Posts Tagged ‘regulated power supply’

Transistors – Voltage Regulation, Final Chapter

Monday, November 19th, 2012

     Last time we learned how the transistor opens a path for electric current to flow from the collector to the emitter in our example circuit.  It does so by making use of an unregulated power supply.  Now let’s see how the Zener diode fits into the mix.

transistor series voltage regulator

Figure 1


     It just so happens that bipolar transistors, like the one in our example circuit in Figure 1, are designed so that voltage at its emitter is dependent upon the voltage applied at its base.  This makes them ideal for use in voltage regulator circuits where this kind of predictability is required.

     For example, in our transistor series voltage regulator, the Zener diode is connected to the transistor’s base, B.  When the branch current flows from RLimiting down through the diode, a Zener voltage, VZener, is established.  Since the diode is connected to the transistor, VZener voltage is also applied to the transistor’s base.  Thus the transistor’s emitter voltage will be regulated according to the Zener voltage.

     Bipolar transistors are designed by manufacturers to typically operate with a standardized voltage difference of 0.6 volts between the base and emitter.  This is represented in Figure 1 as VBE, where BE stands for base-emitter.  VBE is standardized at a known quantity of 0.6 volts to simplify things within the industry and aid engineers in their calculations to design transistor circuits, as we’ll now see.

     With the Zener diode connected to the transistor base in our example circuit, the voltage difference is denoted as:

VBE = VZener   VE

where VE is the emitter voltage.  Rearranging terms to solve for VE, we get:

VE = VZener – VBE

Inserting VBE,  which we know is standardized at 0.6 volts:

VE = VZener – 0.6 volts

Since the emitter is physically connected to the output terminal of the transistor series voltage regulator, the emitter voltage is going to be equal to the output voltage, VOut.

     We learned earlier in this series of articles that VZener is a reliable source of consistent voltage.  Because it is present in our transistor series voltage regulator, our example circuit will produce a nice, constant regulated output voltage of VZener – 0.6 volts, a voltage that is useful for many of today’s applications.  However the transistor series voltage regulator provides us with a major advantage over the Zener diode voltage regulator circuit.

     The advantage of a transistor series voltage regulator lies in the fact that  RLimiting is on a separate branch all to its own within the regulator circuit, and because of this it no longer acts as a roadblock to limit the main path of current flow, as happens within the Zener diode voltage regulator circuit discussed previously.  Refer to the red path shown in Figure 1.  With RLimiting in this position the transistor series voltage regulator is able to feed more current to the external supply circuit than is possible through the Zener diode voltage regulator alone.  This means it can be used in more power hungry applications like energizing today’s TVs and modern kitchen appliances.

     That wraps up our discussion on transistors.  Next time we’ll begin a new topic, how medical devices can be designed using systems engineering, a systematic approach that ensures that designed devices satisfy both user and regulatory requirements.



Transistors – Voltage Regulation Part XVI

Monday, November 5th, 2012

     We’ve been discussing the Zener diode voltage regulator circuit, its advantages and disadvantages.  We learned that the limiting resistor, RLimiting, creates a major disadvantage in the operation of the circuit, effectively acting as a roadblock to restrict current flow.  Let’s see how to improve on that. 

     Figure 1 illustrates a transistor series voltage regulator circuit.


transistor series voltage regulator with Zener diode

Figure 1


     In this circuit the transistor is known as a bipolar transistor.   Like the FET we discussed earlier, it has three electrical connections, however on the bipolar transistor the connections are referred to as the collector, base, and emitter.  These are labeled C, B, and E in Figure 1.

     The bipolar transistor acts as a valve, resting within the main path of current flow.  That is, it controls the flow of electric current traveling from the collector to the emitter, as well as the voltage available at the emitter.  The transistor is designed so that current flows in one direction only, from collector to emitter.  We’ll talk more about that in our next article.

     The limiting resistor, RLimiting, is located on a branch of the circuit leading to the Zener diode and the transistor base.  Next time we’ll connect an unregulated power supply and external supply circuit to our transistor series voltage regulator.  This will enable us to see how placing RLimiting on the branch, rather than along the main current path, results in a major advantage over using the Zener diode voltage regulator alone.  


