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How Microwave Cooking Works

by Marshall Brain

The microwave oven is one of the great inventions of the 20th century - millions of homes in America have one. Microwave ovens are popular because they cook food incredibly quickly. They are also extremely efficient in their use of electricity because a microwave oven heats only the food - nothing else.

A microwave oven uses microwaves to heat food. Microwaves are

radio waves. In the case of

microwave ovens, the commonly used radio wave frequency is roughly 2,500 megahertz (2.5 gigahertz). Radio waves in this frequency range have an interesting property: they are absorbed by water, fats and sugars. When they are absorbed they are converted directly into atomic motion - heat. Microwaves in this frequency range have another interesting property: they are not absorbed by most plastics, glass or ceramics. Metal reflects microwaves, which is why metal pans do not work well in a microwave oven.

How Microwave Ovens Cook Food

You often hear that microwave ovens cook food "From the inside out." What does that mean? Here's an explanation to help make sense of microwave cooking.

Let's say you want to bake a cake in a conventional oven. Normally you would bake a cake at 350 degrees F or so, but let's say you accidentally set the oven at 600 degrees instead of 350. What is going to happen is that the outside of the cake will burn before the inside even gets warm. In a conventional oven, the heat has to migrate (by conduction) from the outside of the food toward the middle (See the HSW article entitled

How a Thermos Works for a good explanation of conduction and

other heat transfer processes). You also have dry, hot air on the outside of the food evaporating

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Howstuffworks "How Microwave Cooking Works"

moisture. So the outside can be crispy and brown (e.g. - bread forms a crust) while the inside is moist.

In microwave cooking, the radio waves penetrate the food and excite water and fat molecules pretty much evenly throughout the food. There is no "heat having to migrate toward the interior by conduction". There is heat everywhere all at once because the molecules are all excited together. There are limits of course. Radio waves penetrate unevenly in thick pieces of food (they don't make it all the way to the middle), and there are also "hot spots" caused by wave interference, but you get the idea. The whole heating process is different because you are "exciting atoms" rather than "conducting heat".

In a microwave oven, the air in the oven is at room temperture, so there is no way to form a crust. That is why foods like "Hot Pockets" come with a little cardboard/foil sleeve. You put the food in the sleeve and then microwave it. The sleeve reacts to microwave energy by becoming very hot. This exterior heat lets the crust become crispy as it would in a conventional oven.

Links

Microwave ovens are described by several interesting links on the web. Try these:

● For an excellent discussion of the different parts of a microwave oven and how they work

together, click here.

● For a great collection of in-depth questions and answers about microwaves, click here.

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The website below is governed by its own terms and

conditions; its owner has no affiliation with

HowStuffWorks.com.

How Does A Microwave Oven Work?

Basic Theory of Operation

Excerpts from the book The Complete Microwave Oven Service Handbook

--NOW available on CD-ROM (

CLICK HERE)

and from the video

You Can Fix Your Microwave Oven, Plus VCR Know-How

Debussy's L'Îsle Joyeuse for your listening enjoyment.

(Use right mouse button on note icon to start and control music)

How A Microwave Oven Works

Microwave ovens use various combinations of electrical circuits and mechanical devices to produce and control an output of 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 .

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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 microwave energy.

Basically, here is how it works: As shown in Figure 1, electricity from the wall outlet travels through the power cord and enters the microwave oven through a series of fuse and safety protection circuits. These circuits 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 .

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Generally, the control system includes either an electromechanical relay or an electronic switch called a triac

as shown in Figure 2 . 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 ( Figure 3 ), 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 3000 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.

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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

Learn more about the myths & mysteries of microwave ovens

Database | FAQ | Links |

Copyright Information

Unless otherwise noted, all materials at this cite (including without limitation all text, html markup, graphics, and graphic elements) are copyrighted ©, 1989-2000 by

J. Carlton Gallawa. The material

available through this site may be freely used for attributed noncommercial educational purposes only. We ask that due credit and notification be given the author.

All materials appearing on this website may not be reproduced, stored in any retrieval system, or used in any way for commercial purposes without the express prior written permission of the copyright holder.

