## Posts Tagged ‘microwave oven’

### Further Inside the Wall Wart

Sunday, September 11th, 2011
 What do wall warts, aka AC wall adapters, and microwave ovens have in common?  Well, in previous blogs discussing microwaves, we saw how a microwave oven’s high voltage circuitry uses a transformer, diode, and capacitor to effectively convert AC voltage into DC voltage.  Wall warts do much the same thing and in very much the same way.      If you will recall from our discussion of microwave ovens in the past few weeks, the transformer in a high voltage circuit transforms 120 volts into a much higher voltage, say 4000 volts, in order to make things work.  The diode and capacitor within both the microwave and the wall wart are key to facilitating this magical act, but in the wall wart it happens at a much lower voltage, about 12 volts.      Last week we began exploring the inner workings of the wall wart.  We discovered how its transformer converts the 120 volts emanating from your average wall outlet to the 12 volts required by most electronic devices.  These voltages are shown at Points A and B in Figure 1 below.  The fact that the voltage being put out results in waves of energy which alternate between a positive maximum value, zero, and a negative maximum value, makes it an unacceptable power source for most electronic devices.  They require voltage that doesn’t alternate, and this is where the wall wart’s diode bridge and capacitor come into play. Figure 1 – The Workings of the Wall Wart Transformer      The wall wart’s diode bridge consists of four electronic components, namely the diodes, which are connected together.  This diode bridge goes a bit further than the single diode present in a microwave oven, because it doesn’t merely eliminate negative aspects of alternating voltage.  It actually transforms negative voltage into positive voltage.  The result is a series of 12 volt peaks as shown at Point C of Figure 1.  In fact, we end up with twice as many voltage peaks, and this is important, as you’ll see below.      We still have the problem of zero voltage gaps to address.  You see, over time the voltage at Point C of Figure 1 keeps changing between 0 volts and positive 12 volts.  This can lead to problems, because many electronic devices require a consistent voltage of greater than zero to operate properly.  For example, a light emitting diode (LED) might develop an annoying flicker, or you might end up hearing an irritating hum while listening to the radio.  These annoyances are virtually eliminated by feeding voltage from the diode bridge into the capacitor, which gets rid of the zero voltage gaps between the voltage peaks.      Like a microwave’s capacitor, the one within a wall wart charges up with electrical energy as the voltage from the diode bridge nears the top of a peak.  Then, as voltage begins its dive back to a zero value, the capacitor discharges its electrical energy to fill in the gaps between peaks.  The result is the rippled voltage pattern at Point D of Figure 1.  With the gaps filled in, the voltage is at, or close enough to, the 12 volts required to keep an electronic device operating properly when it is connected to the wall wart’s low voltage power cord.      Well, that’s it for our look at the wall warts that power our myriad of electronic devices.  Next time we’ll switch to a totally topic and look at some of the basics of food manufacturing equipment design. ____________________________________________

### Electrocution by Microwave Oven

Sunday, August 21st, 2011

### The Microwave Oven — More on How AC Becomes DC

Monday, August 15th, 2011
 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. ____________________________________________

### 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. ____________________________________________

### The Microwave Oven Becomes Reality

Sunday, July 31st, 2011

### Microwave Radar and Melted Candy

Sunday, July 24th, 2011

### The Heart of the Microwave Oven

Monday, July 18th, 2011
 In the world of inventions it happens with some regularity that an invention to do one thing unexpectedly leads to a device that does something completely different.  Take for example Edison’s phonograph.  At the time, he was working on an invention to record the dots and dashes of Morse code telegraph messages so they could be sent out repeatedly without an operator having to tap them out each time and possibly making mistakes while doing so.  Little did he know that this would lead to the evolvement of the phonograph and recording industries.      Another invention “by mistake” took place when the resonant cavity magnetron, originally developed for use with microwave radar, led to the development of the microwave oven.  Last week we talked about how long wave radar, the first type of radar to be developed, was effectively used by the British to repel enemy air attacks during World War II.  But long wave radar was large and cumbersome to employ, and it soon evolved into an improved version, the shorter wave, or, as we know it, microwave radar.  So what is this resonant cavity magnetron that led to its creation?  A pop bottle can give us a clue.      Blow across the top of an empty glass pop bottle (or soda bottle, depending on the part of the country you’re from) and a familiar resonant sound results.  The sound is created by an effect known as cavity resonance, and other bona fide musical instruments make use of this phenomenon to produce musical sounds.  How this works is that where a cavity exists, when air molecules are introduced into it, the molecules are caused to resonate in and out of the cavity many times per second.  This creates a sound at a certain frequency, that frequency depending on the shape and dimensions of the cavity, as well as the size of its opening.      The resonant cavity magnetron, or magnetron for short, is actually a high powered vacuum tube that operates in a very similar fashion to a pop bottle, or any other musical device making use of a cavity, but instead of using air molecules to generate sound waves, it uses electrons to generate short wavelength radio waves, called microwaves.  The magnetron contains a series of cavities that are arranged in a circle with the openings pointing inward towards the center, as shown in Figure 1. Figure 1 – Interior View of a Resonant-Cavity Magnetron      Next week we’ll see what this interesting looking device has in common with the simple act of blowing air across the top of a pop bottle and what this all has to do with microwaves. _____________________________________________

### The Origins of the Microwave Oven

Sunday, July 10th, 2011