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Ever been jolted with electric current? Like the time you’d just gotten out of the shower and went to plug in a lamp with damp hands? So what do you think the voltage was that caused that nasty biting feeling that resulted from your momentary lapse in good judgment? Once, while operating a subway car at a railroad museum at which I was a member, I was inadvertently “electrocuted.” I went to turn on the lights inside the car, and unbeknownst to me the light switch was faulty. When I touched it I instantly became connected to the car’s 600 volt lighting circuit. With just a split second of contact the current passed through the tip of my right index finger, along my right arm, down the right side of my body, and out the tip of my big toe, finally exiting into the metal railcar’s body. The current actually burned a hole where it had exited through my boot. The experience was both frightening and painful, but fortunately did not result in any real injury. I was lucky that the current had bypassed my heart, because if it hadn’t, I might not be alive today. That was 600 volts. Now imagine being jolted by the 4000 volts present in a microwave oven’s internal high voltage circuitry. Last week we discovered how the high voltage circuit in a microwave oven converts the ordinary, everyday 120 volts alternating current (AC) present in our homes into a much higher voltage approximating direct current (DC). This is done by an internal component known as the capacitor. The capacitor is capable of storing large amounts of electrical energy, and this can result in microwave ovens presenting a danger even when unplugged. A microwave oven capacitor is shown in Figure 1. If you happened to touch its wire terminals while it’s still charged, its power can rapidly discharge high voltage electrical current throughout your body. The electrical current from the capacitor can even stop your heart from beating, and this is exactly what caused the demise of a person featured on a soon to be released Discovery Channel program, Curious and Unusual Deaths. While being interviewed as an expert for the program, I was asked to explain this rather unique phenomenon of latent stored energy, and how it may present a threat.
Figure 1 – A Microwave Oven Capacitor Remember, a microwave oven capacitor can remain charged with dangerous electrical energy for hours, even days, after the microwave oven plug is pulled from the wall outlet. The bottom line here is that you should not attempt to fix your microwave oven, unless you are trained and certified to do so. Next week we’ll switch to a different topic, namely an electrical device known as a “wall wart.” That’s the black plastic adapter you plug into electrical outlets to power your cell phones, laptops, and other small electronics. ____________________________________________ |
Posts Tagged ‘curious and unusual deaths’
Electrocution by Microwave Oven
Sunday, August 21st, 2011
The Microwave Oven Becomes Reality
Sunday, July 31st, 2011| The evolution of cooking methods has been interesting indeed, from the open fires of primitive man, who must have decided at some point that cooked meat tasted better than raw, on to wood fired stoves, fossil fuel-based cooking, whether coal, propane, or gas, and let’s not forget electric range tops. Standing on its own in the modern kitchen is the microwave oven. What will be next? The space age food pill dispensing stations of the futuristic cartoon family, The Jetsons?
We’ve been talking about resonant cavity magnetrons and the purpose they serve within a microwave radar system. We also learned about Dr. Percy Spencer’s discovery and how microwave radar transmissions emanating from a magnetron can cook food, not to mention melt candy bars.
Figure 1- Microwaves Melt Candy Bars and Cook Food Although the technologies used in microwave radars and microwave ovens are similar, they do have important differences. It would be both unsafe and impractical to install microwave radar systems into kitchens. Radiation emitted from radar wave guide lacking proper containment would bounce aimlessly around the kitchen, posing a threat to human safety. You see, microwaves don’t know the difference between our bodies and the food we wish to cook. They’ll heat up human tissue just as readily as a bowl of chicken soup. Another issue is that runaway microwaves lose much of their effectiveness through their aimless bouncing about, and much of it would not be directed to the food itself. Dr. Spencer would learn how to corral that energy, making microwave cooking a commercial success. The biggest problem for Dr. Spencer to overcome was containment of the microwaves. They needed to be directed towards food in order to efficiently heat them. His first microwave oven was a metal box containing an opening at the top into which a magnetron wave guide could be inserted. This would then introduce microwaves into the box, and the metal construction of the box would safely contain them. The safety issue had now been resolved because the waves couldn’t escape, they would simply bounce around inside the box and most of their energy would be transferred into any food placed inside. Dr. Spencer’s employer, the Raytheon Corporation, produced the first commercial microwave oven in 1954, and it was appropriately named the “Radarange.” It was huge, almost six feet tall, and weighed in at about 750 pounds! Hardly something that could fit into a home kitchen. Despite its massive size, the Radarange wasn’t all that powerful and couldn’t compete against the compact countertop microwave ovens in use today. It wasn’t until 1967 that technology improved enough to give us the smaller, more efficient units affordable to consumers. This improvement involved using a newly developed semiconductor device called a “diode” within the high voltage electric circuitry that powers the magnetron. We’ll learn more about these technologies in our next post. Also in our next post, we’ll see how high voltage circuits can pose electrocution hazards in a way you‘d never expect. I discussed one of these instances in my recent appearance on The Discovery Channel program, Curious and Unusual Deaths, soon to be aired. _____________________________________________ |
Microwave Radar and Melted Candy
Sunday, July 24th, 2011| Ever discover a melted candy bar in your pocket? You immediately start to think about the sources for the heat that had caused the mess. Did you stand too close to the stove, were you outside in summer heat too long, or did you simply sit on it? Or was it perhaps caused by being in proximity to a whirring device, something which does not seem to generate any heat at all? If you’ve been reading along with us, you know what device I’m talking about.