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Send correspondence to: Microtech, P.O. Box 940, Gonzalez, Florida 32560

Disclaimer: The author assumes no liability for any incidental, consequential or other liability from the

use of this information. All risks and damages, incidental or otherwise, arising from the use or misuse of the information contained herein are entirely the responsibility of the user. Although careful precaution has been taken in the preparation of this material, we assume no responsibility for omissions or errors.

Debussy's L'Îsle Joyeuse courtesy of David Siu and The Classical Midi Connection:

http://www.dtx.

net/~raborn/

As of 12/25/97 you are visitor number 212985

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How Do Microwave Ovens Cook Food?

How Do Microwaves Cook?

Excerpts from the book The Complete Microwave Oven Service Handbook

--NOW available on CD-ROM (CLICK HERE)

and from the video You Can Fix Your Microwave Oven, Plus VCR Know-How

Music of Claude Debussy for your listening enjoyment.

(Use right mouse button on note icon to start and control music)

Microwaves possess three basic characteristics:

1. Just as sunlight shines through a window, microwaves pass right through some materials. Materials such as glass, paper, and plastic are transparent to and generally unaffected by microwaves.

2. Microwaves are reflected by metal surfaces, much as a ball would bounce off a wall. The metal walls of the cooking space actually form a cavity resonator. In other words, the enclosure is designed to resonate the microwaves as they are radiated from the magnetron tube. The principle of resonance may be illustrated using sound waves. When a piano key is struck, it produces sound vibrations or sound waves. Sometimes a note is played on a piano, and an object across the room, perhaps a wineglass, can be heard vibrating and producing the same sound. This is the result of resonance. The resonating characteristics of the wineglass are the same as those of the piano string. Therefore, the wineglass is in tune, or in resonance, with the sound wave produced by the piano string. In the same way, the cooking cavity of a microwave oven is designed to be in "tune" with the resonant characteristics of the microwaves.

❍ Metal racks are physically proportioned so as not to disrupt the energy pattern.

3. Microwaves penetrate and are absorbed by some substances, primarily food products.

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How Do Microwave Ovens Cook Food?

Friction Produces the Heat That Cooks the Food

To illustrate this third characteristic, notice the cooked turkey to the right. The waves of microwave energy are cycling above and below a horizontal baseline. The half cycle below the baseline possesses negative properties, and the half cycle above the line is correspondingly positive. Basically, the effect of this wave, as it alternates between positive and negative, would be like a magnet flipping back and forth.

All liquids and food products, such as this turkey, are made up of molecules. These molecules have positive and negative particles, so they tend to behave like microscopic magnets. As the positive half cycle of the microwave penetrates the food, the negative particles of the molecules are attracted and attempt to align themselves with this positive field of energy. Then, when the microwave energy alternates to the negative half cycle, the opposite occurs -- The negative particles are repelled and the positive particles are attracted, causing a flipping motion (actually, this reaction is the movement of the particles within each molecule, so, technically, they reverse polarity). This might be compared to a room full of people trying to run back and forth, from one side to the other. Obviously, there would be a lot of bumping, rubbing, agitation, and friction.

Now, consider that the actual frequency of the

RF energy used in microwave ovens is 2450 million cycles per second! Moreover, consider that within the course of one of those cycles, the

molecules would actually change their direction (polarity) twice - once for the positive half-cycle and once for the negative half-cycle. This red-hot rate of vibration causes tremendous friction within the food, and - just as rubbing your hands together makes them warm - this friction produces heat.

So the heat is produced directly in the food, but the food is not cooked, as is commonly believed, from the inside out. Actually, the cooking begins just beneath the outer surface and from there inward and outward, with the majority of the energy being expended in the outer layers. The rate and degree of heating depend on the depth and density of the food, as well as its ability to conduct heat. Because the microwave energy is changed to heat as soon as it is absorbed by the food, it cannot make the food radioactive or contaminated. When the

microwave energy is turned off and the food is removed from the oven, there is no residual radiation remaining in the food. In this regard, a microwave oven is much like and electric light that stops glowing when it is turned off.

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How Do Microwave Ovens Cook Food?

Learn more about the myths & mysteries of microwave ovens

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Copyright Information

Unless otherwise noted, all materials at this cite (including without limitation all text, html markup, graphics, and graphic elements) are copyrighted ©, 1989-99 by

J. Carlton Gallawa.

The material available through this site may be freely used for attributed noncommercial educational purposes only. We ask that due credit and notification be given the author.

