Fan PWM/FG/RD Signal Wires: PLC & Thermostat Wiring
DC brushless and EC fans typically have 3-5 wires: besides positive and negative power, PWM controls speed, FG provides tachometer feedback, and RD reports stall alarms. When connecting to PLCs, thermostats, or BMS in industrial control cabinets, the definitions and wiring of these three wires must be correct — otherwise the fan won't spin or alarms trigger falsely. More models: DC axial fans, EC axial fans.
Signal Wire Definitions
| Wire | Full Name | Function | Signal Type |
|---|---|---|---|
| PWM | Pulse Width Modulation | Speed control: 0-100% duty = 0-100% speed | 5V square wave, 25kHz |
| FG | Frequency Generator | Speed feedback: 2 pulses per revolution | 5V square wave, open-drain |
| RD | Rotation Detect | Alarm: low when running, high on stall/stop | 5V logic, open-drain |
Typical Wiring
| Fan Wire | Connect to PLC / Thermostat |
|---|---|
| Red (+12V/24V DC) | Power positive |
| Black (GND) | Power negative (common ground with PLC) |
| Yellow (PWM) | PLC PWM output / Thermostat PWM port |
| Green (FG) | PLC high-speed counter (X0/X1) |
| Blue (RD) | PLC interrupt input (with 10k pull-up resistor) |
Not sure about your PLC model or thermostat output? Send us your wiring diagram and we will confirm the pinout, or contact us for selection support.
Frequently Asked Questions
What is FG signal? How does PLC read it?
FG (Frequency Generator) is tachometer feedback pulses: 2 square-wave pulses per revolution. Connect to a PLC high-speed counter input; pulse frequency ÷ 2 ÷ 60 = RPM.
What is RD signal?
RD (Rotation Detect) is a lock/alarm output. It outputs low when the fan runs normally, and goes high on stall or shutdown. A PLC interrupt input receiving the high level triggers an alarm.
What is the PWM speed control voltage range?
JIEFENGFAN DC fans use 5V logic PWM at 25kHz, with 0-100% duty cycle corresponding to 0-100% speed. Note: this is a PWM square wave, not a 0-10V analog voltage.
Can I speed-control a 2-wire (power-only) fan?
No. 2-wire AC/DC fans only have positive and negative power wires, with no speed control or feedback. For PWM/FG/RD functions, you must choose a 3-wire or 5-wire version, such as JF12038D24HBL-5 (12038 DC model).
How do I connect a thermostat to a PWM fan?
If the thermostat outputs 0-10V analog, you need a PWM converter (available from JIEFENGFAN) to convert to 25kHz PWM. Alternatively, choose an EC fan with built-in 0-10V input.