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The Purpose, Function and Testing of Interlock Switches Used In Microwave Ovens

Excerpt from the distinguished e-Book, The Complete Microwave Oven Service Handbook: Operation, Maintenance, Troubleshooting and Repair on CD-ROM
© 1996-2013 J. Carlton Gallawa . All Rights Reserved Worldwide

Typical interlock switch used in microwave ovensTo ensure safe operation, all microwave ovens are equipped with safety interlock switches . The door-interlock system is one of the most significant safeguards in a microwave oven. While switch configuration, size, shape, and mounting arrangement may vary from one model to the next, the purpose and basic operation remain the same in all models.

The purpose of the interlock system is to interrupt the production of microwave energy when the oven door is opened, and similarly, to prevent any microwave output until the door is firmly and safely closed.

A principal component of the interlock system is the interlock monitor switch . In August of 1974, a Federal safety standard went into effect that required that all microwave ovens be equipped with an interlock monitoring system. The monitor switch, which is also called the safety switch, short switch, sensing switch, and failure detector switch , functions as a fail-safe type of device, in that it disables the microwave oven if an interlock failure should occur. The actual method differs from model to model, but the results are the same:a blown (or opened) fuse.

Generally speaking, the normal sequence of switch operation when the door is opened is as follows. First the primary switch opens its contacts. Second, (yes) the secondary switch opens. Finally, the interlock monitor switch closes its contacts. The fail-safe system works like this:If any of the switches and/or relays included in the monitor loop (or circuit) fail to open their contacts properly when the door is opened, a short circuit is created when the monitor switch closes its contacts. The closed contacts of the monitor switch and the faultily-closed contacts of the defective switch combine to cause an immediate short circuit, which, in one way or another (depending on the model), blows the line fuse, or otherwise disables the oven. All this happens before the door can be opened far enough to allow any dangerous levels of microwave radiation to escape.

All switches have a voltage and current rating. A typical door-interlock switch is rated at 15 amps with 125 or 250 VAC applied. When replacing an interlock switch, these ratings must be met or exceeded, otherwise premature switch failure will result. Another replacement consideration is the operating and release force. This is the relative amount of pressure needed to actuate the switch. Various applications call for differing amounts of operating pressure. Like the voltage and current ratings, this is an important factor when obtaining a replacement switch. For example, if the normal pressure of a latch mechanism is insufficient to depress the actuator button, the switch's operating force is probably too high for that application.

The interlock switch to be tested in this example has a COM. (common) terminal, a N.O. (normally open) terminal, and a N.C. (normally closed) terminal. Interlock monitor switches are usually constructed with only the COM and N.C. terminals. Other switches are made with just the COM and N.O. terminals. The following tests will cover most circumstances. Simply disregard the terminal-to-terminal tests that do not apply.

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 carefully read, fully understand and be prepared to follow the very important safety precautions found by clicking here and please read the disclaimer at the bottom of this page.

How To Test Interlock Switches Used in Microwave Ovens

  1. Unplug the oven and remove the outer cover. DISCHARGE ALL HIGH VOLTAGE CAPACITORS. Procedure
  2. Visually examine the switch terminals and connectors for signs of overheating, such as discoloration, or brittleness.
    • Many times the problem is merely a burned slip-on connector due to a poor crimp joint, or weakened connection. If the switch is in good working order, the repair can be made by cleaning the terminals, and replacing the burned connector. Or, simply cut off the burned connectors and solder the wires directly to the switch terminals.
    Carefully remove the harness leads from the switch terminals. Pry them loose if necessary, but do not exert too much force, or the terminal may break right out of the switch.
    • Many models use a connector with a locking clip in the center of the receptacle terminal. This extruding lever must be pressed down while gently pulling the connector off the terminal.
    Set the ohmmeter to read ohms at a scale of R X 1 As shown in the picture, place one meter probe on the COM terminal and the other probe on the N.O. terminal. With the actuator (or lever, or button) not depressed, the meter should read infinity (an open circuit). Without moving the meter probes, press down on the switch actuator until a "click" is heard. At the point of the click, the meter should swing to a reading of zero ohms (or continuity).
    • Rule of thumb:A healthy "click" usually means the switch is working normally. However, switches with just two terminals, both of which come out the back, do not click when actuated, and neither do switch modules.
    Move the probe from the N.O. terminal to the N.C. terminal. The other probe remains on the COM terminal. Press the actuator and the meter should read infinity. Release the actuator and the meter should read zero ohms. Set the meter to the highest resistance (ohms) scale and measure from the N.C. terminal to the N.O. terminals for a normal reading of infinity.
  3. Measure from each terminal to any metal mounting hardware that is part of the switch assembly for a normal reading of infinity.
Any abnormal readings would indicate that the switch is defective and should be replaced.
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