All materials appearing on this website may not be reproduced, stored in any retrieval system, or used in any way for commercial purposes without the express prior written permission of the copyright holder.

Send correspondence to: Microtech, P.O. Box 940, Gonzalez, Florida 32560

Disclaimer: The author assumes no liability for any incidental, consequential or other liability from the use of this information. All risks and damages, incidental or otherwise, arising from the use or misuse of the information contained herein are entirely the responsibility of the user. Although careful precaution has been taken in the preparation of this material, we assume no responsibility for omissions or errors.

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Microwave Energy--What it is and what it is not

What Are Microwaves?

Excerpts from the book The Complete Microwave Oven Service Handbook

--NOW available on CD-ROM (

CLICK HERE)

and from the video You Can Fix Your Microwave Oven, Plus VCR Know-How

Debussy's L'Îsle Joyeuse for your listening enjoyment.

(Use right mouse button on note icon to start and control music)

Microwaves are very short waves of electromagnetic energy that travel at the speed of light (186,282 miles

per second). Microwaves used in microwave ovens are in the same family of frequencies as the signals used in radio and television broadcasting.

The theory of electromagnetic energy can be illustrated by what happens when a pebble is tossed into a quiet pond. The pebble striking the still surface causes the water to move up and down in the form of ripples, or waves, that radiate in ever-widening circles over the surface of the pond. These waves, which move up and down at right angles to the direction they are traveling, are called transverse waves. Microwaves are examples of transverse waves.

The disturbance resulting from the pebble landing in the water is transmitted through the water in the form of ripples or waves. The water serves merely as a medium through which the disturbance travels. In this sense, these ripples are more like sound waves, which also need a medium to travel through, normally using the molecules that exist in the air or water. That is why, for example, thundering rocket engines that would deafen the ears under normal circumstances, would be inaudible in the quiet vacuum of space.

On the other hand, electromagnetic forms of energy, such as microwaves, radar waves, radio and TV waves, travel millions of miles through the emptiness of space without the need of any material medium through which to travel. This is because, simply put, electromagnetic waves are, in themselves, stored energy in motion.

A Phenomenal Force

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Microwave Energy--What it is and what it is not

Electromagnetic radiation begins with a phenomenon that occurs when electric current flows through a conductor, such as a copper wire. The motion of the electrons through the wire produces a field of energy that surrounds the wire and floats just off its surface. This floating zone or cloud of energy is actually made up of two different fields of energy, one electric and one magnetic. The electric and magnetic waves that combine to form an electromagnetic wave travel at right angles to each other and to the direction of motion. If the current flowing through the wire is made to oscillate at a very rapid rate, the floating electromagnetic field will break free and be launched into space. Then, at the speed of light, the energy will radiate outward in a pulsating pattern, much like the waves in the pond. It is theorized that these waves are made up of tiny packets of radiant energy called photons. Streams of photons, each carrying energy and momentum, travel in waves like an undulating string of cars on a speeding roller coaster.

Is Microwave Radiation the Same as

Radioactive Radiation?

No. There is a very important difference. As illustrated by the frequency spectrum on the right, microwaves used in microwave ovens, similar to microwaves used in radar equipment, and telephone, television and radio communication, are in the non-ionizing range of electromagnetic radiation. Non-ionizing radiation is very different from Ionizing radiation . Ionizing radiation is extraordinarily high in frequency (millions of trillions of cycles per second). It is, therefore, extremely powerful and penetrating. Even at low levels, ionizing radiation can damage the cells of living tissue. In fact, these dangerous rays, have enough energy and intensity to actually change (ionize) the molecular structure of matter. In sufficient doses, ionizing radiation can even cause genetic mutations. As shown on the frequency spectrum, the ionizing range of frequencies includes X-rays, gamma rays, and cosmic rays. Ionizing radiation is the sort of radiation we associate with radioactive substances like uranium, radium, and the fall-out from atomic and thermonuclear explosions.

Non-ionizing radiation is very different. Because of the lower frequencies and reduced energy, it does not have the same damaging and cumulative properties as ionizing radiation. Microwave radiation (at 2450 MHz) is non-

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Microwave Energy--What it is and what it is not

ionizing, and in sufficient intensity will simply cause the molecules in matter to vibrate, thereby causing friction, which produces the heat that cooks the food.

