Tuesday, 3 April 2012

Power supplies part III

Still getting on with the job of designing a linear power supply!

Last post talked about smoothing and why large electrolytic capacitors are used to remove ripple.  This post is going to talk about voltage regulators.

The voltage output from the previous circuit although now nice and smooth will not be constant.  As soon as an electronic 'load' or impedance to use the proper term is placed across the positive and zero volt terminals the voltage will drop because there is not a constant current present to drive the load.  The circuit that fixes this problem is called a voltage regulator.  There are many different types of circuits for making voltage regulators and they all have their place.  I don't intend going into all the different types of regulator here.  If people are interested the web sites below maybe useful:

http://en.wikipedia.org/wiki/Power_supply#Linear_regulated_power_supply - Wikipedia!

http://www.eevblog.com/2011/11/28/eevblog-221-lab-power-supply-design-part-1/ - the excellent EEVBlog by Dave L. Jones!

http://www.allaboutcircuits.com/vol_3/chpt_9/2.html - All about circuits....I highly recommend this site!  Its
great for electronics theory.

Essentially a voltage regulator is a special circuit that provides a constant voltage to a current controlled device which keeps the output to the load constant whether the load or the supply are changing.  We know that the supply isn't constant as the transformer is supplied with an AC voltage which switches polarity every 10ms and the load attached to the output may not be constant either.

The constant voltage output is usually achieved by using a special diode called a Zener diode.  These devices are a special kind of diode which allow current to flow in both directions once a certain voltage is applied to the anode.  Zener diodes are used to provide a constant voltage reference.  Check out all about circuits page in zener diodes for more information here:

http://www.allaboutcircuits.com/vol_3/chpt_3/11.html

The next part of the regulator would be made up of a series pass transistor.  This is normally a high current transistor which is permanently switched on by the zener diode and a resistor and is used to keep the current output constant. So by implementing the circuit below we have a constant voltage regulator.  To keep the theory simple I have selected a 12V zener diode and that is what decides our output voltage for the power supply.

If we were to implement the circuit above it would provide us with a constant 12V (all right 11.643V) output whatever load was attached.  However there are better and easier ways of implementing the above circuit (Series voltage regulator) using an integrated circuit known as the three terminal voltage regulator.

There are literally hundreds of different types of three terminal linear voltage regulators - some of the more popular ones are the ubiquitous 78 series which are a fixed voltage output linear regulator.  Another might be the LT38 series.

For my power supply though I wanted an adjustable voltage output and so I'm going to use an adjustable voltage regulator - the venerable but useful LM317.  There are many manufacturers of this device but they all perform the same function.  The datasheet and application note for this regulator from Texas Instruments are here:

http://www.ti.com/lit/ds/symlink/lm117.pdf - Datasheet

The circuit will now look like this.  This circuit is directly taken from the information provided in the datasheet.  All the information needed to design the circuit is there.  Datasheets are like rosetta stones to electronics design engineers.  When they are well written they can make our lives so much simpler!


You may have noticed I have added some extra components to our circuit.  R1 is what is known as a 'bleeder' resistor.  This is so that when power is removed from the circuit the energy stored in the electrolytic capacitors is safely drained away.  Otherwise the capacitors can remain charged for a very long time and store a lot of energy which will be released when someone (normally me) shorts the output by mistake.  The diodes D4 and D3 are protection diodes for the regulator IC.  They protect the regulator from short circuits and transients caused by capacitors discharging.  The resistor R5 and the LED are for indication purposes.  Its always good to have visual indication that the power supply is switched on!  The variable resistor R3 is to allow the user to vary the output voltage from 1.25V to about 40V.  R4 is part of the voltage divider which sets the output voltage.  C4 and C6 are ripple smoothing capacitors for the output.  Finally I added a voltmeter so that we can see what the output voltage is set at when using the circuit.

Well...thats it for now folks.  More to come soon!  Enjoy and take care - Alex