Hi How long does microwave energy hang around? You have probably been warned by your mother: "Wait a few seconds (or minutes) after the beep for all the microwaves to disappear". There is no scientific basis for such a recommendation. Once the beep has sounded (or the door has opened), it is safe. This is because:
1. There is no such thing as residual microwave radiation from a microwave oven - it is either being produced or is non-existent. 2. There is little energy storage in the microwave generator compared to the amount being used. The typical high voltage capacitor - the only component that can store energy - has a capacity of less than 15 W-s (Watt-seconds) even for the largest ovens. Power consumption is typically 800 to 1,500 W depending on oven size. Therefore, the capacitor will be fully drained in much less than .1 second - long before the beep has ended or the door has cleared the front panel. (Based on the numbers, above, for a 1,500 W oven with a capacitor storing 15 W-s, it is more like .01 seconds!) WARNING: This only applies to a *working* microwave oven! If there is no heat, the magnetron may not be drawing any current from the HV power supply and the HV capacitor can remain charged for a long time. In this case, there is a very real risk of potentially lethal electrical shock even after several minutes or more of being unplugged There *is* a wire mesh embedded in the glass panel. Since the holes in the mesh are much much smaller than the wavelength of the 2.45 GHz microwaves (about 5 inches or 12.5 cm), it is essentially opaque to microwaves and essentially all the energy is reflected back into the oven cavity. (From: Filip (I'll buy a vowel) Gieszczykiewicz ([EMAIL PROTECTED]).) Greetings. Did you ever see a "mesh" satellite disk up close? You will note that it looks much like it's made out of simple wire mesh that you can get in a hardware store (in the USA, it's called "chicken fence" :-). The reason this works is that the wave that the dish picks up is longer than the hole in the mesh. Consider bouncing a tennis ball on the "wire mesh" in the microwave - it WOULD work because the ball is bigger than the holes. The wave in the microwave is about 2.5cm "long" ... as long as the holes are smaller than that (actually, you want them as small as possible - without affecting the "watching the food" - to minimize any stray and harmonic waves from escaping... like bouncing tennis and golf and ping-pong balls and marbles off the mesh - you want to catch all the possible sizes - yet still be able to see through it) they will not let anything out of the oven. BTW, it's not really "glass" but rather a 'sandwich' of glass, from the outside, wire mesh (usually a sheet of metal which is either stamped or drilled with a hole pattern - like a color TV CRT mask!), and followed by a sheet of glass or plastic to make sure that food splatters and vapor condensation are easy to clean - imagine scraping the mesh! How a microwave oven works The operation of a microwave oven is really very simple. It consists of two parts: the controller and the microwave generator. A schematic diagram of the microwave generating circuitry and portions of the controller is usually glued to the inside of the cover. The controller is what times the cooking by turning the microwave energy on and off. Power level is determined by the ratio of on time to off time in a 10-30 second cycle. The microwave generator takes AC line power. steps it up to a high voltage, and applies this to a special type of vacuum tube called a magnetron - little changed from its invention during World War II (for Radar). Controller The controller usually includes a microcomputer, though very inexpensive units may simply have a mechanical timer (which ironically, is probably more expensive to manufacture!). The controller runs the digital clock and cook timer; sets microwave power levels; runs the display; and in high performance ovens, monitors the moisture or temperature sensors. Power level in most microwave ovens is set by pulse width control of the microwave generator usually with a cycle that lasts 10-30 seconds. For example, HIGH will be continuous on, MEDIUM may be 10 seconds on, 10 seconds off, and LOW may be 5 seconds on, 15 seconds off. The power ratios are not quite linear as there is a 1 to 3 second warmup period after microwave power is switched on. However, some models use finer control, even to the point of a continuous range of power. These are typically "inverter" models which use a more sophisticated type of power supply than the simple high voltage transformer, capacitor, rectifier, system described below. However, there have been some back in the 1970s that did this with a 1 second or so pulse width modulated cycle, fast enough to have the same effect as continuous control for all practical purposes. The operating voltages for the controller usually are derived from a stepdown transformer. The controller activates the microwave generating circuitry using either a relay or triac. Sensors More sophisticated ovens may include various sensors. Most common are probes for temperature and moisture. A convection oven will include a temperature sensor above the oven chamber. Since these sensors are exposed to the food or its vapors, failures of the sensor probes themselves are common. Cooling fans Since 30 to 50 percent of the power into a microwave oven is dissipated as heat in the Magnetron, cooling is extremely important. Always inspect the cooling fan/motor for dust and dirt and lubricate if necessary. A couple of drops of electric