Posts Tagged ‘voltage regulator’

Transistors – Voltage Regulation Part XV

Sunday, October 28th, 2012

     We’ve been discussing the advantages of using limiting resistors within Zener diode regulating circuits to lessen the probability of circuit failure. Today we’ll continue our discussion, focusing on the Zener diode’s advantages and disadvantages.  See Figure 1.

Zener diode voltage regulator circuit

Figure 1


     Figure 1 discloses the simplicity of a voltage regulator employing a Zener diode.  There are only two components, a limiting resistor, RLimiting, and the Zener diode itself, which also makes the entire assembly cost effective to manufacture. 

     Despite the obvious advantages, there is one major disadvantage to the Zener diode voltage regulator.  Ironically, the limitation imposed by RLimiting on the current IPS itself creates an operational dilemma.  When electronic devices are connected to the output terminals of the regulator, RLimiting  and its current-limiting action becomes a disadvantage.  See Figure 2.

electronic voltage regulator

Figure 2


     The amount of current I available to flow through to electronic devices is limited, sometimes too much, and the net result is that the Zener diode voltage regulator can only be used to power electronic devices drawing small amounts of current.  It is unacceptable for many applications, such as powering kitchen appliances or flat screen TVs.

     Next time we’ll see how to improve upon the Zener diode voltage regulator circuit by adding a transistor.  This will eliminate the road block imposed by RLimiting, thus allowing higher, but still regulated, current to flow through to the output terminals.


Transistors – Voltage Regulation Part XIV

Monday, October 22nd, 2012

     As we’ve come to know through this series of blogs, all electronic components pose some degree of internal resistance to the electric current flowing through them.  This resistance results in electrical energy being converted into heat energy, heat which poses potential problems to sensitive components like electronic circuit boards.  If things get hot enough, components fail and fires may ignite. 

     To address these issues engineers design circuits with resistors whose job it is to limit the current flowing to electrical components.  In this article we’ll see how a limiting resistor protects a Zener diode from this fate, allowing it to continue doing its job of regulating voltage.    

     In our last blog we applied Ohm’s Law to our regulated power supply circuit, which makes use of a Zener diode.  See Figure 1.power supply

Figure 1


     Ohm’s Law gave us the following equation to determine the amount of current, IPS, flowing from the unregulated power supply portion, through the current limiting resistor RLimiting, and making its way into the rest of the circuit:

IPS = (VUnregulatedVZener) ÷ RLimiting

     We learned last week that for the circuit to work, the voltage of the unregulated power supply portion of the circuit, VUnregulated, must be greater than the Zener voltage, VZener.

     Looking at the equation above, we see that the voltage difference is divided by RLimiting, the value of the limiting resistor in the circuit.  This limiting resistor is there to constrain the current flowing to the Zener diode, allowing the diode to keep things under control within the circuit. 

     Basic mathematical principles hold that if a smaller number is divided by a bigger number, the resulting answer is an even smaller number.  Applying this principle to the equation above, if RLimiting is a big number, then IPS must be a smaller number.  On the other hand the smaller RLimiting gets, the bigger IPS becomes. 

     So what does it take for our circuit to fail?  Remove the limiting resistor as shown in Figure 2 and the value for RLimiting disappears.  In other words, RLimiting becomes zero.

zener diode with no limiting resistor

Figure 2


     In this case our Ohm’s Law equation becomes:

IPS = (VUnregulatedVZener) ÷ 0 =

     The resulting answer is said to go to infinity, or , as it is represented mathematically.  In other words, without a limiting resistor being employed within our circuit, IPS will become so large it will overwhelm the diode’s current handling capacity and lead to circuit failure. 

     Next time we’ll go over some advantages and disadvantages of this Zener diode voltage regulating circuit, and why the disadvantages outweigh the advantages for many applications.


Transistors – Voltage Regulation Part XII

Sunday, October 7th, 2012

     Let’s continue our discussion with regard to the example circuit discussed last time and 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.

Zener diode voltage regulator

Figure 1


     To recap our discussion from last week, the unregulated power supply portion of the circuit in Figure 1 generates an unregulated voltage, VUnregulated.  Then the Zener diode, which acts as a voltage regulator, takes in VUnregulated and converts it into a steady output voltage, VOutput.  Because these output terminals are connected to the ends of the Zener diode, VOutput  is equal to the voltage put out by it, denoted as VZener.

