Overview
This document outlines the electrical configuration, function, and diagnostic procedures for the Fujitsu Outside Air Handling Unit (OAU).
It details each circuit, board, and protection device that forms the electrical control system — essential for commissioning, maintenance, and troubleshooting.
The OAU’s control system is divided into five main sections:
Filter Circuit
Converter & Smoothing Circuit
Power Supply Circuit
Controller Circuit
Switch PCB and Auxiliary Components
1. Filter Circuit
Components: Filter PCB, Fuse (20A & 3.15A), Varistor, Surge Absorber, Common Mode Choke Coil, Capacitors.
Functions:
Protects internal circuits from power supply noise and voltage surges.
Prevents inverter noise from leaking to the main power supply.
Provides overvoltage and surge protection through the varistor and surge absorber.
Key Specifications:
| Component | Specification |
|---|---|
| Common Mode Choke Coil | 20A, 2mH |
| X Capacitor | AC275V, 1.0µF |
| Y Capacitor | AC250V, 0.01µF |
| Varistor | AC470V |
| Surge Absorber | AC3000V |
| Fuse1 | 20A |
| Fuse2 | 3.15A |
Inspection:
Check continuity across fuses, measure 230V AC between W305 and W306.
If open, inspect for shorted components on the primary or solenoid relay circuit.
2. Converter & Smoothing Circuit
Components: Diode Bridge, Capacitor, Posistor, Reactor.
Functions:
Converts AC (230V) to DC (325V).
Smooths voltage ripple using a high-capacity capacitor.
Limits inrush current using the Posistor (PTC thermistor).
Improves power factor and reduces harmonic distortion via the Reactor.
Specifications:
| Component | 5–8 HP | 10 HP |
|---|---|---|
| Diode Bridge | 600V / 15A | 600V / 25A |
| Capacitor | 450V / 1000µF | 450V / 1200µF |
| Reactor | 8A / 13mH | 14A / 6mH |
| Posistor | 300Vrms / 12Arms | — |
Inspection:
Diode Bridge output should be DC 325V (230V × √2).
Capacitor must not show bulging, leakage, or deformation.
Posistor resistance ≈ 40Ω ±25% (at 25°C).
Reactor must not be open-circuit; continuity test required.
3. Power Supply Circuit
Component: Power Supply PCB.
Function: Converts high-voltage DC (325V) to low-voltage DC outputs for control and motor circuits.
Outputs:
DC15V (for motor control)
DC12V / DC5V (for relay, WRC, and control PCB)
Fuse: 3.15A / 250V (protects the power supply IC and primary circuit).
Diagnostic Point:
If no DC output is observed, check:
DC325V input on connector C600
Primary and secondary voltages around the Switching Power Supply IC
4. Controller Circuit
Components: Controller PCB and Communication PCB.
Controller PCB Functions:
Manages the refrigerant cycle (EEV, SV21, and compressor control signals).
Collects thermistor data for heat exchanger and air temperature control.
Interfaces with remote controllers and outdoor units.
Communication PCB Functions:
Handles data communication between indoor, outdoor, and RB units.
Provides system error reporting via wired remote controller.
Inspection Points:
CN4: 12VDC / 5VDC to control PCB.
CN8/CN9: 5VDC for thermistor sensors.
Green LED indicator: ON = normal microprocessor function.
5. Switch PCB and Peripheral Components
Functions:
Configures refrigerant address and remote controller settings.
DIP switches adjust function settings; rotary switches assign address numbers.
Peripheral Electrical Components:
Fan Motors: 5/8/10 HP models — resistance between pins 4–6 = ≥300kΩ.
EEV (Electronic Expansion Valve): Resistance = 200 ±10% Ω at 20°C.
Solenoid Valve (SV21): Resistance = 1324Ω at 20°C.
Thermistors: Measure per temperature/resistance conversion table (e.g., 15°C ≈ 26.3kΩ).
6. Fault Indication (New Error Codes)
| Error Code | Description | Possible Cause |
|---|---|---|
| 39.1 | Motor 1 DC Power Supply Error | Low DC voltage (<163V) or diode bridge fault |
| 39.2 | Motor 2 DC Power Supply Error (8HP only) | Same as above |
| 4.A | Air Temp Thermistor Error | Sensor open/short |
| 5.9 | Fan Motor Error | Fault in fan rotation or drive signal |
7_Electrical parts for OAU_2014… |
7. Summary and Service Notes
Before servicing or commissioning:
Confirm DC325V input, fuse integrity, and LED status.
Check for no liquid leakage or swelling on capacitors.
Verify all thermistors and motors are within resistance tolerance.
Ensure communication with the outdoor unit is active.
Technician Tip: Always wait at least 5 minutes after isolating power before touching capacitors, as they hold charge after disconnection.
Conclusion
The OAU’s electrical system integrates multiple safety and conversion layers to ensure reliable performance under high static pressure and fresh air load.
Understanding the role and interrelation of each PCB — from filter protection to control logic — is crucial for accurate diagnostics and efficient maintenance.