A PLC (Programmable Logic Controller) consists of hardware components such as a CPU (Central Processing Unit), memory, I/O (Input/Output interfaces), power module, and communication module. Among these, the I/O interface is a critical component that connects the controller to external devices, responsible for signal input (Input) and output (Output). Below, we will discuss dry contact inputs and wet contact inputs in relation to input signals.
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Classification of I/O Interfaces |
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DI/DO (Digital Input and Output)
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DI (Input): Receives switch signals (such as buttons, limit switches), with a voltage typically of 24V DC or 220V AC.
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DO (Output): Controls relays, indicator lights, etc.
AI/AO (Analog Input and Output)
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AI (Input): Receives continuous signals (such as temperature and pressure sensors), with standard signals of 4-20mA or 0-10V.
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AO (Output): Outputs continuous signals to control variable frequency drives, proportional valves, etc.
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Dry Contacts vs. Wet Contacts |
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1. Dry Contact
A dry contact is a passive switch that has two states: closed and open, with no polarity between the two contacts, allowing for interchangeability. The primary function of this contact in a circuit is to control the connectivity of the circuit through the opening and closing of the contacts.
Electrical Characteristics
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Passive: The contacts themselves have no voltage and require an external power source to operate.
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No Polarity: Can be connected to AC or DC circuits.
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Good Isolation: Physically isolated contacts with strong anti-interference capability.
PLC Wiring
DI modules typically provide power (e.g., 24V) for dry contacts. When the contacts close, a circuit is formed, allowing current to flow into the PLC. As shown in the figure below, a dry contact input is essentially a short circuit between DI and COM.
Application Scenarios
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Simple switch control: such as emergency stop buttons and limit switches.
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High reliability situations: Dry contacts pose no leakage current risk, suitable for safety circuits (e.g., safety relays).
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AC/DC mixed systems: Dry contacts have no polarity and can be compatible with different power types.
2. Wet Contact
A wet contact is an active switch that has its own power source (voltage/current). It has two states: powered and unpowered, with polarity between the two contacts, meaning it cannot be reversed. The role of a wet contact in a circuit is to control the flow of current through the opening and closing of the contacts.
Electrical Characteristics
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Active: Requires external power supply (e.g., 24V DC).
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Polarity: Must distinguish between NPN (sinking) or PNP (sourcing) outputs.
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Direct signal drive: No need for PLC to provide power, but must match input type.
PLC Wiring:
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NPN type (active low): Output connects to PLC’s DI, common connects to 0V (GND).
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PNP type (active high): Output connects to PLC’s DI, common connects to 24V+.
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Example (NPN Proximity Switch): Sensor brown wire (+24V) → Power + Sensor blue wire (0V) → Power – Sensor black wire (signal) → PLC DI (When the sensor is triggered, it outputs a low-level signal) |
Application Scenarios
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High-speed detection: such as counting objects on a production line (photoelectric sensors).
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Precision control: Analog wet contacts (such as 4-20mA signals) used for temperature and pressure monitoring.
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Industrial automation: Three-wire sensors (such as cylinder magnetic reed switches) directly interfacing with PLCs.
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Selection Recommendations |
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Prefer dry contacts:
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In situations requiring high isolation and compatibility with AC/DC (e.g., emergency stop circuits).
Prefer wet contacts:
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In situations requiring fast response or direct driving of PLC inputs (e.g., sensor signals).
Note on compatibility:
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The NPN/PNP type of wet contacts must match the PLC input module (Japanese PLCs commonly use NPN, while European PLCs commonly use PNP).
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Common Issues |
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Dry contact misjudgment:
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During long-distance transmission, line impedance may cause the PLC to fail to detect the signal; it is recommended to add a relay for isolation.
Wet contact short circuit:
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Connecting the output of an NPN sensor directly to 24V+ will burn it out; strict adherence to polarity in wiring is required.
Recommended Reading:
Five Common Serial Communication Protocols in Power Systems
The Three Industrial Brothers: PLC, RTU, DTU
DLT698 and DLT645 Baud Rates are Different, Don’t Get ConfusedCan DLT698 Completely Replace DLT645?IEC104 Protocol Master-Slave Interaction ProcessIEC104 Implementation of One Master and Multiple SlavesDLT645: The Most Specialized Communication Protocol in the Power IndustryDLT645 Protocol Does Not Require In-Depth Study, Just Master These PointsBesides DLT698, What Other Communication Protocols Exist in the Power Industry (Object-Oriented)
The Three Major Members of the Modbus Protocol Family
Learning the Second Step of the Host Computer: The Golden Combination of RS485 and Modbus
Modbus Protocol: What is the Difference Between Using RS232 and RS485?