Last time we talked about an effect known as cavity resonance and how a sound is created, much like a musical note, when we blow across the top of a glass pop bottle containing some air space. Our breath causes the air molecules to bounce in and out of the bottle’s cavity, producing the sound. Microwave technology works in much the same way, making use of an electronic device called a resonant-cavity magnetron. But instead of generating a musical tone, like our breath does over the bottle, the magnetron produces short wavelength radio waves, known as microwaves, and it was initially developed to generate these microwaves for radar systems. So, how does the magnetron work? The magnetron contains a series of cavities arranged in a circle, their openings pointed towards the center as shown in Figure 1. Engineers determined that when a high voltage, say 4000 volts, is applied to the magnetron, it begins to boil off electrons through a filament, called a cathode, located at its center. Once free of the cathode, the electrons want to flow to a part of the magnetron called the anode. This is because the cathode is positively charged and the anode is negatively charged, and as we know, electrons like to flow from positive to negative. The anode is also the part of the magnetron containing the cavities, and we’ll see the significance of this in a moment.
Figure 1 – Interior View of a Resonant-Cavity Magnetron Before the electrons can take their desired straight path to the anode, they are deflected by powerful magnets located on either end of the magnetron. These magnets force the electrons to move in a circular pattern over the openings in the cavities. Like the air molecules passing over the top of a pop bottle when you blow across it, the electrons move over each cavity opening in the magnetron, creating not musical tones, but microwaves. The microwaves are then collected from the magnetron using an antenna and directed along a tube called a wave guide. The microwaves leave the wave guide when they are transmitted by radar systems. The radar system then transmits the microwaves towards moving objects they wish to track. These tracked objects are as diverse as airplanes, ships, and weather patterns. See Figure 2.
Figure 2 – Microwave Radar Transmission So, how does the microwave oven fit into our discussion? In 1946 an engineer by the name of Percy Spencer was working on a radar magnetron for the Raytheon Corporation, a producer of electronic technology for industry and defense. During the course of his work he unexpectedly exposed himself to microwaves from a wave guide, and he couldn’t help but notice that the candy bar in his pocket had melted. Putting two plus two together, he realized the microwaves had caused the candy bar to heat up. Dr. Spencer further experimented and came to the conclusion that microwaves can cook foods far more quickly than conventional ovens, and the modern day kitchen appliance was soon born. Next time we’ll look at how Dr. Spencer’s microwave cooking discovery was developed into the microwave oven we find in most kitchens. We’ll also see how even an unplugged microwave oven can pose an electrocution hazard, as I explained in the Discovery Channel program I was recently featured on, Curious and Unusual Deaths. _____________________________________________ |
Green Screens and Makeup Chairs
Monday, June 20th, 2011| Imagine being a typical guy, the kind that hates to shave and would never dream of wearing anything that even remotely resembles makeup. That was me, before being contacted by The Discovery Channel to act as a subject matter expert on the topic of steam power plants and electrocutions, part of a series they’ll be airing in the near future to be called Curious and Unusual Deaths. Little did I know when agreeing to appear on the program that I was also agreeing to venture into an area of human experience most males would like left to the females, that of donning facial makeup.
Upon my arrival in Toronto to meet up with the producers I was treated like a celebrity. A chauffer had been sent to greet me at the airport, and I was driven to a fancy downtown hotel. The next morning I was driven to the video studio and spent some time in the makeup chair to get gussied up. My imperfections muffled, I was invited to take a seat in front of a green screen, and for those of you who don’t know what the function of this marvel of photography is, let me briefly explain. The green screen enables the editors to digitally impose any kind of background behind me at a later time of their choosing, anything from the Grand Canyon to footage of World War II. As of this time I have no idea what images will be shown in back of me as I speak. It’ll be interesting to see where I end up! Once the cameras and lights were adjusted in the studio, there was a clack from the director’s clapper board, accompanied by a shout of “roll ‘em!” And we were off shooting, just like in Hollywood. Two video cameras shot me simultaneously, one placed directly in front and one to my left. One of the show’s producers sat next to the front-positioned video camera during shooting. She needed to be within earshot, because she soon proceeded to ask me pointed questions about the science behind each incident being investigated. It was up to me to ad-lib the responses, bearing in mind that the subject matter was technical and had to be conveyed to an audience of average technical background in a manner in which they could understand it. Every now and then we’d take a break from filming so the crew could tend to the video equipment. It seems that adjustments had to be continually made to both cameras and lighting. Videotapes had to be changed, and the makeup artist would use these as opportunities to tweak my makeup. All in all it was an enjoyable experience. Everyone acted professionally and was easy to work with, and the producers expressed their satisfaction with the part I had played. I was told that I explained the technical subject matter in a way that anyone could understand, and I thanked them. But after all, as I shared with them, that’s my job! It was nice to hear that I was doing it well. Next time we’ll get into how GFCI outlets help to prevent you from getting electrocuted and from becoming a potential subject on Curious and Unusual Deaths. _____________________________________________
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