Transistors – Voltage Regulation Part XI

Monday, October 1st, 2012
     Without limits on our roadways things would get quickly out of hand.  Imagine speeding down an unfamiliar highway and suddenly coming upon a sharp curve.  With no speed limit sign to warn you to reduce speed, you could lose control of your car.  Limits are useful in many situations, including within electronic circuits to keep them from getting damaged, as we’ll see in a moment.

     Last time we introduced the Zener diode and the fact that it performs as a voltage regulator, enabling devices connected to it to have smooth, uninterrupted operation at a constant voltage.  Let’s see how it works.

Unregulated Power Supply

Figure 1


     In Figure 1 we have an unregulated power supply circuit introduced in a previous article in this series.  We learned that this power supply’s major shortcoming is that its output voltage, VOutput, is unregulated, in other words, it’s not constant.  It varies with changes in the direct current supply voltage, VDC.

     It also varies with changes in, RTotal, which is the total internal resistance of components connected to it.  RTotal changes when components are turned on and off by microprocessor and digital logic chips. When VOutput is not constant, those chips can malfunction, causing the device to operate erratically or not at all.

     But we can easily address this problem by adding a Zener diode voltage regulator between the unregulated power supply and the external supply circuit.  See the green portion of Figure 2.

Zener Diode Voltage Regulator

Figure 2


     Our power supply now consists of a Zener diode and a limiting resistor, RLimiting.  The limiting resistor does as its name implies, it limits the amount of electric current, IZ, flowing through the Zener diode.  Without this limiting resistor, IZ could get high enough to damage the diode, resulting in system failure.

     Next time we’ll see how the Zener diode works in tandem with the limiting resistor to control current flow and hold the output voltage at a constant level.


Transistors – Voltage Regulation Part X

Monday, September 24th, 2012
     Through the ages it’s been common practice to name important discoveries after those who discovered them.  For example, James Watt was a mechanical engineer who improved the steam engine by finding a solution to the problem of steam condensing into water inside the engine, a phenomenon which resulted in the engine cooling and reducing its efficiency.  Thus it was fitting that a metric unit of power, the watt, was named in his honor.  Today we’ll become acquainted with the man behind the naming of the Zener diode, Clarence Zener, and take a look at his contributions with regard to the function of this electrical component.

     Last time we began our discussion on electrical components known as diodes and saw how they’re used on circuit paths to govern the flow of current.  The Zener diode is a particular type of diode and a key component in transistorized voltage regulator circuits, as we’ll see later.  For now, let’s see how it works.     The symbol for the Zener diode is almost identical to that of a standard diode, introduced in my previous blog, but the Zener version has a bent line going through it resembling a distorted letter “z.”  See Figure 1.

Zener diode voltage regulator

Figure 1       


      Electric current flows through the Zener Diode just as it does through a standard diode.  But when the current flows in reverse, that’s where the similarity ends.  See Figure 2.

  Zener diode

Figure 2    


     When current tries to flow in the reverse direction, the Zener diode acts as an electrical conductor and allows current to pass through it.  In other words, it doesn’t block current flow as standard diodes do.

      At this point, you may be asking, “What’s so special about that?”  Perhaps you’ve made the connection that it behaves no differently than a metal wire.  But that isn’t entirely correct.

     You see, when current passes in the reverse direction through the Zener diode, it maintains a constant voltage.  This is called the Zener Voltage and is denoted as VZener.  The significance here is that within the circuit, any electronic component connected across the leads of a Zener diode will be supplied with a constant, unchanging voltage.  Thus the Zener diode works as a voltage regulator, enabling devices connected to it to have smooth, uninterrupted operation at a constant voltage.  It should be noted that this phenomenon only happens when the current flowing through the Zener diode is flowing in reverse.

     Next time we’ll look at a basic regulated power supply circuit to see how a Zener diode is incorporated in order to maintain a consistent output voltage.