motor oil or 3-in-One will go a long way. If there are any belts, inspect for deterioration and replace if necessary. An oven that shuts off after a few minutes of operation could have a cooling problem, a defective overtemperature thermostat, a bad magnetron, or is being operated from very high AC line voltage increasing power to the oven. One interesting note: Since 30 to 50 percent of the power goes out the vents in the back as heat, a microwave oven is really only more efficient than conventional means such as a stovetop or gas or electric oven for heating small quantities of anything. With a normal oven or stovetop, wasted energy goes into heating the pot or oven, the air, and so on. However, this is relatively independent of the quantity of food and may be considered to be a fixed overhead. Therefore, there is a crossover point beyond which it is more efficient to use conventional heat than high tech microwaves. Microwave generator This is the subsystem that converts AC line power into microwave energy. The majority of microwave ovens use a brute force approach which consists of 5 parts: high voltage (HV) transformer running off the AC line, HV rectifier diode, HV capacitor, magnetron, waveguide to oven chamber. (A few employ solid state inverter in place of the simple HV transformer. These will be discussed later.) The most common microwave generator consists of the following: * High Voltage Transformer. Typically has a secondary of around 2,000 VRMS at 0.5 to 1 amp - more or less depending on the power rating of the oven. There will also be a low voltage winding for the Magnetron filament (3.3 V at 10 A is typical). You cannot miss this as it is the largest and heaviest component visible once the cover is removed. There will be a pair of quick-connect terminals for the AC input, a pair of leads for the Magnetron filament. and a single connection for the HV output. The HV return will be fastened directly to the transformer frame and thus the chassis. These transformers are designed with as little copper as possible. The primary for 115 VAC is typically only 120 turns of thick wire - thus about 1 turn per volt input and output (this is about 1/4th as many turns as in a "normal" power transformer. (It's usually possible to count the primary turns by examining how it is wound - no disassembly required!) So there would be about 3 turns for the magnetron filament and 2080 turns for the high voltage winding for the transformer mentioned above. The reason they can get away with so few turns is that it operates fully loaded about 90 percent of the time but is still on the hairy edge of core saturation. The HV components are actually matched to the HV transformer characteristics. Performance will suffer if the uF value of a replacement HV capacitor is not close to that of the original. There is also generally a "magnetic shunt" in the core of the transformer. This provides some current limiting, possibly to compensate for various magnetron load conditions. However, it's not enough to provide any reduction in the likelihood of electrocution should you come in contact with the HV winding! * Rectifier - usually rated 12,000 to 15,000 PRV at around 0.5 amp. Most commonly, this will be rectangular or cylindrical, about 0.5 inch long with wire leads. Sometimes, it is a box bolted to the chassis. One end will be electrically connected to the chassis. * Capacitor - 0.65 to 1.2 uF at a working voltage of around 2,000 VAC. Note that this use of 'working voltage' may be deceiving as the actual voltage on the capacitor may exceed this value during operation. The capacitor is metal cased with quick-connect terminals on top (one end). Always discharge the capacitor as described below before touching anything inside once the cover is removed. * Magnetron - the microwave producing tube includes a heated filament cathode, multiple resonant cavities with a pair of permanent ceramic ring magnets to force the electron beams into helical orbits, and output antenna. The magnetron is most often box shaped with cooling fins in its midsection, the filament/HV connections on the bottom section, and the antenna (hidden by the waveguide) on top. Sometimes, it is cylindrical in shape but this is less common. The frequency of the microwaves is usually 2.45 GHz. When salvaging parts from dead microwave ovens, save the HV components (transformer, capacitor, and diode) as a group (assuming all are still good). Then, if a repair is needed to another oven it may be better to replace all 3 both because this eliminates uncertainty if more than 1 part failed or is marginal, and they will have been designed to have the best compatibility. Repair or replace? 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. However, if you can do the repair yourself, the equation changes dramatically as your parts costs will be 1/2 to 1/4 of what a professional will charge and of course your time is free. The educational aspects may also be appealing. You will learn a lot in the process. Many problems can be solved quickly and inexpensively. Fixing an old microwave for the dorm room may just make sense after all. Installation and Preventive Maintenance Microwave oven installation and use To assure safety and convenient, follow these recommendations: * Read your users manual from cover to cover especially if this is your first microwave. What a concept! If nothing else, you may discover that your oven has features you were not aware were even possible. In any case, there may be requirements or suggestions that are specific to your model and will enable you