Exerpts from the CD-ROM:
THE COMPLETE MICROWAVE OVEN SERVICE HANDBOOK: Operation, Maintenance, Troubleshooting and Repair

Copyright © 1989-2013 J. Carlton Gallawa . All Rights Reserved Worldwide

8.13.05 Triac Drive Circuit

The schematic below shows a typical triac drive circuit. As noted, the triac is an electronic switch (or solid state relay) that is capable of controlling heavy AC loads with a high duty cycle.

In a microwave oven, the magnetron is generally supplied by rectified high voltage obtained with a 50/60 Hz transformer. In models that use a triac, the voltage supplied to the magnetron power supply is controlled by the triac, which is in series with the primary winding of the transformer.

The description of operation is as follows. When the oven door is closed and a cook cycle is initiated, the main microprocessor sends a signal to transistor T1 causing the circuit to switch on. Next, at a synchronized time that depends on the model, optocoupler PH1 / IC1 (sometimes called photocoupler) activates and provides an output "gate" voltage that triggers the triac, switching it on and thereby providing a current path to the primary winding of the high-voltage transformer. The oven lamp, blower motor and stirrer motor (in some models) are controlled by relay RL1, which is driven by transistor T2.

Current flow through the optocoupler circuit is controlled by a 220-ohm 1-watt limiting resistor. To protect the circuit from inductive kickback when the load is switched off, a surge absorber (or snubber) is placed in parallel with triac main current carrying terminals. The varistor absorbs the potentially destructive energy of incoming transient pulses, thereby protecting vulnerable circuit components.

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Copyright © 1989-2013 J. Carlton Gallawa . All Rights Reserved Worldwide