Copyright Information

Unless otherwise noted, all materials at this cite (including without limitation all text, html markup, graphics, and graphic elements) are copyrighted ©, 1989-2000 by J. Carlton Gallawa. The material available through this site

may be freely used for attributed noncommercial educational purposes only. We ask that due credit and notification be given the author.

All materials appearing on this website may not be reproduced, stored in any retrieval system, or used in any way for commercial purposes without the express prior written permission of the copyright holder.

Send correspondence to: Microtech, P.O. Box 940, Gonzalez, Florida 32560

Disclaimer: The author assumes no liability for any incidental, consequential or other liability from the use of this information. All risks and damages, incidental or otherwise, arising from the use or misuse of the information contained herein are entirely the responsibility of the user. Although careful precaution has been taken in the preparation of this material, we assume no responsibility for omissions or errors.

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The Interlock Switches Used In Microwave Ovens

The Purpose and Function of Interlock Switches

Used In Microwave Ovens

To ensure safe operation, all microwave ovens are equipped with safety interlock switches . The door-interlock system is one of the most significant safeguards in a microwave oven. While switch configuration, size, shape, and mounting arrangement may vary from one model to the next, the purpose and basic operation remain the same in all models.

The purpose of the interlock system is to interrupt the production of microwave energy when the oven door is opened, and similarly, to prevent any microwave output until the door is firmly and safely closed.

A principal component of the interlock system is the interlock monitor switch . In August of 1974, a Federal safety standard went into effect that required that all microwave ovens be equipped with an interlock monitoring

system. The monitor switch, which is also called the safety switch, short switch, sensing switch, and failure detector switch , functions as a fail-safe type of device, in that it disables the microwave oven if an interlock failure should occur. The actual method differs from model to model, but the results are the same:a blown (or opened) fuse.

Generally speaking, the normal sequence of switch operation when the door is opened is as follows. First the primary switch opens its contacts. Second, (yes) the secondary switch opens. Finally, the interlock monitor switch closes its contacts. The fail-safe system works like this:If any of the switches and/or relays included in the monitor loop (or circuit) fail to open their contacts properly when the door is opened, a short circuit is created when the monitor switch closes its contacts. The closed contacts of the monitor switch and the faultily-closed contacts of the defective switch combine to cause an immediate short circuit, which, in one way or another (depending on the model), blows the line fuse, or otherwise disables the oven. All this happens before the door can be opened far enough to allow any dangerous levels of microwave radiation to escape.

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The Interlock Switches Used In Microwave Ovens

All switches have a voltage and current rating. A typical door-interlock switch is rated at 15 amps with 125 or 250 VAC applied. When replacing an interlock switch, these ratings must be met or exceeded, otherwise premature switch failure will result. Another replacement consideration is the operating and release force. This is the relative amount of pressure needed to actuate the switch. Various applications call for differing amounts of operating pressure. Like the voltage and current ratings, this is an important factor when obtaining a replacement switch. For example, if the normal pressure of a latch mechanism is insufficient to depress the actuator button, the switch's operating force is probably too high for that application.

The interlock switch to be tested in this example has a COM. (common) terminal, a N.O. (normally open) terminal, and a N.C. (normally closed) terminal. Interlock monitor switches are usually constructed with only the COM and N.C. terminals. Other switches are made with just the COM and N.O. terminals. The following tests will cover most circumstances. Simply disregard the terminal-to-terminal tests that do not apply.

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 Interlock Switches

1. Unplug the oven and remove the outer cover.

2. DISCHARGE ALL HIGH VOLTAGE CAPACITORS.

(Procedure)

3. Visually examine the switch terminals and connectors for signs of overheating, such as discoloration, or brittleness.

❍ Many times the problem is merely a burned slip-on connector due to a poor crimp

joint, or weakened connection. If the switch is in good working order, the repair can be made by cleaning the terminals, and replacing the burned connector. Or, simply cut off the burned connectors and solder the wires directly to the switch terminals.

4. Carefully remove the harness leads from the switch terminals. Pry them loose if necessary, but do not exert too much force, or the terminal may break right out of the switch.

❍ Many models use a connector with a locking clip in the center of the receptacle

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The Interlock Switches Used In Microwave Ovens

terminal. This extruding lever must be pressed down while gently pulling the connector off the terminal.