Transistors – Voltage Regulation Part IX

Sunday, September 16th, 2012
     One way streets frustrate me, and I usually end up wasting a lot of time and gas driving in circles to get to my destination.  Generally speaking, I prefer a two way street.  Electric current flowing through electronic circuits is somewhat analogous to traffic flow.  There are circuit paths that act like one way streets and others that act like two-way.

     An electrical component called a diode can be used on circuit paths to govern the flow of current.  They are a key component in basic transistorized voltage regulator circuits, as we’ll see later.  For now, let’s get a basic understanding of how they work.

     Diodes are typically made of a semiconductor material, such as the element germanium.  These materials behave in a complex way that fall along the lines of quantum physics.  Esoteric phrases such as electron-hole theory, crystalline atomic lattice theory, and impurity doping are some of the concepts involved and would require a book onto themselves to explain.  For the purposes of this article all we have to know is that semiconductors have two properties.  The first property is that of an electrical conductor, that is, a material which allows electric current to pass through it.  Copper wire is a good example of this.  The second property is that of an electrical insulator, which blocks the flow of electric current.  Materials such as glass, wood, and rubber fall into the insulator category.

     A photo of a diode is shown in Figure 1, along with its symbol used in electrical schematics.

A diode and a diode schematic symbol.

Figure 1


     When electric current flows through a diode in one direction, as shown in Figure 2, the semiconductor material inside of it acts as a conductor, ushering it along a single path.

diode one way current flow

Figure 2


     When current tries to flow through the diode in the reverse direction, the semiconductor material acts as an insulator.  That is, it blocks the flow of current as shown in Figure 3.

diode stops reverse flow of electric current

Figure 3


     So we see that diodes can act like one way streets, restricting current flow.  But, not all diodes work this way.  Next week we’ll introduce a special kind of diode called the Zener diode, which allows current to flow in two different directions, and we’ll see how this functionality is put to work in regulated power supplies.


Transistors – Voltage Regulation Part II

Sunday, July 29th, 2012
     I joined the Boy Scouts of America as a high schooler, mainly so I could participate in their Explorer Scout program and learn about electronics.  I will forever be grateful to the Western Electric engineers who volunteered their personal time to stay after work and help me and my fellow Scouts build electronic projects.  The neatest part of the whole experience was when I built my first regulated power supply with their assistance inside their lab.  But in order to appreciate the beauty of a regulated power supply we must first understand the shortcomings of an unregulated one, which we’ll begin to do here.

     Last time we began to discuss how the output voltage of an unregulated power supply can vary in response to power demand, just as when sprinklers don’t have sufficient water flow to cover a section of lawn.  Let’s explore this concept further.

Figure 1


     Figure 1 shows a very basic representation of a microprocessor control system that operates three components, an electric relay (shown in the blue box), buzzer, and light.  These three components have a certain degree of internal electrical resistance, annotated as RR, RB, and RL respectively.  This is because they are made of materials with inherent imperfections which tend to resist the flow of electric current.  Imperfections such as these are unavoidable in any electronic device made by humans, due to impurities within metals and irregularities in molecular structure.  When the three components are activated by the microprocessor chip via field effect transistors, denoted as FET 1, 2 and 3 in the diagram, their resistances are connected to the supply circuit.

     In other words, RR, RB, and RL create a combined level of resistance in the supply circuit by their connectivity to it.  If a single component were to be removed from the circuit, its internal resistance would also be removed, resulting in a commensurate decrease in total resistance.  The greater the total resistance, the more restriction there is to current flow, denoted as I.  The greater the resistance, the more I is caused to decrease.  In contrast, if there is less total resistance, I increases.

     The result of changing current flow resistance is that it causes the unregulated power supply output voltage to change.  This is all due to an interesting phenomenon known as Ohm’s Law, represented as this within engineering circles:

V = I × R

where, V is the voltage supplied to a circuit, I is the electrical current flowing through the circuit, and R is the total electrical resistance of the circuit.  So, according to Ohm’s Law, when I and R change, then V changes.

     Next time we’ll apply Ohm’s Law to a simplified unregulated power supply circuit schematic.  In so doing we’ll discover the mathematical explanation to the change in current flow and accompanying change in power supply output voltage we’ve been discussing.