The High Voltage Relay Used In Microwave Ovens

The high voltage relay , also called the variable power switch, is a special type of relay. It is operated by the control panel, but it performs its switching in the high voltage section.

typical high voltage relays Most models of microwave ovens control the output power by governing the on-off time of the magnetron tube. This is most commonly done by cycling on and off the line voltage that is applied to the primary side of the high-voltage transformer. Models that use a high voltage relay accomplish this power control by actually switching the high voltage on and off. This is quite a feat for this little reed relay, so it relies on very carefully timed signals from the control panel.

The high voltage relay can experience two basic types of failures:1) An open operating coil, 2) A structural breakdown resulting in internal arcing and sometimes a crippling backfeed of current to the control panel.

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 .
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How To Test The High Voltage Relay

Unplug the oven.
DISCHARGE ALL HIGH VOLTAGE CAPACITORS. Procedure
First examine the relay for physical defects, such as cracks in the glass, or a smoky appearance. Check the casing for lacerations or blistering.
Carefully remove all attached leads.
With the ohmmeter set to the appropriate scale, measure across the relay coil for a normal resistance of about 200 to 400 ohms, depending on the brand.
Set the meter to the highest resistance scale and make the following measurements:

From one contact terminal (or wire) to the other
From each contact terminal to each coil terminal
From each of the four terminals to chassis ground

A reading of less than infinity on any of these measurements indicates that the relay is defective and should be replaced. Before doing so, however, it would be good to first inspect the relay-drive circuit on the control panel for damaged components. Control panel repairs might also be necessary.
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