| The world of electricity is full of mysteries and often unanticipated outcomes, and if you’ve been reading along with my blog series you have been able to appreciate and come to some understanding of a fair number of them. This week’s installment will be no exception.
Last week we looked briefly at the high voltage circuit within a microwave oven. We discovered that the circuit contains a transformer that raises 120 volts alternating current (AC) to a much higher voltage, around 4000 volts AC. The circuit then transforms the AC into direct current (DC) with the help of electronic components known as a diode and capacitor. Let’s take a closer look at how the diode and capacitor work together to make AC into DC. Let’s follow an AC wave with the aid of a device called an oscilloscope. An oscilloscope takes in an electronic signal, measures it, graphs it, and shows it on a display screen so you can see how the signal changes over time. An AC wave is shown in Figure 1 as it would appear on an oscilloscope. Figure 1 – Alternating Current Wave You can see that each wave cycle starts with a zero value, climbs to a positive maximum value, then back to zero, and finally back down to a maximum negative value. The current keeps alternating between positive and negative polarity, hence the name “alternating current.” Within the microwave oven’s high voltage circuitry the transformer does the job of changing, or transforming if you will, 120 volts AC into 4000 volts AC. This high voltage is needed to make electrons leave the cathode in the magnetron and move them towards the anode to generate microwaves. But we’re not done with the transformation process yet. The magnetron requires DC to operate, not AC. DC current remains constant over time, maintaining a consistent positive value as shown in Figure 2. It is this type of consistency that the magnetron needs to operate.
Figure 2 – Direct Current The microwave’s diode and capacitor work together to convert the 4000 volts AC into something which resembles 4000 volts DC. First the diode acts like a one-way valve, passing the flow of positive electric current and blocking the flow of negative current. It effectively chops off the negative part of the AC wave, leaving only positive peaks, as shown in Figure 3.
Figure 3 – The Diode Chops Off The Negative Part of the AC Wave Between the peaks are gaps where there is zero current, and this is when the capacitor comes into play. Capacitors are similar to batteries because they can be charged with electrical energy and then discharge that energy when needed. Unlike a battery, the capacitor charges and discharges very quickly, within a fraction of a second. Within the circuitry of a microwave oven the capacitor charges up at the top of each peak in Figure 3, then, when the current drops to zero inside the gaps the capacitor comes into play, discharging its electrical energy into the high voltage circuit. The result is an elimination of the zero current gaps. The capacitor acts as a reserve energy supply to fill in the gaps between the peaks and keep current continually flowing to the magnetron. We have now witnessed a mock DC current situation being created, and the result is shown in Figure 4.
Figure 4 – The Capacitor Discharges to Fill In The Gaps Between Peaks The output of this approximated DC current looks like a sawtooth pattern instead of the straight line of a true DC current shown in Figure 2. This ripple pattern is evidence of the “hoax” that has been played with the AC current. The net result is that the modified AC current, thanks to the introduction of the diode and energy storing capacitor, has made an effective enough approximation of DC current to allow our magnetron to get to work jostling electrons loose from the cathode and putting our microwave oven into action. You now have a basic understanding of how to turn AC into an effective approximation of DC current. Next week we’ll find out how this high voltage circuit can prove to be lethal, even when the microwave oven is unplugged. ____________________________________________ |
Posts Tagged ‘high voltage circuit’
The Microwave Oven — More on How AC Becomes DC
Monday, August 15th, 2011
The Microwave Oven High Voltage Circuit—How AC Becomes DC
Sunday, August 7th, 2011| My mom was a female do-it-yourselfer. Toaster on the blink? Garbage disposal grind to a halt? She’d take them apart and start investigating why. Putting safety first, she always pulled the plug on electrical appliances before working on them. Little did she know that this safety precaution would not be enough in the case of a microwave oven. Let’s see how even an unplugged microwave can prove to be a lethal weapon and, yes, we’re going to have to get technical.
Last week we talked about the magnetron and how it needs thousands of volts to operate. To get this high of a voltage out of a 120 volt wall outlet–the voltage that most kitchen outlets provide–the microwave oven is equipped with electrical circuitry containing three important components: a transformer, a diode, and a capacitor, and just like the third rail of an electric railway system these items are to be avoided. If you decide to take your microwave oven apart and you come into contact with high voltage that is still present, you run the risk of injury or even death. But how can high voltage be present when it’s unplugged? Read on. First we need to understand how the 120 volts emitting from your wall outlet becomes the 4000 volts required to power a microwave’s magnetron. This change takes place thanks to a near magical act performed by AC, or alternating current. In the case of our microwave components, specifically its diode and capacitor, AC is made to effectively mimic the power of DC, or direct current, the type of current a magnetron needs. This transformation is made possible through the storage of electrical energy within the microwave’s capacitor. Next week we’ll examine in detail how this transformation from AC to DC current takes place, as seen through a device called an oscilloscope. ____________________________________________
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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. _____________________________________________ |
