Microwave ovens rely on electromechanical relays to switch high-voltage circuits (like the magnetron and the timer). When your microwave stops heating or behaves erratically, a faulty relay is often the culprit. Testing the relay helps you diagnose the problem accurately before replacing any parts. In this guide, we’ll walk you through the complete process of testing a microwave relay. We’ll use the MEISHUO MPD series relay as a reference—a popular choice for home appliances, including microwave ovens, thanks to its 10A switching capability, compact size (18.4×10.2×15.5 mm), and compliance with international safety standards.

Tools You’ll Need

• Digital multimeter (with resistance and voltage measurement)
• Adjustable DC power supply (0–60 VDC recommended)
• Insulation resistance tester (megger, 500 VDC)
• High-voltage withstand tester (optional, for dielectric strength checks)
• Screwdriver and basic hand tools
• Alligator clips and test leads

⚠Safety First Before opening any microwave, unplug the unit and discharge the high-voltage capacitor. Even when unplugged, capacitors can store lethal charges. Wait several minutes or use a proper discharging tool.

Step 1: Visual Inspection

Remove the relay from the PCB (or isolate it from the circuit). Examine the relay case for:

• Cracks or melting
• Burn marks or discoloration
• Corroded or bent pins

If any physical damage is visible, the relay should be replaced immediately.

Step 2: Measure the Coil Resistance

The coil is an electromagnet that actuates the contacts. Using a multimeter in resistance mode, measure across the coil pins. Compare your reading with the Coil Data table from the MEISHUO MPD datasheet. Example: MPD Coil Specifications (at 23°C)

Rated Voltage (VDC)	Coil Resistance (Ω) ±10%	Operate Voltage (VDC)	Release Voltage (VDC)
5	55.6	≤3.75	≥0.25
12 (Standard)	180	≤9	≥0.6
24 (Standard)	720	≤18	≥1.2
48 (Standard)	2880	≤36	≥2.4

• If the measured resistance iswithin ±10% of the rated value, the coil is likely good.
• Anopen circuit (infinite resistance) or a short (near zero) means the coil is damaged.
• Also check for any resistance between coil pins and the case/contacts – it should be infinite.

Step 3: Check Contact Resistance (Voltage Applied)

Now apply the rated coil voltage (e.g., 12 VDC for a 12 V relay) using your DC power supply. You should hear a distinct click as the contacts close. With the coil energized, measure the resistance between the common (COM)and normally open (NO) terminals. According to the MPD datasheet, the initial contact resistance should be ≤100 mΩ (measured at 1 A, 6 VDC).

• A readingbelow 100 mΩ indicates healthy contacts.
• Higher resistance (several ohms) or an open line suggestspitted or burnt contacts.

De-energize the coil; the COM–NO path should return to infinite resistance (open circuit). If it remains closed, the contacts are welded.

Step 4: Insulation Resistance Test

Use an insulation resistance tester (megger) set to 500 VDC to check isolation between:

• Contact to coil(between any contact pin and any coil pin)
• Between open contacts(if the relay has a changeover contact, but for 1 Form A it’s simply COM–NO open)

According to the MPD datasheet, the insulation resistance should be ≥1000 MΩ at 500 VDC.

• Lower values may indicate contamination or breakdown of the insulating materials.

Step 5: Dielectric Strength Test (Optional but Recommended)

If you have access to a hipot tester, you can verify the relay’s ability to withstand high voltage without breakdown. Use the limits from the datasheet:

Test Points	Test Voltage (AC, 1 min)
Between coil and contacts	4000 VAC
Between open contacts	1000 VAC

No arcing or excessive leakage current should occur. This test confirms the integrity of the insulation system.

Step 6: Operate and Release Voltage Test

This test ensures the relay picks up and drops out at the correct voltages, which is critical for reliable operation in a microwave control circuit.

1. Connect the coilto your variable DC supply and a voltmeter across the coil.
2. Gradually increase voltagefrom zero until the contacts close (you hear the click or measure continuity). Record this voltage as the operate voltage.
3. Gradually decrease voltageuntil the contacts open. Record this as the release voltage.

Compare with MPD specifications:

• Operate voltagemust be ≤75% of rated voltage (e.g., ≤9 V for a 12 V relay).
• Release voltagemust be ≥5% of rated voltage (e.g., ≥0.6 V for 12 V).

If the operate voltage is too high, the relay may not energize reliably in the appliance. If the release voltage is too low, it might not drop out when de‑energized.

Step 7: Operate and Release Time (Advanced)

In most field repairs, timing isn’t measured, but if you have an oscilloscope and a signal generator, you can verify:

• Operate time ≤10 ms
• Release time ≤5 ms

These values come from the MPD characteristics table and confirm the relay’s mechanical response.

Interpreting the Results

Test	Pass Condition (MPD)	Fail Indication
Coil resistance	Within ±10% of datasheet value	Open/short → coil faulty
Contact resistance	≤100 mΩ (energized)	High resistance → contacts worn
Insulation resistance	≥1000 MΩ @500 VDC	Lower → insulation breakdown
Dielectric strength	No breakdown @ specified voltage	Arcing → unsafe
Operate voltage	≤75% of rated voltage	Too high → weak coil or mechanism
Release voltage	≥5% of rated voltage	Too low → may not release properly

If the relay fails any of these tests, replacement is necessary.

Why Choose MEISHUO Relays for Your Microwave?

At MEISHUO, we specialize in manufacturing high‑quality relays for home appliances. The MPD seriesoffers:

• 10A switching capabilityat 250 VAC / 30 VDC
• Compact, sealed constructionfor PCB mounting
• Wide operating temperature–40°C to +105°C
• Global approvals: UL, CQC (certificates E358149, CQC10002049964)
• Long electrical and mechanical life(1×10⁵ operations)

Whether you’re repairing a microwave or designing a new control board, MEISHUO relays deliver consistent performance and reliability.

Final Thoughts

Testing a microwave relay systematically helps you avoid unnecessary part replacements and ensures the appliance operates safely. Always compare your measurements against the manufacturer’s datasheet—like the MEISHUO MPD specifications provided here. If you need a replacement, visit our website or contact our sales team for genuine MEISHUO relays. Stay safe, and happy troubleshooting!