DIY Appliance: Repair TRIAC

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Repairing your own appliances today requires that you have a good understanding of electronics, especially solid-state electronics. Without a working knowledge of solid-state components, you are really limited as to what you can do. Almost all state of th

You need to know how solid-state components work and how to test them, otherwise you will be nothing more than what we use to refer to in the TV repair business back in the 1960s as a “Parts Changer.” A “Parts Changer” was a person who did not understand how the individual components in a circuit worked and how they worked together to perform a certain task, so what they did was to change one part after another until they found the one that was causing the problem. That was very time consuming for the technician and very costly for the customer. You are trying to save money by repairing your own appliances, so you do not want to be a “Parts Changer.”

One of the solid state components that you need to know about is the TRIAC. A TRIAC is actually a solid state relay found in most microwave ovens today and is used to control the voltage supplying the primary winding on the high-voltage transformer. The TRIAC may be mounted right on the PC Timer Board or may be mounted off the board as a separate component, either way it does the same thing, it acts as an on and off switch for the HV Transformer's primary winding. By controlling the current flowing in the primary, it controls the oven's cook time.

What do TRIACS look like?

The TRIACs used in microwave ovens, more often than not, will look like one of these in this picture.

On the microwave oven's schematic, it will appear as this schematic symbol.

If the TRIAC is part of the control board it will not show on the schematic as a discreet component. Very rarely will you find a schematic of the circuit board provided. Most schematic will simple show how the other component connect to the board. Not to worry. You will still be able to test the TRIAC and replace it if it is defective. It will be worth the time it will take to learn the testing procedure and the time it takes to perform the actual testing because it will save you money. A TRIAC will cost you about $10 whereas the complete controller board may cost you $100 or more to replace.

Tools and supplies to test and replace a TRIAC.

  • PC Board soldering iron

  • Vacuum Desoldering pump

  • Rosin Core solder

  • Digital Multimeter (DMM)

  • Nut-drivers

  • Screwdrivers

  • Long-nose pliers

  • High-voltage capacitor discharging tool/jig

  • Proper replacement TRIAC

Testing a Microwave Oven TRIAC.

  • Unplug the microwave oven and remove the wraparound cover.

  • Carefully discharge the HV Capacitor.

  • Make a sketch of the way the wires connect to the TRIAC before removing them.

  • Identify the TRIAC terminals. The terminals are usually marked—G (Gate), T1, and T2. They can also be identified by their physical size with the Gate being the smallest and T2 being the largest.

  • Set your DMM to the Ohms range if it is an auto-ranging meter, or to the R X 100 range if it is a manual ranging meter.

  • Take a reading between the gate terminal and T1. Reverse the meter probes and take a second reading between the Gate terminal and T1. If the TRIAC is a normal reading in both directions will be between 10 and 200 Ohms depending on the model TRIAC.

  • Now set your DMM to its highest scale and take readings between T1 and T2, T2 and Gate, and from each terminal to chassis ground. A good TRIAC will register infinite resistance in each of these tests. Infinite resistance will register as an “O.L.” on your DMMs LCD.

  • Any reading significantly different than these indicate a defective TRIAC and it should be replaced.


1 comment

Taylor Rios
Posted on Apr 23, 2012