5. Set the ohmmeter to read ohms at a scale of R X 1

6. As shown in the picture, place one meter probe

on the COM terminal and the other probe on the N.O. terminal. With the actuator (or lever, or button) not depressed, the meter should read infinity (an open circuit).

7. Without moving the meter probes, press down on the switch actuator until a "click" is heard. At the point of the click, the meter should swing to a reading of zero ohms (or continuity).

❍ Rule of thumb:A healthy "click" usually

means the switch is working normally. However, switches with just two terminals, both of which come out the back, do not click when actuated, and neither do switch modules.

8. Move the probe from the N.O. terminal to the N.C. terminal. The other probe remains on the COM terminal.

9. Press the actuator and the meter should read infinity.

10. Release the actuator and the meter should read zero ohms.

11. Set the meter to the highest resistance (ohms) scale and measure from the N.C. terminal

to the N.O. terminals for a normal reading of infinity.

12. Measure from each terminal to any metal mounting hardware that is part of the switch assembly for a normal reading of infinity.

Any abnormal readings would indicate that the switch is defective and should be replaced.

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The Interlock Switches Used In Microwave Ovens

A Complete Course In Commercial &

Residential Microwave Oven Repair

Step up to a new level of

instruction!

J. Carlton Gallawa. The

material available through this site may be freely used for attributed noncommercial educational purposes only. We ask that due credit and notification be given the author.

All materials appearing on this website may not be reproduced, stored in any retrieval system, or used in any way for commercial purposes without the express prior written permission of the copyright holder.

Send correspondence to: Microtech, P.O. Box 940, Gonzalez, Florida 32560

Disclaimer: The author assumes no liability for any incidental, consequential or other

liability from the use of this information. All risks and damages, incidental or otherwise, arising from the use or misuse of the information contained herein are entirely the responsibility of the user. Although careful precaution has been taken in the preparation of this material, we assume no responsibility for omissions or errors.

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How To Test the Triac

The Purpose of the Triac

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.

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How To Test the Triac

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:

a. From T1 to T2. b. From T2 to the gate c. 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.

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.

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How To Test the Triac

● Send us E-mail: microtech@gallawa.com

CLICK HERE

● If you would like to learn more about repairing

commercial, industrial and residential microwave ovens and become part of a multimillion dollar service industry, we invite you to take a look at Microtech's highly acclaimed CD-ROM.

● Take the finest repair video ever produced,

combine it with the ultimate textbook on microwave oven repair, meticulously update every part and you have "THE COMPLETE

MICROWAVE OVEN SERVICE HANDBOOK v. 2002 on CD-ROM." This is THE definitive step-

by-step instructional CD to making safe, successful and profitable repairs on all types of microwave ovens.

● From the common tools you'll need and

clearly outlined safety procedures, right down to the final profit-producing repair. This stateof-the-art courseware gives you the training you need in a progressive, straight-foward and easy-to-use format that's guaranteed to take you to the skill--and income--level you want.

● It takes every page of this 400-page, fact-

crammed course to teach you all you need to know about this profitable field of repair.

Disclaimer: The author assumes no liability for any incidental, consequential or other liability from the use of this information. All risks and damages, incidental or otherwise, arising from the use or misuse of the information contained herein are entirely the responsibility of the user. Although careful precaution has been taken in the preparation of this material, we assume no responsibility for omissions or errors.

Unless otherwise noted, all materials at this cite (including without limitation all text, html markup, graphics, and graphic elements) are copyrighted © 1989-2002 by J. Carlton Gallawa. The material available through this site

may be freely used for attributed noncommercial educational purposes only. We ask that due credit and notification be given the author.

All materials appearing on this website may not be reproduced, stored in any retrieval system, or used in any way for commercial purposes without the express prior written permission of the copyright holder.

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How To Test the Triac

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High Voltage Transformer Test

The Purpose of the High Voltage Transformer in

Microwave Ovens

Excerpts from the book The Complete Microwave Oven Service Handbook --NOW available on CD-ROM (CLICK HERE)

and from the video

Gallawa

It might be said that the HIGH-VOLTAGE TRANSFORMER is the "muscle" of the microwave oven. With an input of 120 VAC (or 240 VAC in many commercial models) applied to the primary winding, the high-voltage transformer (also referred to as power or plate transformer) steps up that primary voltage to a very high voltage. This high voltage is then boosted even higher by the voltage-doubling action of the capacitor and diode. The resulting voltage, about 3000 - 5000 volts DC (depending on the model), is available at the high voltage (output) tap (see illustration below).

