Testing a microwave oven transformer (MOT) requires caution because these transformers output high voltage (2,000-4,000V) and can be dangerous if mishandled. Below is a safe step-by-step guide to testing a microwave transformer.

Safety Warnings:

• Unplug the microwave before working on it.

• Discharge the high-voltage capacitor (if present) with an insulated screwdriver.

• Wear insulated gloves and use proper tools.

• Never touch exposed terminals while power is applied.

• Work in a dry environment to avoid short circuits.

Tools Needed:

1. Multimeter (for resistance and voltage tests)

2. Insulated screwdriver

3. Low-voltage AC power source (optional, for bench testing)

4. Light bulb (for load testing, optional)

Step 1: Identify the Transformer Terminals

A microwave transformer has three main windings:

1. Primary winding (connects to 120V/240V AC input) – Thicker wires, low resistance (~0.5–2Ω).

2. High-voltage (HV) secondary winding (outputs 2,000–4,000V) – Thin wires, high resistance (~50–150Ω).

3. Filament winding (powers the magnetron, ~3.3V AC) – Very low resistance (~0.1–1Ω).

 (Example image, actual terminals may vary.)

Step 2: Check for Shorts or Open Circuits (Resistance Test)

1. Set your multimeter to Ω (resistance mode).

2. Test the primary winding:

• Place probes on the two primary terminals.

• Expected: 0.5–2Ω (if open or ∞, the winding is bad).

3. Test the HV secondary winding:

• Place one probe on the HV terminal and the other on the transformer core (ground).

• Expected: 50–150Ω (if ∞, the winding is open).

4. Test the filament winding:

• Place probes on the two filament terminals.

• Expected: 0.1–1Ω (if ∞, it’s faulty).

If any winding shows 0Ω (dead short) or ∞ (open), the transformer is bad.

Step 3: Power-Up Test (Optional, Advanced Users Only)

Danger: Lethal voltage! Use extreme caution.

1. Isolate the transformer (remove from microwave).

2. Apply 120V AC to the primary winding (use a fused power cord).

3. Measure the output voltages:

• HV secondary: Should read 2,000–4,000V AC (use a high-voltage probe).

• Filament winding: Should read ~3.3V AC.

If no voltage or very low voltage, the transformer is faulty.

Step 4: Load Test (Optional)

• Connect a high-voltage diode & capacitor (like in a microwave) and check for proper voltage under load.

• Alternatively, use a neon bulb to check for HV presence (it should glow faintly).

Common Microwave Transformer Failures:

Good Transformer: Correct resistances, proper voltage output.
Bad Transformer:

• Open winding (∞ resistance).

• Shorted winding (0Ω or very low resistance).

• Burnt smell or visible damage.

• No high-voltage output when powered.

Final Advice:

• If the transformer is bad, replace it—microwave transformers are not repairable.

• Never bypass safety features (like the thermal fuse).

• Consider using a Variac for safer low-voltage testing.

• The above content is published by demiks, please specify, demiks is a substation equipment manufacturer, specializing in the production of high-voltage tester, transformer tester, circuit breaker test equipment, relay tester, SF6 gas analyser, cable fault tester and other products, if you have power test testing needs, please feel free to contact demiks power science and technology limited company or send an email! Give us: contact@demikspower.com