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 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 THE HIGH VOLTAGE CAPACITOR IS FULLY DISCHARGED
Test Procedure

   1. Unplug the oven.
   2. DISCHARGE ALL HIGH VOLTAGE CAPACITORS.
   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.


How To Discharge The High Voltage Capacitor

The capacitor is discharged by creating a short circuit (direct connection) between the two capacitor terminals and from each terminal to chassis ground (bare metal surface).

  1. Do this by touching the blade of an insulated-handled screw driver to one terminal, then slide it toward the other terminal until it makes contact and hold it there for a few seconds. (This can result in a rather startling "pop!" Note:If there is a spark, the capacitor is evidently holding a charge, thus it is most likely not defective)
  2. Repeat the procedure to create a short between each capacitor terminal and chassis ground.
  3. If the capacitor has three terminals, use the same procedure to create a short circuit between each terminal and then from each terminal to ground.
  4. Older Amana-made models (generally those manufactured before 1977) have red, round filter capacitors mounted in the base of the magnetron tube which can also hold a charge. Ground each magnetron terminal by creating a short circuit to chassis ground using the blade of a screwdriver as explained above.

Capacitor Test Procedure

  1. Unplug the oven.
  2. DISCHARGE ALL HIGH VOLTAGE CAPACITORS.
  3. Note the wiring and carefully remove all leads from the capacitor terminals. (If there is a bleeder resistor, it need not be removed. But, bear in mind that some measurements will reflect the meg-ohm resistance of the resistor)
  4. Set the ohmmeter to its highest resistance scale.
  5. Measure from one terminal to the other for a normal reading of infinity (or the value of the bleeder resistor).
  6. Now reverse the leads. The meter should momentarily deflect toward the zero mark, then slowly drift back to infinity.
  7. Reverse the leads once again. This should produce the same meter deflection.
  8. Next measure from each terminal to the capacitor's metal case for a normal reading of infinity. (If there is an internal diode, the meter readings will reflect the diode's forward bias resistance. 
  9. A visual inspection will also reveal certain defects, such as:
    • Evidence of arcing or burning at the insulators
    • The presence of an oily film or smell suggests a dielectric (non-conductive medium) leak
    • A bulging case indicates dielectric breakdown

Any such defects or abnormal readings would require replacement of the capacitor


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