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How To Test the High Voltage Rectifier (Diode) Used In Commercial & Residential Microwave Ovens The Gulf Coast's one and only commercial microwave oven repair specialist
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IMPORTANT SAFETY INFORMATION: Microwave ovens are among the most dangerous appliances to work on. Before attempting any troubleshooting, testing or repairs, for your personal safety, we strongly urge you to carefully read the very important safety precautions found by clicking here and Please read our disclaimer at the bottom of this page. common diodes or rectifiers used in microwave ovens

The high-voltage rectifier (diode) works along with the high-voltage capacitor to effectively double the already-high voltage that is provided by the power transformer. This powerful voltage, about 3000 - 5000 volts DC (depending on the model), is applied to the magnetron tube, causing it to produce the microwave energy that cooks the food.

Testing the HV diode requires an ohm meter with at least a 6 volt battery in order to accurately measure the front to back resistance of the diode. Meters with insufficient battery power may read infinite resistance (open) in each direction, mistakenly showing a good diode as being open.

However, the following resistance tests will conclusively reveal a diode that is shorted. In most cases, defective diodes, whether shorted or open, will show some physical signs of the defect, such as a burned crack, a blistered spot, or it may even be split in two. Also, a shorted diode will usually give off a pungent electrical burning odor.

Before making this or any other test:

ALWAYS MAKE SURE THE MICROWAVE OVEN IS UNPLUGGED AND ALL HIGH VOLTAGE CAPACITORS ARE FULLY DISCHARGED

Test Procedure

  1. Unplug the oven.
  2. DISCHARGE ALL HIGH VOLTAGE CAPACITORS. (Procedure)
  3. Carefully remove the lead that goes to the capacitor (the ground connection may remain attached)
  4. Set the ohmmeter to read ohms at a scale of R X 10,000 or higher.
  5. Measure the resistance across the terminals of the diode by touching the positive meter probe to the anode and the negative probe to the cathode (the cathode is the side that goes to ground, usually marked by an arrow, dot or stripe).
  6. A normal diode, depending on make and model, should read about 50,000 to 200,000 ohms.
    (Note:The polarity of the meter probes, with regard to forward and reverse bias readings, may be relative to the type of meter being used.)
  7. Reversing the leads should produce a reading of infinity (open), unless there is a bleeder resistor across the diode, in which case the reading would show the [megohm] value of the resistor.
  8. If continuity is read in both directions, the diode is shorted. If infinity is read in both directions, the diode is open. In each case the diode must be replaced.
In some models the diode is located inside of the high voltage capacitor. In this case, identify the diode terminal and perform the same test as above, measuring from the diode terminal to the capacitor's metal case.

We welcome your comments and suggestions: j.gallawa@cox.net

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