Microwave Oven Transformer

Microwave Oven Transformer
Basic Operation

The microwave is a very effective electrical device. It functions on the basis of a high voltage transformer.

Given below are some operational details about this transformer.

Working principle and microwave oven transformer uses

The transformer has a solid E core with a center leg and an air gap. The coils are wound on this core and they carry current. This unique construction creates two magnetic fields around the core. One travels through the center leg while the other cuts across the secondary core through the periphery.

When there is no current in the primary, all the magnetic field would travel through the periphery and induce current in the secondary. Now as one starts powering the primary coil, the magnetic field would start flowing from the centre leg which will reduce the voltage on the secondary. However the current through the secondary would remain constant. With the right dimensions and design, the transformer would maintain constant current while adjusting the voltage as per the load on the primary.

When taken in the context of a microwave, the transformer is the perfect solution to the kind of input the magnetron requires to function properly.

Testing a faulty High voltage Transformer

Given the complex operation of the transformer, it is not surprising that there can be many potential problems which need to be diagnoses. Some of them can be tested with simple devices like an ohmmeter while others require a more complex procedure. The simple steps are described below.

The basic tests that need to be performed are typically related to the resistances on the various sections of the microwave oven transformer. Before starting the testing, ensure that the microwave has been powered off, capacitor discharged and the terminals are disconnected. Run the following checks:

Primary Resistance: 0.1 W to 0.5 W. Generally, the resistance on the primary would be 0.2 W.
Filament Resistance: This is generally a very low resistance filament and the standard ohmmeter may not be able to measure it. One just needs to ensure that this is not shorted to the chassis.
Transformer Secondary Resistance: 25 W to 150 W. If this turns out to be open circuited, it is a problem and that needs to be fixed.
Filament to primary resistance should be infinite or open circuited.
Filament to high voltage and core resistance should be infinite or open circuited. If this is shorted, it can be repaired easily as the filament is a think wire with a couple of turns. Just ensure that you protect it from overheating
Primary to high voltage as well as the core resistance should be infinite or open circuited.
High voltage and core resistance should be 25 W to 150 W.

Replacing high voltage transformer

Most times the only solution to a fault transformer is to replace the unit completely. While replacing the transformer take utmost care to ensure that the microwave is disconnected from the power plug and let it rest for a couple of minutes. This will discharge the high voltage capacitor.

Now open the outer covering of the microwave and unscrew the transformer. Most times, the chassis holding the transformer may have some other parts screwed to it too. Take care to ensure that you mark the right wires so that you do not mess the connections while fixing the new transformer.

The only challenge would be when the new transformer is not the same model as the old one. You would need to study the wiring as well as figure out a way to mount it safely.

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