## Posts Tagged ‘circuit’

### 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.

## 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.

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### Transistors – Voltage Regulation Part XVII

Monday, November 12th, 2012

Last time we learned about a new type of transistor called a bipolar transistor and how it controls the flow of electric current traveling from the collector to the emitter within our circuit.  We also saw how the bipolar transistor is integrated within a Zener diode voltage regulator circuit to make a new type of circuit called a transistor series voltage regulator.

Now let’s see how this all works by hooking our circuit up to both an unregulated power supply and an external supply circuit as shown in Figure 1.

## Figure 1

When voltage VUnregulated is applied to our transistor series voltage regulator circuit by way of an unregulated power supply, electric current flows through RLimiting into the base, B, of the transistor.  The transistor senses this current and responds by opening a path for current to flow from its collector, C, to its emitter, E.  With this path established, current flows freely from the unregulated power supply, through the transistor’s collector and emitter, on to the output terminal, and finally to the external supply circuit.  Total resistance of this circuit is said to be RTotal

At this point you’re probably wondering why the bipolar transistor base and Zener diode are connected to RLimiting.  Next time we’ll conclude our series by seeing how this connection is crucial to the functionality of our transistor series voltage regulator.

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### 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.

## 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.

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### Transistors – Voltage Regulation Part XIII

Monday, October 15th, 2012

Last time we learned how the Zener diode, an excellent negotiator of current, is involved in a constant trade off, exchanging current for voltage so as to maintain a constant voltage.  It draws as much current through it as is required to maintain a consistent voltage value across its leads, essentially acting as voltage regulator in order to protect sensitive electronic components from power fluctuations.

Now let’s revisit our example power supply circuit and see how Ohm’s Law is used to determine the amount of electric current, IPS, that flows from the unregulated power supply and why this is important to the function of the Zener diode.  See Figure 1.

## Figure 1

If you’ll recall, Ohm’s Law states that current flowing through a resistor is equal to the voltage across the resistor divided by its electrical resistance.  In our example that would be IPS flowing through to RLimiting.  In fact, the voltage across RLimiting is the difference between the voltages at each of its ends.

Applying this knowledge to our circuit, the voltage on one end is VUnregulated, while the voltage at the other is VZener.  According to Ohm’s Law the equation which allows us to solve for IPS is written as:

IPS = (VUnregulatedVZener) ÷ RLimiting

And if we have a situation where VUnregulated equals VZener , such as when the voltage of an unregulated power supply like a battery equals the Zener voltage of a Zener diode, then the equation becomes:

(VUnregulatedVZener ) = 0

And if this is true, then the following is also true:

IPS = 0 ÷ RLimiting = 0

In other words, this equation tells us that if VUnregulated is equal to VZener, then the current IPS will cease to flow from the unregulated portion of the circuit towards the Zener diode and the external supply circuit.  Put another way, in order for IPS to flow and the circuit to work, VUnregulated must be greater than VZener.

Next week we’ll continue our discussion and see why the resistor RLimiting is necessary in order to prevent the circuit from self destructing.

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### 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.

## 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.

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### 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.

## 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.

## 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.

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### 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.

## 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.

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### Transistors – Voltage Regulation Part VI

Sunday, August 26th, 2012
Believe it or not as a kid in grade school I used to hate math, particularly algebra.  None of my teachers were able to decipher its complexities and render it comprehensible to me or the majority of my classmates.  Then in high school everything changed.  I had Mr. Coleman for freshman algebra, and he had a way of making it both understandable and fun, in a challenging kind of way.  With 40 years of teaching under his belt, Mr. Coleman knew exactly how to convey the required information in an understandable manner, and to this day I find his insights useful in solving engineering calculations.

Last time we began our discussion on Ohm’s Law and how it may be applied to our example circuit to solve for the electrical current flowing through it.  Let’s continue our discussion to see how the Law applies to only one part of the circuit.  Then, we’ll use a little algebra to show how the output voltage of an unregulated power supply is affected by changes in RTotal.

## Figure 1

To help us see things more clearly, in Figure 1 we’ll cover up the inside workings of the unregulated power supply side of the circuit and concentrate on the external supply part of the circuit alone.  Since RTotal is connected to the terminals of the power supply, the voltage applied to RTotal is the same as the power supply output voltage, VOutput.

In my previous article, we learned that according to Ohm’s Law, the current flowing through a resistance is equal to the voltage applied to it, divided by the resistance.  The fact that RTotal is connected to the two output terminals like we see in Figure 1, allows us to use Ohm’s law to solve for the electrical current, I, flowing through  RTotal:

I = VOutput ÷ RTotal

Now let’s pull the cover off of the unregulated power supply again to see what’s going on within the circuit as a whole.

