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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