They rely on external energy for their functionality.
Active Signals: These signals are powered and require a power supply to function. Most analog signals fall under this category, regardless of whether they use two-wire, three-wire, or four-wire configurations.
2. Key Characteristics
Two-Wire and Three-Wire Instruments: These typically use 24V DC power supplies, with the signal and power sharing a common circuit.
Four-Wire Instruments: The power supply circuit is independent of the signal circuit. The signal is isolated and often not grounded, ensuring better performance and safety.
High-Voltage Instruments (e.g., 220VAC): These instruments must use independent circuits for signal return and cannot share common lines.
3. How to Receive Passive and Active Signals
Both two-wire and three-wire analog and digital instruments (DCS, PLC) include terminals for direct wiring. However, power for field instruments is sometimes isolated to avoid interference, using separate groundings or power arrangements.
Four-wire instruments generally feature isolated power supplies without common terminals, which require careful wiring to maintain isolation integrity.
Understanding Passive Contacts and Active Contacts
1. Definitions
Contacts are components that determine the on/off state of a signal.
Passive Contacts (Dry Contacts): These are not self-powered. They require the receiving instrument to provide the power to enable their operation.
Active Contacts (Wet Contacts): These are self-powered, receiving power from their own supply. The term “dry” or “wet” is often used as a borrowed analogy from other domains.
2. Common Types
Passive Contact Examples:
Various switches: limit switches, travel switches, foot switches, rotary switches, temperature switches, and level switches.
Buttons and keypads.
Sensor outputs: water immersion sensors, fire alarms, glass break sensors, vibration detectors, smoke detectors, and condensation sensors.
Outputs from relays and reed switches.
Active Contact Examples:
Passive contact signals that are supplied with power, typically in ranges like DC 0-30V (standard is DC 24V) for industrial controls or AC 110-380V (commonly AC 220V or AC 380V) in electrical circuits.
TTL-level outputs, which can act as wet contacts when buffered (e.g., 7407, 245, 244 ICs).
Outputs from NPN transistors, Darlington transistors, and infrared sensors.
3. Differences Between Passive and Active Contacts
The primary difference lies in whether the contact point has its own power supply:
Passive Contacts: Powered by the receiving instrument. The voltage level depends on the requirements of the receiving device.
Active Contacts: Powered independently, often matching the required triggering or driving voltage of the connected device.
Practical Tips for Wiring and Applications
Safety Precautions:
Always ensure power is turned off when wiring active or passive components.
Use appropriate voltage and current levels for the contact’s specifications to avoid damage.
Interference Avoidance:
Avoid routing signal wires near high-power electrical cables to minimize electromagnetic interference.
For analog signals, consider using shielded cables and proper grounding.
Device Compatibility:
Verify that the signal and power specifications of all connected devices are compatible.
Double-check wiring diagrams and manuals to ensure correct connections.
Examples for Common Scenarios:
For passive contacts, ensure the receiving device can supply sufficient power to detect the signal.
For active contacts, ensure the supplied power does not exceed the receiver’s voltage and current tolerances.
Illustrations and Diagrams
Adding diagrams to show the differences between wiring for two-wire, three-wire, and four-wire instruments as well as the configurations for active and passive contacts will significantly enhance understanding. Ensure diagrams clearly label terminals, power sources, and common lines.
Summary
Understanding the differences between passive and active signals, as well as passive and active contacts, is essential for professionals in automation control. Correct implementation ensures signal accuracy, device safety, and system reliability. By adhering to best practices and safety precautions, you can effectively integrate these components into your control systems.