In most newer models, the transformer also incorporates a second output winding, as shown in the illustration below. This is a stepped-down output that provides the filament voltage (typically 3 to 4 VAC) to the magnetron tube.

Some models use a separate filament transformer . If that is the case, skip steps 3 through 5.

Important Safety Information

Microwave ovens are one of the most dangerous appliances to work on. Before attempting any troubleshooting, testing or repairs, for your personal safety we strongly urge you to carefully read, fully understand and be prepared to follow carefully the very important

safety precautions found by clicking here .

Please read our

disclaimer at the bottom of this page.

WARNING! THESE VOLTAGES CAN BE LETHAL! NEVER, EVER ATTEMPT TO MEASURE THE OUTPUT VOLTAGES OF THE HIGH-VOLTAGE TRANSFORMER.!

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High Voltage Transformer Test

How To Test The HighVoltage Transformer

1. UNPLUG the oven.

2. DISCHARGE ALL HIGH-

VOLTAGE CAPACITORS. (Procedure)

3. Disconnect the high-voltage lead from the transformer's high-voltage terminal (or tap).

4. With an ohmmeter set to the lowest resistance scale, R X 1, measure the resistance from the high-voltage terminal to the transformer chassis (ground).

a. Some early models are equipped

with two or three high-voltage taps. If this is the case, perform the measurement from each tap to chassis ground.

5. The meter should read about 55 to 70 ohms, depending on which high-voltage tap is being measured and the model being tested.

a. A substantially higher or lower reading would indicate that the transformer is defective.

6. Carefully disconnect the leads from the primary (input) terminals

7. Measure from one transformer primary terminal to the other for a normal reading of less than 1 ohm (i.e., .22 ohms)

a. A substantially higher reading would indicate that the primary winding is open.

8. Set the meter to its highest resistance scale and check from each transformer primary terminal to the transformer chassis (ground) for a normal reading of infinity (open circuit).

a. Any measurement of resistance would indicate some degree of a short to ground

9. Set the meter back to its lowest resistance scale. Then carefully disconnect the filament leads and measure from one filament terminal to the other for a normal resistance of less than 1 ohm.

10. Set the meter to its highest scale and measure from each filament terminal to chassis ground. a. Any measurement that is substantially different from the normal readings would indicate a

defective transformer.

Send us E-mail if you have difficulty in locating this or any other replacement part. We will be happy to help you:

microtech@gallawa.com

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High Voltage Transformer Test

Illustration from THE COMPLETE

MICROWAVE OVEN SERVICE HANDOOK

If you would like to learn more about repairing commercial and residential microwave ovens and become part of a multi-million dollar service industry, we invite you to take a look at Microtech's powerful new computerized training course.

● Virtually 1000s of microwave

oven repair case histories with

factory approved troubleshooting and repair procedures

● Factory troubleshooting charts,

error codes, schematics and diagnostic procedures

● Touch panel (key pad) matrix

diagrams for 100s of brands

● Hundreds of vividly colored

schematics, diagrams, illustrations and photos ease the toughest repair chores.

● Fully searchable text for quick

information gathering

● All chapters, sections and

references are hyperlinked for fast and smooth navigating

● FREE technical support

● Full 30-day, no-questions- asked,

money-back guarantee

THE COMPLETE MICROWAVE OVEN SERVICE HANDBOOK 2002

Operation, Maintenance, Troubleshooting and Repair

J. Carlton Gallawa

Unless otherwise noted, all materials at this cite (including without limitation all text, html markup, graphics, and graphic elements) are copyrighted © 1989-2002 by J. Carlton Gallawa. The material available through this site

may be freely used for attributed noncommercial educational purposes only. We ask that due credit and notification be given the author.

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High Voltage Transformer Test

All materials appearing on this website may not be reproduced, stored in any retrieval system, or used in any way for commercial purposes without the express prior written permission of the copyright holder.

