DC Power Supply

Posted by PITHOCRATES - February 13th, 2013

Technology 101

Every DC Power Supply has a Transformer, a Rectifier Circuit and a Voltage Regulation Circuit

Alternating current (AC) power is one of the greatest technological developments of mankind.  It gives us the modern world we live in.  We can transmit it over very long distances.  Allowing a few power plants to power large geographic areas.  Something Thomas Edison’s direct current (DC) power just couldn’t do.  Which is a big reason why he lost the War of Currents to George Westinghouse and Nikola Tesla.  AC power also allows the use of transformers.  Allowing us to take the one voltage produced by a power plant and convert it to any voltage we need.

AC power can power our home lighting.  Our air conditioning.  Our electric stove.  Our refrigerator.  Our doorbell.  Pretty much all of the non-fun things in our house.  Things with electric motors in them.  Heating elements.  Or solenoids.  But one thing AC power can’t do is power the fun things in our homes.  Televisions.  Our audio equipment.  Our cable/satellite boxes.  Pretty much anything that doesn’t have an electric motor, heating element or solenoid in it.  These things that process information or audio and video signals.  Or all of the above.  Things that have circuit boards.  With electronic components.  The kind of things that only work with DC power.

Of course all of these things in our homes plug into AC wall receptacles.  Even though they are DC devices.  So what gives?  How can we use AC power to operate DC devices?  With a little something we call a DC power supply.  And every one of those fun things has one.  Either one built-in.  Or an external power pack at the end of a cord.  Every DC power supply has three parts.  There is a transformer to step down the AC voltage.  A rectifier circuit.  And a voltage regulation circuit.

A Diode is a Semiconductor Device that allows a Current to pass through when there is a Forward Bias

The typical electrical receptacle in a house is 120 volt AC.  An AC power cord brings that into our electronic devices.  And the first thing it connects to is a transformer.  Such as a 120:24 volt transformer.  Which steps the 120 volts down to 24 volts AC.  Where the waveform looks like this.

DC Power Supply AC Input

The voltage of AC power rises and falls.  It starts at zero.  Rises to a maximum positive voltage.  Then falls through zero to a maximum negative voltage.  Then rises back to zero.  This represents one cycle.  It does this 60 times a second.  (In North America, at least.  In Europe it’s 50 times a second.)  As most electronic devices are made from semiconductors this is a problem.  For semiconductor devices use low DC voltages to cause current to flow through PN junctions.  A voltage that swings between positive and negative values would only make those semiconductor devices work half of the time.  Sort of like a fluorescent light flickering in the cold.  Only these circuits wouldn’t work that well.  No, to use these semiconductors we need to first get rid of those negative voltages.  By rectifying them to positive voltages.  When we do we get a waveform that looks like this.

DC Power Supply Rectified

A diode is a semiconductor device that allows a current to pass through when there is a forward bias.  And it blocks current from passing through when there is a reverse bias.  An alternating voltage across a diode alternates the bias back and forth between forward bias and reverse bias. Using one diode would produce a waveform like in the first graph above only without the negative parts.  If we use 4 diodes to make a bridge rectifier we can take those negative voltages and make them positive voltages.  Basically flipping the negative portion of the AC waveform to the positive side of the graph.  So it looks like the above waveform.

All Electronic Devices have a Section built Inside of them called a Power Supply

The rectified waveform is all positive.  There are no negative voltages.  But the voltage is more of a series of pulses than a constant voltage.  Varying between 0 and 24 volts.  But our electronic devices need a constant voltage.  So the next step is to smooth this waveform out a little.  And we can do this by adding a capacitor to the output of the bridge rectifier.  Which sort of acts like a reservoir.  It stores charge at higher voltages.   And releases charge at lower voltages.  As it does it smooths out the waveform of our rectified voltage.  Making it less of a series of pulses and more of a fluctuating voltage above and below our desired output voltage.  And looks sort of like this.

DC Power Supply Capacitor

This graph is exaggerated a little to show clearly the sinusoidal waveform.  In reality it may not fluctuate quite so much.  And the lowest voltage would not fall below the rated DC output of the DC power supply.  Please note that now we have a voltage that is always positive.  And never zero.  As well as fluctuating in a sinusoidal waveform at twice the frequency of the original voltage.  The last step in this process is voltage regulation.  Another semiconductor device.  Typically some transistors forming a linear amplifier.  Or an integrated circuit with three terminals.  An input, an output and a ground.  We apply the above waveform between the input and ground.  And these semiconductor devices will change voltage and current through the device to get the following output voltage (for a 12 volt DC power supply).

DC Power Supply DC Output

All electronic devices that plug into a wall outlet with a standard AC power cord have a section built inside of them called a power supply.  (Or there is an external power supply.  Small ones that plug into wall outlets.  Or bigger ones that are located in series with the power cord.)  And this is what happens inside the power supply.  It takes the 120 volt AC and converts it to 12 volts DC (or whatever DC voltage the device needs).  Wires from this power supply go to other circuit boards inside these electronic devices.  Giving the electronic components on these circuit boards the 12 volt DC power they need to operate.  Allowing us to watch television, listen to music or surf the web.



