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How To Test the Low Voltage (Control) Transformer Used in Microwave Ovens

Excerpts from the book The Complete Microwave Oven Service Handbook
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and from the video You Can Fix Microwave Ovens
Copyright © 1998-2013 by J. Carlton Gallawa

© 1996-2013 J. Carlton Gallawa . All Rights Reserved Worldwide

Low voltage transformers, also referred to as control transformers, serve to step down the line voltage to low voltages that supply power to the electronic control circuitry. These transformers usually have several secondary outputs, which provide an appropriate selection of control voltages as required by the various control components.
In the numerous different models that use a low voltage transformer, the mounting and structure of the transformer can vary considerably. In some cases, it is mounted on the base of the oven. In many models, it is soldered into the printed circuit board, and some models use a transmotor. This is a combination blower motor and low voltage transformer with a secondary winding that provides stepped down voltages. For test purposes, we will use the generic transformer pictured below.
A general resistance check of the low voltage transformer can be performed as follows.

Low Voltage Transformer Test

  1. Unplug the oven. low voltage transformer
  2. DISCHARGE ALL HIGH VOLTAGE CAPACITORS Procedure
    Also ground the bypass (RF) capacitors on the magnetron terminals by momentarily grounding each terminal to the magnetron chassis.
  3. Note the wiring locations, and disconnect all wiring to the solid state relay
  4. Set and zero the ohmmeter on the R X 1 scale.
  5. Measure from terminal to terminal. Generally the resistance of the primary winding is approximately 90 to 110 ohms. If it there is no reading-infinity-the winding is open. The secondary winding (output) resistance readings of various models will vary to some extent, but any reading of infinity-open circuit-between two terminals may indicate an open winding. Experience has shown the primary (input) winding (coil) to be the most susceptible to failure.

    Measuring from any terminal on the primary side to any terminal on the secondary side should produce a reading of infinity. The same is true when measuring from either primary terminal to chassis ground.

    An operational test may be made using the typical secondary voltages noted in the illustration as an approximate guide. However, it is unlikely that the configuration of any one transformer will correspond exactly to the voltages in this generic example.

    USE EXTREME CAUTION DURING LIVE TESTS! DANGEROUSLY HIGH VOLTAGES ARE PRESENT DURING OVEN OPERATION. DO NOT TOUCH ANY COMPONENTS OR WIRING DURING A COOK CYCLE.

Disclaimer:The author assumes no liability for any incidental, consequential or other liability from the use of the above information. All risks and damages, incidental or otherwise, arising from the use or misuse of the information contained herein are entirely the responsibility of the user. Although careful precaution has been taken in the preparation of this material, we assume no responsibility for omissions or errors.

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