Posts Tagged ‘wall wart’

Transistors – Voltage Regulation

Sunday, July 22nd, 2012
     Electrical voltage flow and water flow have a lot in common.  They’re both affected by fluctuations in supply, fluctuations which can adversely impact both performance and equipment integrity.  Take for example a sprinkler that fails to cover a designated section of lawn due to heavy neighborhood demand.  Everybody wants to water on the weekend when it’s been 90 degrees all week, and low water pressure is the result.  There are times when it’s hard to get a glass of water.  By contrast in the winter months, when water demands tend to be lower, water supplies are plentiful.  This scenario of varying water pressure is analogous to what sometimes occurs within electric circuits.

     In my previous blog article on wall warts, I described the operation of a simple power supply consisting of a transformer, diode bridge, and capacitor.  Together, these components converted 120 volts alternating current (VAC) to 12 volts direct current (VDC).  The wall wart power supply is fine for many applications, however it is unregulated, meaning if there are any sudden surges in power, such as spikes or dips caused by lightning strikes or other disturbances on the electric utility system, there could be problems.

     Take for example a power supply that is used in conjunction with sensitive digital logic chips, like the one used in my x-ray film processor design shown in my last blog article.  These chips are designed to run optimally on a constant voltage, like 5 VDC, and when that doesn’t happen input signals can fail to register with the computer program and cause a variety of problems, such as output signals turning on and off at will.  In the film processor the drive motor may start at the wrong time or get stuck in an on modality.  If power surges are high enough, microprocessor chips can get damaged, compromising the entire working unit.

     The output voltage of an unregulated power supply can also vary in response to power demand, just as when sprinklers don’t have sufficient water flow to cover a section of lawn.  Demand for power can change within a circuit when electrical components like relays, lights, and buzzers are turned on and off by digital logic chips.

     Next time we’ll take a look at a basic concept of electrical engineering known as “Ohm’s Law” and how it governs the variable output voltage response of unregulated power supplies.


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.


Inside The Wall Wart

Monday, September 5th, 2011

     What would a cop show be without a crime scene, or better yet the obligatory dissection at the morgue?  Forensic doctors performing autopsies have become commonplace, the clues they provide indispensable.  Forensic engineers such as myself do much of the same thing, working our way backwards through time by dissecting industrial equipment and consumer products left in the wake of fires, injuries, and deaths. 

     Let’s do some forensic dissecting now to see what’s in a wall wart and how it works.  The inside of a basic wall wart is shown in Figure 1.

Figure 1 – Inside The Wall Wart 

     You’ll note that a wall wart has four main components:  a transformer, diode bridge, capacitor, and a printed circuit board (PCB).  The PCB is constructed of plastic resin upon which is mounted copper strips.  This makes a rigid platform base upon which electronic components are attached, namely the transformer, diode bridge, and capacitor.  These components are soldered to the PCB, tying them together both mechanically and electrically.  Now let’s see how the components of the wall wart work together to change the 120 volts coming from your standard wall outlet into the 12 volts needed to power a typical electronic device.   We’ll use an instrument known as an oscilloscope to help us visualize what’s going on.   See Figure 2.

Figure 2 – The Workings of the Wall Wart Transformer

     What is depicted in the graph above is the oscilloscope’s ability to receive an electronic signal, measure it, graph it, and then display it on a screen.  This enables us to see how the signal changes over time.  At Point A, which represents the wall wart plugged into a wall outlet, the voltage alternates between positive 120 volts and negative 120 volts upon entering the wall wart, which will now act as a transformer.

     The wall wart transformer then does as its name suggests, it transforms the 120 volts coming from the outlet into the 12 volts shown at Point B.  You will note that this lower voltage also alternates between positive and negative values, just as the original 120 volts emanating from the wall outlet did.  In one of my earlier blogs I explained that transformers only work when the electricity passing through them alternates over time.  (Click here for a refresher: Transformers )   High voltage alternating electricity in one transformer coil creates magnetic fields that induce alternating electricity at a different voltage in a second transformer coil.  So when you put alternating voltage into the transformer, you get alternating voltage out.  But that’s not the end of the story.  Many electronic devices operate on voltage that doesn’t alternate.  What then?  Will our handy wall wart still be able to bridge the electrical gap to fill our needs?

     Next time we’ll see how the diode bridge and capacitor come into play to deal with the alternating voltage from the transformer in a manner eerily similar to a microwave oven’s high voltage circuit.


Ever Had a Wall Wart?

Sunday, August 28th, 2011

     You might have had warts on your skin.  They’re formed by viruses making a new home.  If you’ve ever had one, you probably didn’t like it and found it hard to get rid of.

     Walls often have warts, too, although you probably didn’t identify them as such.  “Wall Wart” is engineering talk for the black plastic protrusions you often find attached to the exterior of a wall outlet in modern homes.  If you call them anything at all, it’s most likely “AC power adapters.”  A typical wall wart is shown in Figure 1.

Figure 1 – A Typical Wall Wart

     Wall warts provide a handy, portable and easy to use conversionary power source for small electronic devices, including lamps, small appliances, and various modern day electronics.  If you’re like me, you have lots of them scattered on the walls of your home and office.  Most people come to use them when a need arises, say you bought a scanner for your computer.  Beyond that they’re usually not given much thought, but today we’re going to explore them a bit.

     Suppose you’re an engineer and you’ve been asked to design an electronic product for household use.  The product only requires 12 volts of direct current (DC) to operate, but you know that the typical home is wired to supply 120 volts of alternating current (AC).  What can be done to rectify the discrepancy?  Well, there are two distinct choices.

     One of the choices is to design electronic circuitry capable of converting 120 volts AC into 12 volts DC, then place it inside the product.  But is this the best choice?  Not really.  It takes time to design custom circuitry, and doing so will add substantially to the design time and final cost of the product.  This is especially true if the circuitry is produced in small quantities.  Besides, if the electronic product is small, there may not be enough room inside to accommodate this type of circuitry.

     The smarter choice would be to buy a wall wart from another company that specializes in manufacturing them.  They’re produced in huge quantities, so the cost is low.  They also come in standard voltages, like 12 volts DC.  And because the wall wart is external to the product housing, space inside is no longer a concern.  It couldn’t be any easier or cost effective.  Just plug the wall wart into your home electrical outlet, then plug in the product’s 12 volt DC cord.  Done!

     Next time we’ll take a look at what’s going on inside your basic wall wart to see how it works.



Electrocution by Microwave Oven

Sunday, August 21st, 2011

     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.