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Sound Waves, Phonograph, Stylus, Piezoelectric & Magnetic Cartridges, Thermionic Emission, Vacuum Tube, PN-Junction, Transistor and Amplifier

Posted by PITHOCRATES - May 2nd, 2012

Technology 101

The First Phonographs used a Stylus attached to a Diaphragm to Vibrate the Air and a Horn for Amplification 

Sound is vibration.  Sound waves we hear are vibrations in the air.  A plucked guitar string vibrates.  It transfers that vibration to the soundboard on the guitar body.  The vibration of the soundboard vibrates the air inside the guitar body.  Amplifying it.  And shaping it.  Giving it a rich and resonant sound.  Creating music.  And we can reverse this process.  Taking these vibrations from the air.  And putting them into a piece of wax.  Via a vibrating needle.  Or stylus.  Cutting wavy grooves into wax.  And then we can even reverse this process.  By dragging a stylus through those same wavy groves.  Causing the stylus to vibrate.  And if we transfer those vibrations to the air we can hear those sound waves.  And listen to the music they make.

The first phonographs could reproduce sound.  But they didn’t sound very good.  The first phonographs were purely mechanical.  A stylus vibrated a diaphragm.  The diaphragm vibrated the air.  And a horn attached to that diaphragm was the only amplification.  Sort of like cupping your hands around your mouth when shouting.  Which reinforced and concentrated the sound waves.  Making them louder in the direction you were facing.  Which is how these early phonographs worked.  But the quality of the sound was terrible.  And played at only one volume.  Low.

Electric circuits changed the way we listen to music.  Because we could amplify those low volumes.  By changing the vibrations created from those wavy grooves into an electrical signal.  The first phonographs used a piezoelectric cartridge.  Which the stylus attached to.  The piezoelectric cartridge converted a mechanical pressure (the needle vibrating in the wavy groove) into electricity.  Later phonographs used a magnetic cartridge.  Which did the same thing only using a varying magnetic field.  The vibration of the needle moved a magnet or a coil through a magnetic field.  Thus inducing a current in a coil.  Then all you needed was an amplifier and a loudspeaker to make sweet music.

Small Changes in the Control Grid Voltage of a Vacuum Tube make Larger Changes in the Plate Voltage

The first amplifiers used vacuum tubes.  Things that once filled our televisions and stereo systems.  Back in the old days.  Up until about the Seventies.  A vacuum tube operated on the principle of thermionic emission.  Which basically means if you heat a metal filament it will ‘boil off’ electrons.  The basic vacuum tube used for amplification consisted of a cathode and an anode.  Or filament and plate.  And a control grid in between.  Sealed in, of course, a vacuum.  Creating the triode.  The cathode (filament) and anode (plate) created an electric field when connected to a large power source.  The cathode is negative.  And the anode is positive.  When negatively charged electrons are ‘boiled off’ of the cathode the positive anode attracts them.  The greater the heat the greater the thermionic emission.  And the greater the current flow from cathode to anode.  Unless we change the electric field to inhibit the flow of current.  Which is the purpose of the control grid.

Small changes in the control grid voltage will make changes in the large current flowing from cathode to anode.  That is, the larger current replicates the smaller signal applied to the control grid.  This allows the triode to take the low voltage from a phonograph cartridge and amplify it to a higher voltage with enough power to drive a loudspeaker.  Which is similar to diaphragm and horn on the first phonographs.  Only the amplified electric signal moves a lot more air.  And better materials and construction create a better quality sound.  Amplifiers with vacuum tubes make beautiful music.  High-end audio equipment still uses them to this day.  Including almost all electric guitar amps.  So if they have the highest quality why don’t we use them elsewhere?  Because of thermionic emission.  And the heat required to ‘boil off’ those electrons.

Vacuum tubes worked well when plugged into line power.  Such as a radio in a house.  But they don’t work well on batteries.  Because it takes a lot of electric power to heat those filaments.  And you need pretty big batteries to get that kind of electric power.  Like a car battery.  But even a car battery didn’t let you listen to music for long when parked with the engine off.  Because those tubes drained that battery pretty fast.  So there were limitations in using vacuum tubes.  They draw a lot of power.  Produce a lot of heat.  And tend to be pieces of furniture in your house because of their physical size.

Small Changes in the Base Current of a Transistor is Replicated in the Larger Collector-Emitter Current

The transistor changed that.  Making music more portable.  Thanks to semiconductors.  Material with special electric properties.  Based on the amount of electrons in the atoms making up this material.  Atoms with extra electrons make material with a negative charge (N-material).  Atoms missing some electrons make material with a positive charge (P-material).  When you put these materials together the N and the P attract each other.  Electrons cross the junction and fill in the holes that were missing electrons.  And the ‘holes’ cross the junction and fill in the spaces where there were excess electrons.  (When an electron moved, say, from right to left it made a hole and filled a hole.  It made a hole where it once was.  And it filled a hole where it now is.  So it looks like the hole moved from left to right when the electron moved from right to left.)  Neutralizing the N-material and the P-material.  But creating a charged region around the junction.  And it’s this electron flow and hole flow that make these PN junctions work.  When you add a third material you get a transistor.  Made up of three parts (NPN or PNP).  Emitter, base, and collector.

To get the electrons and holes flowing you start applying voltages across the junctions.  A large current will flow from the collector to the emitter.  Similar to the current flow in a tube from cathode to anode.  And a small base current will change that current flow.  Just like the control grid in a vacuum tube.  Small changes in the base current will make similar changes in the larger collector-emitter current.  Just like in a vacuum tube, the larger current replicates the smaller signal applied to the ‘control’.  Or base.  This allows the transistor to take the low-level signal from a phonograph cartridge and amplify it to a higher level.  Just like a vacuum tube.  Only with a fraction of the electric power.  Because there are no filaments to heat. 

Low power consumption and the small physical size allowed much smaller amplifiers.  And amplifiers that everyday batteries could power.  Creating new ways to listen to music.  From the pocket-size transistor radio.  To the bigger stereo boombox.  To the iPod.  Where the basic principle of how we listen to music hasn’t changed.  Just how we vibrate the air that makes that music has.



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