Send correspondence to: Microtech, P.O. Box 940 Gonzalez, FL 32560

Disclaimer: The author assumes no liability for any incidental, consequential or other liability from the use of this information. All risks and damages, incidental or otherwise, arising from the use or misuse of the information contained herein are entirely the responsibility of the user. Although careful precaution has been taken in the preparation of this material, we assume no responsibility for omissions or errors.

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Microwave Oven Repair Safety Precautions

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Microwave Oven Safety Precautions

Microwave Ovens Are Dangerous Appliances to Work On

The microwave oven is one of the most dangerous appliances to repair. The

high-voltage potential combined with the high-current capability of an operating

microwave oven pose a deadly threat to the reckless worker. In addition, microwave ovens are radiation-emitting devices. Normally this does not present a problem. However, improper replacement methods or tampering with safety systems could expose the unwary troubleshooter to dangerous levels of microwave leakage. Therefore, extreme caution and proper procedures MUST be used at all times.

When diagnosing a microwave oven, many problems can be detected merely with careful observation, that is, with the power disconnected. Most tests can be accomplished with the power off and the oven unplugged. We suggest that you leave the "live" testing to the professionals. Nonetheless, certain safety habits must be developed and maintained. If you have experience in troubleshooting electrical equipment, some of these safety precautions may be familiar. However, the high-current potential of the high-voltage circuits in a microwave oven make them possibly life-saving steps.

If you are uneasy or unsure about any of these safety procedures or warnings; or if you feel uncertain as to their importance or your ability to manage them, it would be in your best interest to leave the repair to a qualified professional.

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Microwave Oven Repair Safety Precautions

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Information

● FIRST and ALWAYS, before attempting any repairs, make certain that the unit is not

plugged in.

● Before touching any components or wiring, ALWAYS

DISCHARGE THE HIGH VOLTAGE CAPACITOR! The

high voltage capacitor will quite normally maintain a painfully high-voltagecharge even after the oven is unplugged. Some capacitors employ a bleeder resistor (either externally or internally) that allows the charge to slowly bleed (or drain) off after the oven is unplugged. Do not trust a bleeder resistor--it may be open.

If you forget to discharge the capacitor, your fingers may ultimately provide the discharge path. You only make this mistake a few times, because, while the electric shock is painful, the real punishment comes when you reflexively yank your hand out leaving behind layers of skin on razor-like edges that are there as a reminder to never again forget to discharge the high voltage capacitor.

How To Discharge The High Voltage Capacitor:The capacitor is

discharged by creating a short circuit (direct connection) between the two capacitor

terminals and from each terminal to chassis ground (bare metal surface).

1. Do this by touching the blade of an insulated-handled screw driver to one terminal,

then slide it toward the other terminal until it makes contact and hold it there for a few seconds. (This can result in a rather startling "pop!")

2. Repeat the procedure to create a short between each capacitor terminal and chassis

ground.

3. If the capacitor has three terminals, use the same procedure to create a short circuit

between each terminal and then from each terminal to ground.

4. Older Amana-made models (generally those manufactured before 1977) have red,

round filter capacitors mounted in the base of the magnetron tube which can also hold a charge. Ground each magnetron terminal by creating a short circuit to chassis ground using the blade of a screwdriver as explained above.

NEVER, under any circumstances, touch any oven components or

wiring with your hand or even with an insulated tool during a cook operation.

The high-voltage circuits in a microwave oven generate from 3000

to 5000 volts DC and higher!

This combined with the potential for high current makes the

HIGH VOLTAGE CIRCUITS OF A MICROWAVE OVEN EXTREMELY DANGEROUS TO WORK ON OR AROUND WHEN THE OVEN IS ENERGIZED.

For this reason, most manufacturers pointedly warn that

MEASURING THE HIGH VOLTAGE IS NEITHER NECESSARY NOR ADVISABLE.

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Microwave Oven Repair Safety Precautions

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● It's better (and safer) to avoid making live tests if possible. If such tests become

necessary, and if you are adequately qualified to do so, the test can be performed safely as follows:

1. Make sure the oven is unplugged and that the capacitor discharged.

2. Attach the meter leads to the prospective test points with insulated clip leads.

3. Then step back, plug in the oven, perform the test and observe the meter.

4. In addition, use only one hand whenever possible. Try putting the other hand behind

your back or in your pocket. (Two hands could complete a circuit through your body. Use only one hand and/or an insulated tool, even when the unit is unplugged).

