How to Test a Microwave Transformer

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Updated February 12, 2023

Suppose you’re experiencing cold or lukewarm food from your microwave oven. In that case, you may need to know how to test a microwave transformer. Of course, even the best microwave needs maintenance every once in a while, such as fixing a microwave handle or repairing a door switch. but testing a defective transformer is serious business. So, grab your rubber gloves, a screwdriver, and your multimeter, and we’ll walk you through troubleshooting a high-voltage transformer.


  • The microwave transformer regulates the electric current and allows the microwave magnetron to work.
  • Check the primary winding first, making sure you get a multimeter reading between 2.5 and 5 ohms.
  • Check both high and low-voltage secondary windings, looking for a multimeter reading of between 30 and 150 ohms in the high-voltage winding. Look for less than one ohm for the low-voltage winding.

Testing a Microwave Transformer

Tools: screwdriver, multimeter, rubber gloves

The microwave transformer is often a large, metal, square-shaped unit that maintains the alternating current of the magnetron and your power source. The transformer has a primary winding of thick wire, and below it sits low and high-voltage secondary windings. The integrity of transformer windings is necessary for the system to work, so any breaks in the primary coil will cause a non-working microwave oven. This can cause issues like a microwave that trips the breaker when the door opens.

Insider Tip

Given the complexity of electronics repair, don’t hesitate to contact a local repair shop for higher-level troubleshooting or repair service. Even a defective high-voltage capacitor can contain enough energy to harm or kill an inexperienced repair person.

STEP 1 Safety

Microwaves are dangerous to work on if you aren’t trained in dealing with an entire high-voltage circuit. In addition, as with any microwave oven repair, you run the risk of severe electrical shock. So, if you feel at all unsure about testing the high-voltage transformer, reach out to a certified appliance repair technician before touching your failed high-voltage transformer. That said, most people should know how to reset a microwave without much danger.

STEP 2 Measure Primary Winding

Take your multimeter, set it to the Rx1 scale, and calibrate it. Place your meter leads on both diode terminals that lead to the primary winding. You want an output voltage reading of fewer than 5 ohms.

STEP 3 Check Both Primary Winding Terminals

Use the digital multimeter to check both microwave diodes independently. Place your black lead on the screw holes for the ground, and use the red terminal to measure each terminal. If you get a reading on either, there is a short in the winding.

STEP 4 Check Secondary High-Voltage Winding

Set your multimeter to the Rx10 scale and calibrate it. The high-voltage secondary winding is a single wire from the back of the transformer. Ground one multimeter terminal and place the other in the secondary winding. You want a reading of between 35 and 180 ohms. If you get no reading, the circuit is broken. If you get less than one ohm, there is a short.

STEP 5 Check Secondary Low-Voltage Winding

Set your multimeter back to the Rx1 scale and calibrate it. Place both multimeter terminals across the low-voltage secondary winding. You want a reading of less than one ohm. If there’s no reading, there is a break in the circuit. If your multimeter reads as grounded, your wiring is shorted.

STEP 6 Reinstall or Replace

If your multimeter readings are reasonable, you can reinstall the high-voltage transformer. If not, you need to seek a replacement transformer or a replacement microwave.


Disconnected transformers and other electronic components can still hold an electrical charge, so make sure to leave your microwave unplugged for a minimum of 30 minutes before opening it up.


How many volts does a microwave transformer put out?

The voltage of a microwave oven transformer and the voltage across the capacitor is about 2.1KV, or 2100 volts.

How do I know if my microwave is high-voltage?

Most consumer microwave ovens are high-voltage, so you do not want to touch the inside of your appliance until it is unplugged and left to sit for no less than 30 minutes.

How do you diagnose a bad transformer?

There are some warning signs of a bad transformer. For example, your transformer may be bad if you notice a loud hum or burning smell while your microwave runs. It also will cause your food to be cold.

STAT: A US Department of Energy survey found that Americans in the East North Central US use their microwaves at least 25% more than other Americans. (source)

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