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The Thermistor Used in Microwave Ovens

© 1996-2006 J. Carlton Gallawa . All Rights Reserved Worldwide


The thermistor is a thermal resistor with a high negative temperature coefficient of resistance. In other words, as the temperature increases its resistance decreases.

A thermistor is mounted in the ceiling of most convection models, where it monitors the temperature of the cooking chamber. As the temperature changes, so does the thermistor's resistance. The control circuitry thereby governs the cooking temperature by responding to the changes in the thermistor's resistance.

There are a number of thermistor applications and configurations. Two very common types of thermistors are tested here. As noted in the illustration, we will label them thermistor type "A" and thermistor type "B."

Important Safety Information

Working on a microwave oven is a very dangerous task. Therefore, BEFORE performing any tests, troubleshooting or repairs, for your personal safety, we strongly urge you to carfully read, fully understand and be prepared to follow the very important safety precautions found by clicking here .

Please read the disclaimer at the bottom of this page.

Testing Thermistor Type "A"

  1. Unplug the oven.
  2. DISCHARGE ALL HIGH VOLTAGE CAPACITORS. Procedure
  3. Disconnect the leads from the thermistor and remove it from the oven.
  4. Visually examine the component side of the thermistor for corrosion. See below .
  5. Corroded thermistor leads will cause such symptoms as:
    • "The fan motor has a mind of its own. Sometimes it comes on all by itself for no reason."
    • The oven runs for two minutes then shuts off (if it runs for just one minute, then shuts off-check the damper motor)
    • The oven will not cook in the convection mode.
    • The temperature is lower or higher than preset.
  6. Check the resistance of the thermistor as follows.
    1. Zero the ohmmeter on the R X 10,000 (or R X 10K) scale.
    2. Referring to the illustration, measure from the "COMMON" terminal to the "HIGH" terminal (meter polarity is not a factor). The thermistor resistance should reflect the ambient temperature as shown below:
      Room Temperature Approximate Resistance in Ohms
      68° F (20° C). To 86° F (30° C) 322,000 to 165,000 respectively

  7. Measuring from the "COMMON" terminal to the "LOW" terminal should produce the following readings:

    Room Temperature Approximate Resistance in Ohms
    68° F (20° C). To 86° F (30° C) 68,000 to 35,000 respectively
  8. Measuring from "LOW" to "HIGH" should read infinity.
Example of corroded thermistor

Testing Thermistor Type "B"

  1. Disconnect one lead from the thermistor or remove its respective connector from the control unit.
  2. Measure across the thermistor terminals for the following normal resistance readings in relation to the room temperature:
Room Temperature Approximate Resistance in Ohms
68° F (20° C). To 86° F (30° C) 800,000 to 155,000 respectively
If the readings obtained are substantially different from these, the thermistor should be replaced.

 
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Copyright Information

Unless otherwise noted, all materials at this cite (including without limitation all text, html markup, graphics, and graphic elements) are copyrighted ©, 1989-2006 by J. Carlton Gallawa. The material available through this site may be freely used for attributed noncommercial educational purposes only. We ask that due credit and notification be given the author.

All materials appearing on this website may not be reproduced, stored in any retrieval system, or used in any way for commercial purposes without the express prior written permission of the copyright holder.

Disclaimer: The author assumes no liability for any incidental, consequential or other liability from the use of this 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.

Send correspondence to: Microtech, P.O. Box 940, Gonzalez, FL 32560
© 1996-2006 J. Carlton Gallawa . All Rights Reserved Worldwide

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