Core Definition in One Sentence
Active Signal: Powered by itself. The signal line carries its own voltage/current, requiring no external power supply to transmit data.
Passive Signal: Unpowered. It only represents a switch state or resistance change, and requires external power to operate.
What Is an Active Signal?
Definition
A device with an internal power source outputs voltage or current signals that are inherently energized. These signals can be transmitted directly through signal wires without additional external power.
Common Types
Analog Active Signals: 4-20mA (active), 0-10V, 0-5V
Digital Active Signals: Active high/low level outputs, NPN/PNP transistor outputs
Key Characteristics
Generates its own voltage or current.
Transmits signals via two wires without needing external power.
Used in instruments/transmitters that self-power and send signals out.
More susceptible to interference; consider adding isolation for long-distance transmission.
Practical Example
An isolation transmitter or temperature controller that outputs a 4-20mA signal powered by its own supply is using an active signal.
What Is a Passive Signal?
Definition
A device with no internal power source only provides a contact switch, on/off state, or resistance variation. It carries no power itself and requires an external power supply to form a working circuit.
Common Types
Passive Digital Signals: Relay dry contacts, pushbuttons, limit switches, float switches
Passive Analog Signals: PT100 RTDs, thermocouples, potentiometers
Key Characteristics
No inherent voltage or current; only functions as an on/off switch.
Must be connected to an external power source (e.g., from a PLC or instrument) to complete the circuit.
Dry contacts are typically polarity-independent.
More resistant to interference than active digital signals, making them suitable for long-distance applications.
Practical Example
A relay output is simply a switch that opens or closes. It carries no power; when the PLC supplies voltage to the circuit, the switch closure creates a signal-this is a passive dry contact.
Direct Comparison: Active vs. Passive Signals
表格
| Item | Active Signal | Passive Signal |
|---|---|---|
| Internal Power | Yes, self-powered | No, unpowered |
| External Power Required | No | Yes (mandatory) |
| Nature of Signal | Output voltage/current | Switch state or resistance change |
| Common Forms | 4-20mA, 0-10V, level outputs | Relay dry contacts, PT100, pushbuttons |
| Polarity | Polarized (must connect correctly) | Usually non-polarized |
| Typical Applications | Instrument transmission, analog signal transfer | Digital interlocks, temperature sensing, contact control |
Common Industrial Applications
4-20mA output from temperature controllers or isolation transmitters → Active Signal
Relay alarm output from instruments → Passive Dry Contact
Temperature measurement with PT100 RTDs → Passive Signal
NPN/PNP proximity or photoelectric switches → Active Digital Signal
Pushbuttons or auxiliary contacts on circuit breakers → Passive Signal
Common Mistakes to Avoid
Treating passive dry contacts as active signals and connecting them without power: The circuit will not function.
Adding external power to an active 4-20mA loop: This can damage the instrument's output circuit.
Misidentifying active/passive signals, leading to incorrect PLC module selection or damaged ports.
Quick Summary
Active = Powered by itself, ready to send signals
Passive = Just a switch/resistor, needs external power to work