General Assist Technical Reference
Outside Air Handling Unit
Electrical system configuration, component checks and fault diagnostics
For Fujitsu Outside Air Handling Unit commissioning, maintenance and troubleshooting
This article explains the five main sections of the OAU electrical control system and provides expected voltages, resistance values and inspection points for field diagnosis.
Electrical safety: Work must only be completed by appropriately qualified personnel. Isolate the unit before resistance or continuity testing and confirm the DC bus has discharged before touching electrical components.
1. System overview
The OAU electrical control system is divided into five main sections:
| Section | Primary purpose |
|---|---|
| Filter circuit | Surge protection, noise suppression and incoming supply protection |
| Converter and smoothing circuit | Converts 230 V AC to approximately 325 V DC and smooths the DC bus |
| Power supply circuit | Produces low-voltage DC supplies for motors, relays and control electronics |
| Controller circuit | Controls refrigerant components, sensors and system communication |
| Switch PCB and auxiliary components | Addressing, function settings and connection of field components |
2. Filter circuit
Components: Filter PCB, 20 A and 3.15 A fuses, varistor, surge absorber, common-mode choke coil and capacitors.
- Protects internal circuits from incoming electrical noise and voltage surges.
- Prevents inverter-generated noise from feeding back into the mains supply.
- Provides overvoltage and surge protection through the varistor and surge absorber.
| Component | Specification |
|---|---|
| Common-mode choke coil | 20 A, 2 mH |
| X capacitor | AC 275 V, 1.0 µF |
| Y capacitor | AC 250 V, 0.01 µF |
| Varistor | AC 470 V |
| Surge absorber | AC 3000 V |
| Fuse 1 | 20 A |
| Fuse 2 | 3.15 A |
Inspection: Check continuity across both fuses and measure approximately 230 V AC between W305 and W306. If a fuse is open, inspect for short-circuited components in the primary or solenoid relay circuit before replacement.
3. Converter and smoothing circuit
Components: Diode bridge, smoothing capacitor, posistor and reactor.
- Converts 230 V AC to approximately 325 V DC.
- Reduces DC voltage ripple using a high-capacity capacitor.
- Limits inrush current through the posistor, which is a PTC thermistor.
- Uses the reactor to improve power factor and reduce harmonic distortion.
| Component | 5-8 HP | 10 HP |
|---|---|---|
| Diode bridge | 600 V / 15 A | 600 V / 25 A |
| Capacitor | 450 V / 1000 µF | 450 V / 1200 µF |
| Reactor | 8 A / 13 mH | 14 A / 6 mH |
| Posistor | 300 Vrms / 12 Arms | Not listed |
Inspection points
- Diode bridge output should be approximately 325 V DC from a 230 V AC supply.
- The capacitor must not show bulging, leakage or deformation.
- Posistor resistance should be approximately 40 Ω ±25% at 25°C.
- The reactor must have continuity and must not be open circuit.
4. Power supply circuit
The power supply PCB converts the high-voltage DC bus into low-voltage DC supplies for the control and motor circuits.
| Output | Application |
|---|---|
| 15 V DC | Motor control |
| 12 V DC / 5 V DC | Relays, wired remote controller and controller PCB |
Protective fuse: 3.15 A / 250 V, protecting the power supply IC and primary circuit.
No DC output: Confirm approximately 325 V DC is present at connector C600, then check primary and secondary voltages around the switching power supply IC.
5. Controller and communication circuits
Controller PCB
- Manages the refrigerant cycle, including EEV, SV21 and compressor control signals.
- Collects thermistor data for heat exchanger and air-temperature control.
- Interfaces with remote controllers and the outdoor unit.
Communication PCB
- Handles communication between indoor, outdoor and RB units.
- Provides system error reporting through the wired remote controller.
| Inspection point | Expected condition |
|---|---|
| CN4 | 12 V DC / 5 V DC supply to controller PCB |
| CN8 / CN9 | 5 V DC supply for thermistor sensors |
| Green LED | LED on indicates normal microprocessor operation |
6. Switch PCB and peripheral components
The switch PCB is used to configure refrigerant addresses, remote controller settings and other function settings. DIP switches adjust functions, while rotary switches assign address numbers.
| Component | Expected value or check |
|---|---|
| Fan motors - 5, 8 and 10 HP | Resistance between pins 4 and 6: 300 kΩ or greater |
| Electronic expansion valve | 200 Ω ±10% at 20°C |
| Solenoid valve SV21 | 1324 Ω at 20°C |
| Thermistors | Compare against the applicable temperature/resistance table; example: approximately 26.3 kΩ at 15°C |
7. Fault indication
| Error code | Description | Possible cause |
|---|---|---|
| 39.1 | Motor 1 DC power supply error | Low DC voltage below 163 V or diode bridge fault |
| 39.2 | Motor 2 DC power supply error - 8 HP only | Low DC voltage below 163 V or diode bridge fault |
| 4.A | Air-temperature thermistor error | Sensor open circuit or short circuit |
| 5.9 | Fan motor error | Fan rotation fault or drive-signal fault |
8. Recommended diagnostic sequence
- Confirm the incoming 230 V AC supply and check both filter-circuit fuses.
- Confirm approximately 325 V DC at the converter output and C600 input.
- Inspect the smoothing capacitor for swelling, leakage or deformation.
- Confirm the required 15 V, 12 V and 5 V DC control supplies.
- Check the green controller PCB LED and system communication.
- Measure motors, EEV, solenoid valve and thermistors against the listed values.
- Use the displayed fault code to focus further testing.
Capacitor warning: Wait at least five minutes after isolating the power supply before touching the capacitor or DC bus components. Confirm the voltage has reduced to a safe level with an appropriate meter before proceeding.
Technical values in this article are based on the supplied OAU electrical parts reference. Always confirm the applicable model documentation and wiring diagram before replacing components.