     The Zener diode, an excellent negotiator of current, is essentially involved in a constant trade off, substituting electric current that originates in the unregulated power supply portion of the circuit for voltage, VOutput, that will serve to power the external supply circuit.   In other words, the Zener diode draws as much current, IZ, through it as it needs, its objective being to keep VOutput at a constant level, and it will continue to provide this constant output, despite the fact that VUnregulated varies considerably.

     So, where does the current IZ come from?  From IPS, that is, the current flowing from the unregulated power supply area, as shown in Figure 1.  

     IPS flows through the limiting resistor to a junction within the circuit.  At this junction, IZ splits off from IPS and continues on to the Zener diode, while current I splits off from IPS on its way to the total internal resistance, RTotal, in the external supply circuit.  

     What this means is that when you add IZ and I together, you get IPS.  Mathematically speaking this is represented as:

IPS = IZ + I

     Why solve for IPS?  We’ll see why this is important when we revisit Ohm’s Law next week and gain a fuller understanding of how IPS, VUnregulated, VZener, and RLimiting relate to each other with regard to the Zener diode.  


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 VIII

Sunday, September 9th, 2012
     Back in the early 1970s my dad, a notorious tightwad, coughed up several hundred dollars to buy his first portable color television.  That was a small fortune back then.  The TV was massive, standing at 24 inches wide, 18 inches high, and 24 inches deep, and weighing in at about 50 pounds.  I think the only thing that made this behemoth “portable” was the fact that it had a carrying handle on top.

     A major reason for our old TV being so big and clunky was of course due to limitations in technology of the time.  Many large, heavy, and expensive electronic components were needed to make it work, requiring a lot of space for the circuitry.  By comparison, modern flat screen televisions and other electronic devices are small and compact because advances in technology enable them to work with far fewer electronic components.  These components are also smaller, lighter, and cheaper.

     Last time we looked at the components of a simple unregulated power supply to see how it converts 120 volts alternating current (VAC) to 12 volts direct current (VDC).  We discovered that the output voltage of the supply is totally dependent on the design of the transformer, because the transformer in our example can only produce one voltage, 12 VDC.  This of course limits the supply’s usefulness in that it is unable to power multiple electronic devices requiring two or more voltages, such as we’ll be discussing a bit further down.

     Now let’s illustrate this power supply limitation by revisiting our microprocessor control circuit example which we introduced in a previous article in this series on transistors. 

microprocessor control

Figure 1


     In Figure 1 we have to decide what kind of power to supply to the circuit, but we have a problem.  Sure, the unregulated power supply that we just discussed is up to the task of providing the 12 VDC needed to supply power for the buzzer, light, and electric relay.  But let’s not forget about powering the microprocessor chip.  It needs only 5 VDC to operate and will get damaged and malfunction on the higher 12 VDC the current power supply provides.  Our power supply just isn’t equipped to provide the two voltages required by the circuit.

     We could try and get around this problem by adding a second unregulated power supply with a transformer designed to convert 120 VAC to 5 VAC.  But, reminiscent of the circuitry in my dad’s clunky old portable color TV, the second power supply would require substantially more space in order to accommodate an additional transformer, diode bridge, and capacitor.  Another thing to consider is that transformers aren’t cheap, and they tend to have some heft to them due to their iron cores, so more cost and weight would be added to the circuit as well.  For these reasons the use of a second power supply is a poor option.

     Next time we’ll look at how adding a transistor voltage regulator circuit to the supply results in cost, size, and weight savings.  It also results in a more flexible and dependable output voltage.


Transistors – Digital Control Interface, Part V

Sunday, July 15th, 2012
     ­­­­­Last time we looked at my electric relay solution to a problem presented by a 120 volt alternating current (VAC) drive motor operating within an x-ray film processing machine.  Now let’s see what happens when we press the button to set the microprocessor into operation. 

 electronic control

Figure 1


     Figure 1 shows that when the button is depressed, the computer program contained within the microprocessor chip goes into action, signaling the start of the control initiative.  5 volts direct current (VDC) is supplied to Output Lead 2, and FET 2 (Field Effect Transistor 2) becomes activated, which allows electric current from the 12 VDC supply to course into the 12 VDC electric relay, through the relay’s wire coil, then conclude its travel into electrical ground.

     The electric relay components, including a wire coil, steel armature, spring, and normally open (N.O.) contact, are shown within a blue box in our illustration.  Current flow is represented by red lines.  The control initiative passes from the microprocessor to FET 2, and then to the 12 VDC electric relay, just as the Olympic Torch is relayed through a system of runners.

