The Purpose of the Triac Used in Commercial & Residential Microwave Ovens

The triac is an electronic relay or switch. Triacs come in many shapes, sizes and colors. The black and white illustration to the right shows most of the types of triacs that are commonly used in microwave ovens, along with their standard terminal designations.

Located either externally or fixed within the controller, the triac operates when it receives an electronic "gate" signal from the control circuitry. It then switches to its closed or "on" state, thus providing a voltage path to the primary winding of the high voltage transformer, thereby energizing the cook circuits.

Considering the heavy job that the triac does, it's not surprising that it is a common candidate for failure.

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.

How To Test the Triac

Triacs with three terminals, such as most of those shown above, can be tested by making a series of resistance checks as follows.

Test One

1. Unplug the oven.
2. DISCHARGE THE HIGH VOLTAGE CAPACITOR. (Procedure)
3. First identify the terminals. The three terminals are generally designated as G (gate), T1 and T2. (A rule of thumb:smallest terminal is the gate; medium sized is T1; largest is T2.)
4. Carefully remove all harness leads. A soldered-in varistor or snubber may remain attached providing it's in good condition.
5. Set and zero the ohmmeter to a scale capable or reading about 40 ohms.
6. Measure from the gate to T1 , note the reading, then reverse the leads.
7. In each measurement, a normal reading would be in the range of 10 to 200 ohms , depending on the model.
8. Next, set the meter to its highest resistance scale. Each of the following readings should produce a normal reading of infinity:
   1. From T1 to T2.
   2. From T2 to the gate
   3. From each terminal to chassis ground

Note:These readings are approximate and may vary with manufacturer, but generally speaking, any results that are significantly different would point to a defective triac.

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Test Two

A second way to test the triac is to evaluate its gate-firing capability:

1. Unplug the oven.
2. DISCHARGE THE HIGH VOLTAGE CAPACITOR. (Procedure)
3. Remove all harness leads. Set the meter to a scale capable of reading about 50 ohms.
4. Attach the negative meter lead to T1 and the positive lead to T2 .
5. Now, using a screwdriver blade, create a momentary short between T2 and the gate . This brief contact should turn the triac "on," thus producing a meter reading of about 15 to 50 ohms.
6. Next, disconnect one of the meter leads, then re-connect it. The meter should return to a reading of infinity .
7. Finally, reverse the meter leads and repeat the tests. The results should be the same.

• Any abnormal tests would suggest a defective triac.
• Replacement triacs are generally available at electronic and appliance parts distributors.
• Send us E-mail: j.gallawa@cox.net

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