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Microwave ovens use various combinations of electrical circuits and mechanical devices to produce and control microwave energy for heating and cooking. Generally speaking the systems of a microwave oven can be divided
into two fundamental sections, the control section and the high-voltage section.

The control section consists of a timer (electronic or electromechanical), a system to control or govern the power output, and various interlock and protection devices. The components in the high-voltage section serve to step up the house voltage to high voltage. The high voltage is then converted into microwave energy.

Basically, here is how it works: electricity through the power cord enters the microwave oven through a series of fuse and safety protection circuits, which include various fuses and thermal protectors that are designed to deactivate the oven in the event of an electrical short or if an overheating condition occurs.

If all systems are normal, the electricity passes through to the interlock and timer circuits. When then oven door is closed, an electrical path is also established through a series of safety interlock switches. Setting the oven timer and starting a cook operation extends this voltage path to the control circuits.

Generally, the control system includes either an electromechanical relay or an electronic switch called a triac. Sensing that all systems are "go," the control circuit generates a signal that causes the relay or triac to
activate, thereby producing a voltage path to the high-voltage transformer.

By adjusting the on-off ratio of this activation signal, the control system can govern the application of voltage to the high-voltage transformer, thereby controlling the on-off ratio of the magnetron tube and therefore the output power of the microwave oven. Some models use a fast-acting power-control relay in the high-voltage circuit to control the output power.

In the high-voltage section, the high-voltage transformer along with a special diode and capacitor arrangement serve to increase the typical household voltage, of about 115 volts, to the shockingly high amount of approximately 3,000 volts! While this powerful voltage would be quite unhealthy-even deadly-for humans, it is just what the magnetron tube needs to do its job-that is, to dynamically convert the high voltage in to undulating waves of electromagnetic cooking energy.

The microwave energy is transmitted into a metal channel called a waveguide, which feeds the energy into the cooking area where it encounters the slowly revolving metal blades of the stirrer blade. Some models use a type of rotating antenna while others rotate the food through the waves of energy on a revolving carousel. In any case, the effect is to evenly disperse the microwave energy throughout all areas of the cooking compartment. Some waves go directly toward the food, others bounce off the metal walls and flooring; and, thanks to special metal screen, microwaves also reflect off the door. So, the microwave energy reaches all surfaces of the food from every direction.

All microwave energy remains inside the cooking cavity. When the door is opened, or the timer reaches zero, the microwave energy stops--just as turning off a light switch stops the glow of the lamp.

Most common problems

Bad interlock switches or door misalignment can cause fuses to blow or no operation when the start button is pressed. Locate and replace defective switches and/or realign door.

Arcing in oven chamber: clean oven chamber and waveguide thoroughly. Replace carbonized or damaged waveguide cover. Smooth rough metal edges. Touch up interior paint.

Blown fuse due to power surge or old age: Replace fuse.

Erratic touchpad operation due to spill - let touchpad dry out for a week

Repairs

With small to medium size microwave ovens going for $60-100 it hardly makes sense to spend $60 to have one repaired. Even full size microwave ovens with full featured touchpanel can be had for under $200. Thus, replacement should be considered seriously before sinking a large investment into an older oven. Most people do not do anything to maintain a microwave oven. While not much is needed, regular cleaning at least will avoid potentially expensive repairs in the future:

a.. Clean the interior of the oven chamber after use with a damp cloth and a drop of detergent if necessary. Built up food deposits can eventually carbonize resulting in sparks, arcs, heating, and damage to the mica waveguide cover and interior paint - as well as potentially more serious damage to the magnetron. If there is any chance of food deposits having made their way above the waveguide cover in the roof of the chamber, remove the waveguide cover and thoroughly clean inside the waveguide as well.
b.. Periodically check for built up dust and dirt around the ventilation holes or grilles. Clean them up and use a vacuum cleaner to suck up loose dust. Keeping the ventilation free will minimize the chance of overheating.
c.. Inspect the cord and plug for physical damage and to make sure the plug is secure and tight in the outlet - particularly if the unit is installed inside a cabinet (yes, I know it is difficult to get at but I warned you about that!). Heat, especially from a combination microwave/convection oven or from other heat producing appliances can damage the plug and/or cord.

Microwave ovens are probably the most hazardous of consumer appliances to service. Very high voltages (up to 5000 V) at potentially very high currents (AMPs) are present when operating - deadly combination. These dangers do not go away even when unplugged as there is an energy storage device - a high voltage capacitor - that can retain a dangerous charge for a long time. If you have the slightest doubts about your knowledge and abilities to deal with these hazards, replace the oven or have it professionally repaired.