to get the most performance from your new microwave. * Select a stand-alone unit rather than a built-in if possible. It will be cheaper to buy, cheaper and easier to service, and possibly more reliable since ventilation and adjacent heat producing appliances will not be as much of a factor. * Select a convenient location - easy access and not too high or too low. This is particularly important if the door of the oven opens down instead of to the left side (only a few models are built this way, however). * Put the microwave oven on its own dedicated 3 wire grounded circuit. Temporary use of a 3 to 2 prong adapter is acceptable only if the outlet box is properly grounded to begin with (BX, Romex, or conduit with ground) AND the adapter's ground wire or terminal is securely attached to the outlet box ground screw. Make sure the outlet is in good condition in either case. Check that the plug (or adapter) fits tightly and that there is no appreciable heating of the outlet during use of the microwave oven. If there is, spread the metal strips of each of the prongs apart if possible and/or replace the outlet. A grounded outlet is essential for safety. Microwave ovens are high power devices and a separate circuit will eliminate nuisance fuse blowing or circuit breaker tripping when multiple appliances are being used at the same time. It will also minimize the possibility of Radio Frequency Interference (RFI) between it and any electronic equipment which might be on the same circuit. A GFCI is not needed as long as the outlet is properly grounded and may result in nuisance tripping with some microwave ovens. Inexpensice outlet testers are available at hardware stores, home centers, and electrical parts distributors, to confirm that the outlet is properly wired and grounded. * Allow adequate ventilation - do not push it up against the wall or wedge it under a tight fitting wall cabinet (or inside one for that matter!). Leave at least 2 inches on all sides and top if possible. * Do not let children use the microwave oven unless properly supervised. It is very easy to cause a fire through the use of excessive times or power settings. Even something as simple as microwave popcorn can explode and/or catch fire if heated for too long - e.g., 5 minutes instead of my precisely determined 3:41 on high :-). Microwave oven maintenance 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: * Clean the interior of the oven chamber after use with a damp cloth and some 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. * Clean the exterior of the cabinet and touchpad in a similar manner. DO NOT use a spray where any can find its way inside through the door latch or ventilation holes, or a dripping wet cloth. Be especially careful around the area of the touchpad since liquid can seep underneath resulting in unresponsive or stuck buttons or erratic operation. Do not use strong solvents (though a bit of isopropyl alcohol is fine if needed to remove sticky residue from unwanted labels, for example). * 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. If there is evidence of overheating at the outlet itself, the outlet (and possibly the plug as well) should be replaced. * Periodically check for built up dust and dirt around the ventilation holes or grills. Clean them up and use a vacuum cleaner to suck up loose dust. Keeping the ventilation free will minimize the chance of overheating. * Listen for any unusual sounds coming from inside the oven. While these appliances are not exactly quiet, grinding, squealing, scraping, or other noises - especially if they were not there when the oven was new - may indicate the need for some more extensive maintenance like belt replacement or motor lubrication. Attending to these minor problems now may prevent major repairs in the future. * Keep your kitchen clean. Yes, I know, this isn't exactly microwave specific but cockroaches and other uninvited guests might just like to take up residence inside the electronics bay of the oven on the nice warm controller circuit board or its neighborhood and they aren't generally the tidiest folks in the world. If it is too late and you have a recurring problem of cockroaches getting inside the electronics bay, tell them to get lost and then put window screen over the vents (or wherever they are entering). Such an open mesh should not affect the cooling of the electronic components significantly. However, the mesh will likely clog up more quickly than the original louvers so make sure it is cleaned regularly. If possible, clean up whatever is attracting the unwanted tenants (and anything they may have left behind including their eggs!!). CAUTION: Do not spray anything into the holes where the door latch is inserted or anywhere around the touchpad as this can result in internal short circuits and costly damage - or anywhere else inside, for that matter. If you do this by accident, immediately unplug the oven and let it dry out for a day or two. ********************************************************************** This message and its attachments may contain legally privileged or confidential information. If you are not the intended recipient, you must not disclose or use the information contained in it. If you have received this e-mail in error, please notify the sender immediately by return e-mail and delete the e-mail. Any content of this message and its attachments which does not relate to the official business of Eraring Energy must be taken not to have been sent or endorsed by Eraring Energy. 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