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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Music, Radio Transmitters, Radio Receivers, CD Players, Compression, MP3 Players, Internet, YouTube, Live Streaming and Music on Demand

Posted by PITHOCRATES - February 29th, 2012

Technology 101

The Roaring Twenties brought Electrical Power and Broadcast Radio into our Homes

We take music for granted today.  We can listen to pretty much anything we want to.  At any time.  In any place.  In the home.  In the car.  At the gym.  It’s nice.  You can listen to some of the most beautiful music at your convenience and leisure.  It wasn’t always like this, though.  During the time Edvard Grieg composed his masterpieces few could listen to them.  Unless you attended a live performance.  Which weren’t that readily available.  Unless you lived in a big city.  Where a symphony orchestra could include some of his music in a performance.  But you had to listen to what they played.  And what they played was the only music you were familiar with.  Unless you had a friend with a piano.  Who could read sheet music.  And was a concert-level pianist.  Again, something not that common.

But today you can click on a computer link and listen to almost any obscure piece of music there is.  From Grieg’s beautiful Bådnlåt (At the Cradle), lyric piece for piano, Op. 68/5.  To something really esoteric like Sparks’ As I Sit Down To Play The Organ At The Notre Dame Cathedral.  You can listen to them.  You can buy them.  Download them to a portable MP3 player.  And take them anywhere.  Just imagine trying to do this in 1899.  Going to the lake.  And wanting to listen to Grieg’s new lyric piece for piano.  Opus 68.  Number 5.  At the Cradle.  Unless you took a piano and a concert-level pianist with you that just wasn’t going to happen.  But this all changed.  Beginning around the dawn of the 20th century.

Nikola Tesla had recently won his war with Thomas Edison.  His AC power replaced Edison’s DC power as the standard.  And in the 1920s we were electrifying the country.  We began to generate and transmit AC power across the land.  To businesses.  And to homes.  Where we could plug in the new electrical appliances coming to market.  We were working on another new technology during this time.  Something that could plug in at home to the new electrical power.  The radio.  This technology had something to do with electromagnetic fields and waves.  Transmitted between antennas.  One on a transmitter.  And one on a receiver.  As long as the transmitter and the receiver were tuned to the same frequency.  The first use of this new technology was in the form of a wireless telegraph.  Which few people had in their homes.  These were more useful to communicate with others who were not connected by telegraph lines.  Like ships at sea.  Where we sent Morse code (those dots and dashes that spelled words).  Which worked well.  As long as all the ships didn’t tried to communicate at the same time on the same frequency.  But transmitting speech or music was a different manner.  Because everyone talks more or less in the same band of frequencies.  And notes played on one violin tend to play at the same frequency on another violin.  So if some radio transmitters broadcasted different concerts at the same time you wouldn’t hear a nice concert on your radio.  You’d hear a cacophony of noise.  To get an idea what that would sound like open up three or four browser windows on your computer.  And play a different song on YouTube in each.  What you hear will not be music.  But noise.

In the Eighties we traded our Phonograph Needles for Laser Beams in our CD Players

Of course, this didn’t stop the development of commercial broadcast radio.  For we tune radio transmitters and radio receivers to the same resonant frequency.  The transmitter transmitting at one frequency all of the time. While the radio receiver could tune in to different frequencies to listen to different radio broadcasts.  When you turned the radio tuning dial you changed what resonant frequency your receiver ‘listened’ to.  Which was basically a filter to block all frequencies but the tuned frequency from entering your radio.  We call that frequency the carrier signal.  Typically just a plain old sinusoidal wave form at a one frequency that we imprint the information of the speech or music on.  The transmitter takes the music waveform and modulates it on the carrier signal.  Then broadcasts the signal on the broadcast antenna.  The receiver then captures this signal on its antenna.  And demodulates it.  Pulling the musical imprint from the carrier signal.  And restoring it to its original condition.  Which the radio than amplifies and sends to a speaker.  I left some steps out of the process.  But you get the gist.  The key to successful broadcast radio was the ability to transform the source signal (speech or music) into another signal.  One that we could transmit and receive.  And transform back into the source signal.

The Roaring Twenties was a Neil Armstrong moment on earth.  It was one giant leap for mankind.  For it was in this decade that the modern world began.  Thanks to Nikola Tesla and his AC power.  Which allowed us the ability to plug in radios in our homes.  And power the great radio transmitters to get the signal to our houses.  Tesla, incidentally, created radio technology, too.  Well, Tesla, and Guglielmo Marconi.  (Patent disputes flared between these two greats about who was first.)  Great technological advancement.  Created during a time of limited government and low taxes.  That unleashed an explosive amount of creativity and invention.  The Eighties was another such decade.

The Eighties launched the digital age.  The world of bits and bytes.  1s and 0s.  Digital watches.  Clocks.  Calculators.  PCs.  And, of course, our music.  For the Eighties gave us the compact disc.  The CD.  Music that didn’t wear out like our vinyl records.  And didn’t pop or hiss with age.  Because a CD player didn’t have a phonograph needle.  That rode the groves on our vinyl records.  It had something far more futuristic.  A laser beam.  That reads information encoded into the CD.  Information encoded onto a reflective layer through a series of pits.  During playback the laser either reflects or doesn’t reflect.  This information is than processed into a series of 1s and 0s.  Then converted into the analog waveform of the source material.  And becomes music again.

The Eighties gave us the Digital Age which led to the Internet and Music on Demand

This process is similar to the process of broadcast radio.  Not in any technological way.  But by changing a source signal into something else.  And then converting it back again.  In the case of the CD we sample an analog signal (i.e., an audio recording).  By taking ‘snapshots’ of it at regular intervals.  Then convert these snapshots into a digital format.  And then transfer this digital information to the reflective layer on a CD.  Those 1s and 0s.  When we play it back the laser reads these 1s and 0s.  Then converts these digital snapshots back into the original audio signal.  Sort of like modulating and demodulating a signal.  Only instead of modulating we’re converting from analog to digital.  Then vice versa.

The quality of the digital format depends on how much information each snapshot contains.  And the interval we sample them at.  Larger chunks of information taken in short intervals contain a lot more information.  And improve the quality of the sound.  But it will also take up a lot of space on those CDs.  Limiting the number of songs we can encode on them.  Which lead to compression.  And MP3s.  Which worked on the premise that there’s a lot of music in music.  But we don’t necessarily hear all of that music.  Some sounds mask out other sounds.  Certain frequencies we barely hear.  So while the CDs tried to reproduce the music as faithfully as possible, we learned that we could discard some of the information in the music without reducing the quality of the music much.  This saved a lot of space on CDs and portable MP3 players.  Allowed faster downloads on the Internet.  And live streaming.

The Roaring Twenties changed our world.  Modernized it.  And gave us many things.  Including broadcast radio.  And music in our homes we never had before.  And the Eighties also changed our world.  Further modernizing it.  Giving us the digital age.  That led to the Internet.  And music on demand like we never had before.  Where we can listen to anything.  No matter how obscure.  It’s now all available at our fingertips.  To listen online.  Or to buy and download to a portable device.  From Grieg to Sparks.  And everything in between.



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