Electronics PDF

Capacitors hold charge when disconnected from power supply. Dielectric keeps charge from jumping from one plate to another. Lightening is a giant capacitive charge discharging. 1 Farad is equal to 1 amp of current at 1 volt for 1 second. Capacitors we work with are typically measured in Micro- Farad...

Capacitors hold charge when disconnected from power supply. Dielectric keeps charge from jumping from one plate to another. Lightening is a giant capacitive charge discharging. 1 Farad is equal to 1 amp of current at 1 volt for 1 second. Capacitors we work with are typically measured in Micro- Farads (µF) and Pico Farads (pF). Common uses of capacitors are camera flashes, lasers, decoupling noise, smoothing power supplies, timing etc. Capacitors - RC Time: Capacitors take time to charge and discharge, according to the amount of current. The charge/discharge time of capacitors is controlled using resistors. Charge time (to 63.2% of supply voltage) and discharge time (to 36.8% of supply voltage) is nicely equal to R*C (in seconds). RC Time allows us to control the rate that things happen in circuits, which turns out to be very useful. Capacitor Types: Three major types of capacitors are ceramic, electrolytic, and tantalum. Ceramic capacitors are small in size and value, ranging from a few Pico Farads to 1 µF. Not polarized, so either end can go to ground. Value is given by a code somewhat like that of resistors. Electrolytic capacitors look like small cylinders and range in value from 1 µF to several Farads. Very inaccurate and change in value as the electrolytic ages. Polarized, cathode must go to ground. Cathode is marked with a minus sign on case. Value is usually written on case. 10 Tantalum capacitors are similar in size to ceramic but can hold more charge, up to several hundred µF. Accurate and stable, but relatively expensive. Usually polarized anode is marked with a plus sign. Semiconductors: It is probably the most important discovery in electronics which happened last century. Without this discovery we wouldn't have televisions, computers, space rocket, CD players, etc. Unfortunately it's also one of the hardest areas to understand in electronics. The reason that makes metals such good conductors is that they have lots of electrons which are so loosely held that they're easily able to move when a voltage is applied. Insulators have fixed electrons and so are not able to conduct. Certain materials, called semiconductors, are insulators that have a few loose electrons. They are partly able to conduct a current. The free electrons in semiconductors leave behind a fixed positive charge when they move about (the protons in the atoms they come from). Charged atoms are called ions. The positive ions in semiconductors are able to capture electrons from nearby atoms. When an electron is captured another atom in the semiconductor becomes a positive ion. This behavior can be thought of as a 'hole' moving about the material, moving in just the same way that electrons move. So now there are two ways of conducting a current through a semiconductor, electrons moving in one direction and holes in the other. The holes don't really move of course. It is just fixed positive ions grabbing neighboring electrons, but it appears as if holes are moving. 11 In a pure semiconductor there are not enough free electrons and holes to be of much use. Their number can be greatly increased however by adding an impurity, called a donor. If the donor gives up some extra free electrons we get an n-type semiconductor (n for negative). If the donor soaks up some of the free electrons we get a p-type semiconductor (p for positive). In both cases the impurity donates extra current carriers to the semiconductor. Adding impurities is called doping. In n-type semiconductors there are more electrons than holes and they are the main current carriers. In p-type semiconductors there are more holes than electrons and they are the main current carriers. The donor atoms become either positive ions (n-type) or negative ions (p-type). The most common semiconductors are silicon (basically sand) and germanium. Common donors are arsenic and phosphorus. When we combine n-type and p-type semiconductors together we make useful devices, like transistors, diodes and chips. The Diode: Simplest useful semiconductor that allows current flow from anode to cathode but not in reverse. Cathode goes to ground. 12 Diode applications examples 1) Reverse polarity protection. 2) Reverse biased diode in parallel with an inductive load will snub the blowback current generated by the collapsing magnetic field. 3) Rectifier converts AC into DC. A diode consists of a piece of n-type and a piece of p-type semiconductor joined together to form a junction. Electrons in the n-type half of the diode are repelled away from the junction by the negative ions in the p-type region, and holes in the p-type half are repelled by the positive ions in the n-type region. A space on either side of the junction is left without either kind of current carriers. This is known as the depletion layer because there are no current carriers in this layer, so current can flow. The depletion layer is, in effect, an insulator. 