Midea Water Heater MWH-38P5 Not Working (No Power at All)
Symptoms:
- No power at all when switched ON.
- Suspect the Main Control Board malfunction.
Board Analysis:
1) Relay (MPQ4-S-112D-A)
- Testing the MPQ4-S-112D-A (a 12VDC, 30A 4-pin normally open relay) requires a multimeter and a 12V power source.
- Identify the Pins
- The relay has 4 pins.
- Coil Pins: These receive the 12V signal to activate the relay.
- Switch (Load) Pins: These are "Normally Open" (NO), meaning they only connect when power is applied to the coil.
- Coil Resistance Test (Multimeter Only)
- Use this to check if the internal electromagnet is intact.
- Set your multimeter to the Resistance (Ω) setting.
- Measure across the two coil pins.
- Result: A healthy MPQ4-S-112D-A coil should read approximately 160Ω (±10%) Virtual Expo. A reading of "OL" (infinite) or 0Ω indicates a dead coil.
- Audible Click Test (12V Power Source)
- Listen for the mechanical movement of the switch.
- Apply 12VDC to the coil pins.
- Result: You should hear a distinct "click" sound as the internal contact moves. If it doesn't click at ≥9V, the relay is seized or faulty.
- Load Continuity Test (Power + Multimeter)
- This confirms the switch actually closes and conducts electricity.
- Set your multimeter to Continuity (Beep) or Resistance (Ω).
- Place probes on the two load pins (the switch side).
- Without Power: The meter should show "OL" (no connection).
- With 12V applied to Coil: The meter should beep or show nearly 0Ω. If the relay clicks but the resistance remains high, the internal contacts are likely burnt or oxidized.
- Summary: Tested the relays (both units) are working fine.
2) Capacitor
- Testing the capacitor with a multimeter.
- EC1: Malfunction (OL)
- EC2: Working
- EC3: Working
- Summary: EC1 need to be replaced with new part.
3) Voltage Regulator
- Set the multimeter to Resistance (Ω) or Continuity Mode.
- Regulator: U1 (UTC7805)
- Input to GND (Pin 1 to Pin 2): Should not show a short circuit (0Ω) or constant beep).
- Output to GND (Pin 3 to Pin 2): Should not show a short circuit.
- Input to Output (Pin 1 to Pin 3): Should not show a direct short, though some resistance is expected.
- Summary: Tested the regulator (UTC7805) is working fine.
- Testing the components with a multimeter.
- Diode: D5/D6/D7/D8/D9/D10 (~0.5V)
- Zener Diode: ZD1 (~0.7V)
- Resistor: R31 (~390kΩ), R37 (~10Ω)
- Optocoupler: U2 (Sharp PC817)
- Check Pin 1 & 2 (multimeter set to Diode Mode)
- Forward (1: Red, 2: Black): ~1.0V
- Reverse (2: Red, 1: Black): OL
- Summary: All other parts above are working fine.
5) Transformer (240VAC to 12VAC)
- Testing the components with a multimeter
- Testing a 240VAC to 12VAC step-down transformer involves checking for structural integrity, measuring coil resistance to ensure they are not "open" or broken, and verifying the output voltage while under power.
- Transformer (Dazhong Electronic, Model: EI35-115200, 200mA)
- Primary Coil: OL / High Resistance (>10MΩ) (typically 500Ω and 2kΩ)
- Secondary Coil: Low resistance (7.3Ω)
- Isolation: OL
- Output Voltage: Not tested (due to safety concern)
- Summary: Suspect transformer primary coil is broken
Analysis:
A failure of the 1000µF capacitor is directly related to transformer and power supply issues in the Midea MWH-38P5. In this unit, the 1000µF capacitor typically serves as the primary filter (smoothing) component for the low-voltage DC power supply generated by the transformer.
If this 1000µF capacitor fails, it can cause the following chain of events:
- Voltage Ripple and Instability: A 1000µF capacitor's main job is to "smooth" the pulsing DC voltage from the rectifier into a steady flat line. If it fails (becomes "open"), a large ripple voltage is applied directly to the control board, which can cause erratic behavior or damage sensitive semiconductors.
- Overloading the Transformer: If the capacitor shorts internally, it creates a direct path to ground for the transformer’s secondary winding. When a transformer is forced to work against a failing filter circuit (like a bad 1000µF cap), the heat builds up in the primary winding. Because the primary wire is hair-thin, it acts like a fuse and breaks.
- System "Stuttering": A weakened capacitor (lowered capacitance) may cause the unit to turn on and off rapidly as the voltage drops every time a relay tries to engage, leading to the "unstable power" symptom often reported by users.
Parts Sourcing:
- Relay (MPQ4-S-112D-A) - for backup purposes only
- Transformer (12VAC, 350mA) - nearest specs available
Parts Replacement:
- Solder the new capacitor (1000uF/25V) on board
- Replace with new transformer (12VAC, 350mA)
- Left: After received new transformer, found out the Pin-to-Pin distance is 2.0cm (possible to modify pins location)
- Right: Unable to source transformer (unlisted by seller) with Pin-to-Pin distance is 1.5cm
- Modified the transformer pins in order able to fit into PCB existing thru holes
- Check the transformer's pin alignment (after modification) on board
- Soldered new transformer (after modification) on board
- Tested the old Main Control Board is working on Midea Water Heater MWH-38P5
New Main Control Board Replacement:
- New Main Control Board (for Model MWH-38P3 / MWH-38P5)
- In parallel, proceed to source the new board directly, the new board will be kept for future used.
Summary:
- Main Control Board is suspected malfunction due to failure of the 1000µF capacitor which is causing transformer (primary winding) coil broken (open circuit).
- After fixing done (changed new 1000uF/25V capacitor and equivalent 12VAC 350mA transformer) on old Main Control Board, the old board is now working well.
- Midea Water Heater MWH-38P5 had been fixed (with old Main Control Board) and resume back to normal.
- In summary, I am now having both old and new Main Control Board in working condition.
Tools Required:
Cost Involved:
- Capacitor 1000uF / 25V (2pcs): US$0.35
- Relay (MPQ4-S-112D-A, 2pcs): US$2.50
- Transformer (12VAC, 350mA): US$5.00
- New Main Control Board: US$20 ~ US$25
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