## Figure 2

In Figure 2 we can see that the current, I, flowing through RTotal is the same current flowing through the balance of the circuit.  In the preceding blog we found that value to be:

I = VDC ÷ (RInternal + RTotal)

We can combine the above two equations for I to develop an algebraic relationship between VOutput and RInternal, RTotal, and VDC:

VOutput ÷ RTotal   =  VDC ÷ (RInternal + RTotal)

Then, by rearranging terms and applying the cross multiplication principle of algebra we can solve for VOutput.  This involves multiplying both sides of the equation by RTotal:

VOutput =  RTotal × (VDC ÷ (RInternal +RTotal))

This equation tells us that although RInternal doesn’t fluctuate, VOutput will fluctuate when RTotal does.  This fact is demonstrated in our equation when we make use of algebra.  That is to say, when a term changes on one side of the equation, it causes the other side of the equation to change as well.  In this case, when RTotal  changes, it causes VOutput to change in proportion to the fixed values of VDC and RInternal.

Next time we’ll look at another shortcoming of unregulated power supplies, more specifically, how one supply can’t power multiple electrical circuits comprised of different voltages.

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### Transistors – Voltage Regulation Part V

Sunday, August 19th, 2012
I’m sure you’ve seen the television commercials warning about harmful interactions between prescription medications.  By the same token electronic circuitry can also be adversely affected by certain combinations of electrical components, as we’ll discuss in today’s blog.

Last time we looked at a circuit schematic containing an unregulated power supply.  This power supply was connected to an external supply circuit containing a number of components such as electric relays, buzzers, and lights.  Each of these components has a resistance factor, and combined they have a total resistance of RTotal.  We saw that when RTotal increases, the electrical current, I, decreases, and when RTotal decreases, I increases.

In contrast to this increasing/decreasing activity of the total resistance RTotal,  the fixed internal resistance of the unregulated power supply, RInternal, doesn’t fluctuate.  Let’s explore Ohm’s Law further to see how the static effect of RInternal  combines with the changing resistance present in RTotal to adversely affect the unregulated power supply output voltage, VOutput, causing it to fluctuate.

## Figure 1

In Figure 1 RTotal and RInternal are operating in series, meaning they are connected together like sausage links.  In this configuration their two resistances add together as if they were one larger resistor.

Generally speaking, Ohm’s Law sets out that the current, I, flowing through a resistor in an electrical circuit equals the voltage, V, applied to the resistor divided by the resistance R, or:

I = V ÷ R

In the case of the circuit represented in Figure 1, the resistors RInternal and RTotal are connected in series within the circuit, so their resistances must be added together to arrive at a total power demand.  Voltage is applied to these two resistors by the same voltage source, VDC.  So, for the circuit as a whole Ohm’s Law would be written as:

I = VDC ÷ (RInternal + RTotal)

But, Ohm’s Law can also be applied to individual parts within the circuit, just as it can be applied to a single kitchen appliance being operated on a circuit shared with other appliances.  Let’s see how this applies to our example circuit’s RTotal next week.

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### Transistors – Voltage Regulation Part IV

Sunday, August 12th, 2012
We’ve all popped a circuit breaker sometime in our lives, often the result of making too heavy of an electrical demand in a single area of the house to which that circuit is dedicated.  Like when you’re making dinner and operating the microwave, toaster, mixer, blender, food processor, and television simultaneously.  The demand for current on a single circuit can be taxed to the max, causing it to pop the circuit breaker and requiring that trip to the electrical box to flip the switch back on.

Last time we began our discussion on unregulated power supplies and how they’re affected by power demands within their circuits.  Our schematic shows there are two basic aspects to the circuit, namely, its direct current source, or VDC,  and its internal resistance, RInternal.  Now let’s connect the power supply output terminals to an external supply circuit through which electrical current will be provided to peripheral devices, much like all the kitchen gadgets mentioned above.

## Figure 1

The external supply circuit shown in Figure 1 contains various electronic components, including electric relays, lights, and buzzers, and each of these has its own internal resistance.  Combined, their total resistance is RTotal, as shown in our schematic.

Current, notated as I, circulates through the power supply, through the external supply circuit, and then returns back to the power supply.  The current circulates because the voltage, VDC, pushes it through the circuit like pressure from a pump causes water to flow through a pipe.

RTotal and I can change, that is, increase or decrease, depending on how many components the microprocessor has turned on or off within the external supply circuit at any given time.  When RTotal increases, electrical current, I, decreases.  When RTotal decreases, electrical current I increases.

Next time we’ll continue our discussion on Ohm’s Law, introduced last week, to show how the static effect of RInternal  interacts with the changing resistance present in RTotal to adversely affect an unregulated power supply’s output voltage.

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