DIY Series 99: Midea Water Heater MWH-38P5 Not Working

Midea Water Heater MWH-38P5 Not Working (No Power at All)

Symptoms:

• No power at all when switched ON.
• Tested incoming supply at water heater terminal block is working.
• 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.

4) Other Parts (Diode / Resistor / Optocoupler)

• 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:

• Capacitor 1000uF / 25V

• 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 footprint (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:

• Screwdriver
• Multimeter
• Soldering Tools
• Cutter

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

DIY Series 98: Joven Water Heater SA10E Not Working

Joven Water Heater SA10E Not Working

The Joven SA10E (3.6kW) is an instant water heater that uses a complex safety and control circuit to handle its 15A current. It does not wire a 10A thermostat directly to the heater; instead, it uses a PCB (Printed Circuit Board) with an EELS (Electronic Earth Leakage Sensor) to manage the switching safely. 

Joven SA10E Circuit Overview

The circuit for a 3.6kW Joven heater typically follows this path: 

• Main Power Inlet: 240V AC power enters through a 20A double-pole switch on your wall.
• EELS (Safety Board): Both Live and Neutral wires pass through a double-pole EELS system. If it detects a leakage of more than 10mA, it instantly cuts power.
• Flow Sensor (SFS): A magnetic sensor detects water flow. The heater will not activate unless water is flowing, preventing the element from burning out.
• Stepless Electronic Power Control: A knob on the front controls a Triac on the PCB. This electronic component acts as the "acting" switch, varying the power to the heater element based on your temperature setting.
• Thermostat with Safety Cut-Out: This is the "high-limit" thermostat. If the water exceeds a safe temperature (e.g., due to a PCB failure), it trips to prevent scalding and fire. 

Symptoms:

• No power at all when switch ON. Sometimes EELS (Green LED) will be ON for a while, then turn OFF again. The heater (Red LED) will not be turn ON, even temperature control knob is turn with water flow in.
• When press "Test/Reset" button, the EELS (Green LED) will be turn ON and then turn OFF immediately.
• Suspect the Main Control Board or thermostat malfunction.

Debugging:

• If you hear a mechanical click inside the unit when you press the "Reset" button, it confirms that either the EELS relay or the high-limit thermal cut-out has physically re-engaged.
• Since the EELS light is still staying off despite that click, here is the diagnosis:
• If the thermostat fails, it breaks the circuit completely. No power reaches the EELS board, so the light stays off.
• Use a multimeter to check for continuity across the thermostat. If it's "open" even after clicking, it must be replaced.

Findings:

• Checking the Main Control Board (Power Supply Board), found no issue.
• Trial and error to replace thermostat, see if it is the culprit. After changing thermostat, the Joven Water Heater (SA10E) resume back to normal.

Parts Sourcing:

• Found the thermostat written ATT S102, try to look for the original parts, but not available for sale at online store.

• Found another thermostat, KSD303 having same shape and size. Rating at 250VAC/10A, 150°C.

• Use Google Gemini to check compatibility. For a 10A application at 250VAC, both the ATT S102 and KSD303 are technically suitable, as 10A is a standard rating for both series. At this current level, the choice depends primarily on whether you need a direct manufacturer-specified replacement (ATT S102) or a more widely available, generic industrial alternative (KSD303)

• While some manufacturers may use a 10A thermostat in a 3.6kW water heater, it is only safe if it is part of a relay or contactor system. In such a setup, the 10A thermostat does not carry the full 15A–16A load itself but instead acts as a "trigger" for a higher-rated switching device. 

Photos:

• Joven Water Heater (SA10E) - bought in Year 2017

• Thermostat located at the top of the water tank

• Remove the thermostat (ATT S102)

• Replace with new thermostat (KSD303)

Tools Required:

• Multimeter
• Screwdriver
• Hex wrench (to remove thermostat)

Cost Involved:

• Thermostat (KSD303): US$1

How a 10A Thermostat Works with a 3.6kW Heater?

If your water heater was originally built with a 10A thermostat, it likely operates in one of the following ways:

• Indirect Switching: The 10A thermostat controls a contactor or relay that is rated for 20A or 25A. The contactor handles the heavy 15A current, while the thermostat only handles a very small control current.
• Safety Cutout Only: Some 10A thermostats are used strictly as a secondary safety limit (high-limit switch) designed to trip and stay off in an emergency, rather than cycling the heater on and off daily.
• Low-Voltage Controls: In modern digital or "smart" systems, the thermostat may operate on low voltage (e.g., 5V or 12V) to send a signal to an electronic board, which then switches the main power.

In the Joven SA10E, the 10A thermostat is likely used as a secondary safety trigger on the control board:

• It does not carry the 15A load directly.
• It sends a signal to the PCB/Relay/Triac to cut off the main power if things get too hot. 
• Typical thermal cut-out profile:
• Standard Thermostat (Auto Cut-Off) @ 55°C: Anti-Scald protection
• Manual Cut-Out @ 80°C–95°C: Secondary safety limit
• Thermostat Max Rating @ 150°C: For dry-burn protection in heating elements