● After any adjustment or repair on a microwave oven, manufacturers require that a final

microwave leakage check be performed to ensure that the unit does not emit excessive radiation.

RF leakage test procedure:

1. If there is evidence that the unit has been previously tampered with by someone of

questionable competence, be prepared to check the leakage before energizing the oven.

2. Most amateurs, and many professionals, do not own an RF leakage meter. These

meters are available at appliance and electronic parts suppliers, ranging in price from about $10.00 up to many hundreds of dollars for the certified models used at the professional level.

3. Inexpensive RF detection devices are also available at many retail stores such as

Walmart and Radio Shack.

4. In lieu of purchasing a certified RF leak checker, you can take your repaired unit to a

factory authorized servicer who will very likely check the leakage for free.

● Do not work alone. Make sure another person is nearby in case of an emergency.

● Use care when lifting and carrying a microwave oven. Remember, most of the weight is

usually on one side (the control panel side).

1. When lifting a microwave oven, keep your back straight and use your legs, not your

back, to do the lifting.

2. Before carrying, secure the power cord and insure a clear path to your destination.

3. When lifting an oven in or out of an automobile, lift it on to the seat--do not slide it.

Better yet, place a piece of cardboard on the seat first.

● Remove your watch and other jewelry.

1. Watches that are susceptible to magnetism will be damaged by the intense

magnetic field surrounding the magnetron tube.

2. Jewelry is electrically conductive and serious injury could result.

● Wear rubber soled shoes.

● Never defeat or tamper with the safety interlock switches or the fuse.

● Do not touch printed circuit board components or circuitry unless you are properly

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Microwave Oven Repair Safety Precautions

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grounded.

1. Static electricity can damage certain static sensitive components.

● Be aware that opening or otherwise modifying your equipment may void any

manufacturer's warranty.

If you fully understand these precautions and are prepared to carefully observe them, proceed to the

Master Repair Database.

If you are uneasy or unsure about any of these safety procedures, or feel uncertain as to their importance, it would probably be in your best

interest to leave the repair to a qualified professional.

Disclaimer: The author assumes no liability for any incidental, consequential or other liability from the use of this information. All risks and damages, incidental or otherwise, arising from the use or misuse of the information contained herein are entirely the responsibility of the user. Although careful precaution has been taken in the preparation of this material, we assume no responsibility for omissions or errors.

You are visitor number 90695

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Output Power Test

How To Check the Output Power (wattage) of a Microwave Oven

Over the years manufacturers have used several different methods to rate the output wattage of microwave ovens. First, there was the traditional method. Then, in 1989-90 came the JIS (Japanese Industrial Standard). Using the JIS method, ovens rated at 700 watts using the traditional method became 750-watt ovens. In 1990-91 the industry changed to the international IEC-705 standard. This pushed the wattage ratings even higher. For example, models rated at 700 traditional watts were instantly turned into 800-watt ovens using the IEC-705 formula.

The following test will provide a suitably accurate measurement of the output power of any microwave oven. Variations or errors in performing this test will produce uncertain results. If the line voltage (from the electrical outlet) is low, the magnetron output will be correspondingly low.

Equipment needed:

● Microwave safe container with 1000 mL (1 Liter) gradation.

● Fahrenheit thermometer (Amana part # R0157397), or centigrade thermometer (Amana part #

M95D5)

Procedure:

1. Pour exactly 1000 mL (1 Liter) of cool tap water into he container. Using the thermometer, stir the water, then measure and record the temperature. For accurate results the water should be about 60 degrees F (20 degrees C).

2. Place the container on the center of the oven cooking shelf (do not leave the thermometer in the container and remove any metal racks), and heat the water (at full power) for 63 seconds. Use the second hand of a watch, not the oven timer.

3. After the heating time is completed, immediately remove the container, stir the water, re-measure and record the temperature of the heated water.

4. Subtract the starting water temperature (step 2) from the ending water temperature (step 3) to obtain the temperature rise.

5. To determine the output power in watts, multiply the total temperature rise by a factor of:

38.75 , if you're using a Fahrenheit thermometer; 70 , if you're using a centigrade thermometer.

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SHARP Microwave Diagram

Specifications and Main Features

Frequently Asked Questions

User Manual