13 Consider what would happen if we connected a small voltage to the diode. Connected one way it would attract the current carriers away from the junction and make the depletion layer wider. Connected the other way it would repel the carriers and drive them towards the junction, so reducing the depletion layer. In neither case would any current flow because there would always be some of the depletion layer left. Now consider increasing the voltage. In one direction there is still no current because the depletion layer is even wider (reverse biased), but in the other direction the layer disappears completely and current can flow (forward biased). Above a certain voltage the diode acts like a conductor. As electrons and holes meet each other at the junction they combine and disappear. Thus a diode is a device which is an insulator in one direction and a conductor in the other. Diodes are extremely useful components. We can stop currents going where we don't want them to go. For example we can protect a circuit against the battery being connected backwards which might otherwise damage it. Zener Diodes: Conducts in reverse-bias direction at a specific breakdown voltage. It is used to provide reference voltage. 14 LED (Light Emitting Diode) Light emitting diodes (LEDS) are special diodes that give out light when they conduct. The fact that they only conduct in one direction is often incidental to their use in a circuit. They are usually just being used as lights. They are small and cheap and they last practically forever, unlike traditional light bulbs which can burn out. The light comes from the energy given up when electrons combine with holes at the junction. The color of the light depends on the impurity in the semiconductor. It is easy to make bright red, green and yellow LEDS but technology can't make cheap LEDS of other colors like white or blue. The symbol and a few examples of this type is shown below (Note the cathode on the component is shown as a flat edge or the The Transistor Transistors underpin the whole of modern day electronics. They are found in watches, calculators, microwaves, hi-fi's. A Pentium computer chip contains over a million transistors. Transistors work in two ways. They can work as switches (turning currents on and off) and as amplifiers (making currents bigger). When acting as an amplifier they operate in the linear mode and as a switch they are forced into saturation (on) or cut off (off). Transistors are sandwiches of three pieces of semiconductor material. A thin slice of n-type or p-type semiconductor is sandwiched between two layers of the opposite type. This gives two junctions rather than the one found in a diode. If the thin slice is n-type the transistor is called a p-n-p transistor, 15 and if the thin slice is p-type it is called an n-p-n transistor. The middle layer is always called the base, and the outer two layers are called the collector and the emitter. In an n-p-n transistor (more common), electrons are the main current carriers (because n- type material predominates). When no voltage is connected to the base then the transistor is equivalent to two diodes connected back to back. Recall that current can only flow one way through a diode. A pair of back-to-back diodes can't conduct at all. If a small voltage is applied to the base (enough to remove the depletion layer in the lower junction), current flows from emitter to base like a normal diode. Once current is flowing however it is able to sweep straight through the very thin base region and into the collector, only a small part of the current flows out of the base. The transistor is now conducting through both junctions. A few of the electrons are consumed by the holes in the p-type region of the base, but most of them go straight through. Electrons enter the emitter from the battery and come out of the collector. To see how a transistor acts as a switch, a small voltage applied to the base will switch the transistor on, allowing a current to flow in the rest of the transistor. NPN and PNP Transistor components look identical to each other the only way to tell the difference is by the component number. The symbol and a few examples of this type are shown below: 16 Transistor Basics: Use three layers of silicon and can be used as a switch or an amplifier. Processor chips are lots and lots of transistors in one package. Transistors have three leads - the base, collector and emitter. Bipolar versus Field Effect Transistors: There are two main families of transistors, Bipolar and FET. FETs are more popular, waste less power (therefore run cooler), and are cheaper than bipolar. FETs can be easily damaged by static electricity, so this explains why bipolar types are used for teaching and training students. The basic operation of bipolar and FETs are the same. NPN versus PNP: In NPN, the base is at a higher voltage than the emitter, current flows from collector to emitter. A small amount of current also flows from base to emitter. NPN Voltage at base controls amount of current flow through transistor (collector to emitter). In PNP, the base is at a lower voltage than the emitter, current flows from emitter to collector. A small amount of current also flows from emitter to base. PNP. Voltage at base controls amount of current flow through transistor (emitter to collector). The arrow represents the direction of current flow. Transistor as switch: Most sensors, processors, microcontrollers can't source enough power to make things happen in the real world. Transistors allow a large amount of current to be controlled by a small change in voltage. Grounds between control circuit and transistor must be common. 17