     We learned in one of my previous articles on industrial control that when an electric relay coil is energized, electromagnetic attraction pulls its steel armature towards the wire coil and the N.O. electrical contact.  In Figure 1 this attraction is represented by a blue arrow.  With the N.O. contact closed the drive motor is connected to the 120 VAC input, and the motor is activated.

microprocessor control

Figure 2


     Figure 2 shows what happens after the button is depressed.  The computer program is activated, directing the microprocessor chip to keep 5 VDC on Output Lead 2 and FET 2 while the prerequisite 40 minutes elapses.  Thus the relay remains energized and the motor remains on during this time.


Figure 3


     In Figure 3, at the end of the 40 minute countdown, the computer program applies 0 VDC to Output Lead 2.  FET 2 then turns off the current flow to the relay and it begins to de-energize, causing the spring to pull the steel armature away from the N.O. contact and the 120 VAC power supply to be interrupted.  The motor is deactivated.

     At the same time, the computer program applies 5 VDC to Output Lead 1 and FET 1 for 2 seconds.  FET 1 turns on the flow of current through the buzzer, causing it to sound off and signal that the x-ray film processing machine is ready for use.

     Next time we’ll look at how transistors are used to regulate voltage within direct current power supplies like the one shown in Figure 3 above.


Transistors – Digital Control Interface, Part II

Sunday, June 24th, 2012
     Not too long ago I was retained as an engineering expert to testify on behalf of a plaintiff who owned a sports bar.  The place was filled with flat screen televisions that were plugged into 120 volt alternating current (VAC) wall outlets.  To make a long story short, the electric utility wires that fed power to the bar were hit by a passing vehicle, causing the voltage in the outlets to increase well beyond what the electronics in the televisions could handle.  The delicate electronics were not suited to be connected with the high voltage that suddenly surged through them as a result of the hit, and they overloaded and failed.

     Similarly, lower voltage microprocessor and digital logic chips are also not suited to directly connect with higher voltage devices like motors, electrical relays, and light bulbs.  An interface between the two is needed to keep the delicate electronic circuits in the chips from overloading and failing like the ill fated televisions in my client’s sports bar.  Let’s look now at how a field effect transistor (FET) acts as the interface between low and high voltages when put into operation within an industrial product.

     I was once asked to design an industrial product, a machine which developed medical x-ray films, utilizing a microprocessor chip to automate its operation.  The design requirements stated that the product be powered by a 120 VAC, such as that available through the nearest wall outlet.  In terms of functionality, upon startup the microprocessor chip was to be programmed to first perform a 40-minute warmup of the machine, then activate a 12 volt direct current (VDC) buzzer for two seconds, signaling that it was ready for use.  This sequence was to be initiated by a human operator depressing an activation button.

     The problem presented by this scenario was that the microprocessor chip manufacturer designed it to operate on a mere 5 VDC.  In additional, it was equipped with a digital output lead that was limited in functionality to either “on” or “off” and capable of only supplying either extreme of 0 VDC or 5 VDC, not the 12 VDC required by the buzzer.

     Figure 1 illustrates my solution to this voltage problem, although the diagram shown presents a highly simplified version of the end solution.

microprocessor control

Figure 1

     The illustration shows the initial power supplied at the upper left to be 120 VAC.  This then is converted down to 5 VDC and 12 VDC respectively by a power supply circuit. The 5 VDC powers the microprocessor chip and the 12 VDC powers the buzzer.  The conversion from high 120 VAC voltage to low 5 and 12 VDC voltage is accomplished through the use of a transformer, a diode bridge, and special transistors that regulate voltage.  Since this article is about FETs, we’ll discuss transistor power supplies in more depth in a future article.

     To make things a little easier to follow, the diagram in Figure 1 shows the microprocessor chip with only one input lead and one output lead.  In actuality a microprocessor chip can have dozens of input and output leads, as was the case in my solution.  The input leads collect information from sensors, switches, and other electrical components for processing and decision making by the computer program contained within the chip.  Output leads then send out commands in the form of digital signals that are either 0 VDC or 5 VDC.  In other words, off or on.  The net result is that these signals are turned off or on by the program’s decision making process.

     Figure 1 shows the input lead is connected to a pushbutton activated by a human.  The output lead is connected to the gate (G) of the FET.  The FET is shown in symbolic form in green. The FET drain (D) lead is connected to the buzzer and its source (S) lead terminates in connection to electrical ground to complete the electrical circuit.  Remember, electric current naturally likes to flow from the supply source to electrical ground within circuits, and our scenario is no exception.

     Next time we’ll see what happens when someone presses the button to put everything into action.