## Posts Tagged ‘electrocution’

### 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

### GFCI Outlets and The Mighty Robot

Sunday, July 3rd, 2011
 Most people aren’t aware of just how important those strange looking wall outlets in our kitchens and bathrooms are, you know, the ones with the little buttons that say Test and Reset.  They’re known as GFCI outlets, that is Ground Fault Circuit Interrupters, and given the right set of circumstances they could save your life.      The GFCI equipped wall outlet, like a mighty robot, continuously watches the flow of electrons (electrical current) passing through, always on the lookout for incongruities between the hot and neutral wires, and ready to jump into action when necessary.  Say, for example, that one of these GFCI equipped outlets has an appliance plugged into it. While the appliance is in safe use there is nothing for the GFCI robot to do.  It simply takes note of the balance of electrons flowing between the hot and neutral conductors, notes that they are equal, and continues to watch for inequalities.   Figure 1 – While the Hand Mixer is Operating Normally, in the GFCI Outlet the Electric Current Flowing in the Hot and Neutral Wires is Equal.   The Robot Takes No Action.       But suppose that there is a problem with the appliance, something that causes a ground fault where the user’s body provides an unintended path to errant electrons flowing from the hot side of the wall outlet.  Those errant electrons are supposed to traverse the neutral wire back through the wall outlet from whence they came, but they have become unruly.  Not to worry, if you are up to code and have an ever vigilant GFCI on that outlet, the robot will immediately notice the anomaly.  Figure 2 – If a Ground Fault Develops in the Hand Mixer and Some Electric Current Flows Through the User’s Body, Then the Robot Notices a Difference In Current Flowing Through the Hot and Neutral Wires in the GFCI Outlet.      The Mighty Robot of the GFCI doesn’t like the fact that the electrons are out of balance, that there are more of them flowing through the hot wire than returning through the outlet via the neutral wire, so within a fraction of a second it will jump into action to correct things.  It hits a lever on a spring loaded mechanism that snaps open an electrical switch connecting the appliance to the hot and neutral sides of the outlet, effectively cutting off the flow of electrons to the appliance.  Cut off from power, the appliance ceases to function, but more importantly, the flow of electrons through the user’s body has been stopped before their body incurs injury, or death. Figure 3- In Response to the Ground Fault, the Robot Opens a Switch in the GFCI Outlet to Cut Off The Flow of Electricity to the Hand Mixer.  The Person Operating the Hand Mixer is Saved.      The GFCI robot, having done its job, now goes into a sleep mode.  It will be reactivated, ready again for its vigilant watch of errant electrons, when the faulty appliance is unplugged and the Reset button is pressed.  This button does what it says, it resets the spring loaded mechanism in the wall outlet, closing the electrical switch, and making the outlet functional again.  The GFCI robot immediately goes back into active monitoring mode.    Now it should be noted that as dependable as GFCI outlets are, they can become defective.  That’s why they have a Test button.  This button should be pressed periodically to see if the robot is still on the job.  If all is in order, the Reset button pops out of the outlet, and anything plugged into that outlet will not operate. When you press the Reset button back in, everything should operate again if there are no fault conditions.      Could the GFCI’s Mighty Robot have prevented the unfortunate incidents discussed during my tenure on The Discovery Channel?  Stay tuned to find out…      That’s it for GFCI outlets.  Next time we’ll take a look at how an invention developed to defend the allies during World War II later morphed into a space age device that cooks our food.  _____________________________________________

### Ground Fault Circuit Interrupters

Sunday, June 26th, 2011