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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. |
Posts Tagged ‘regulated power supply’
Transistors – Voltage Regulation, Final Chapter
Monday, November 19th, 2012
Transistors – Voltage Regulation Part XVI
Monday, November 5th, 2012
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. ____________________________________________ |
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.
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. 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.
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.
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.
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. ____________________________________________ |
