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- 1. TECHNOLOGICAL UNIVERSITY OF THE PHILIPPINES AYALA BOULEVARD, ERMITA, MANILA COLLEGE OF ENGINEERING ASSIGNMET NO.3 Zener Diode, LED, and Seven Segment Display Submitted by: DATE GIVEN: SEPTEMBER 14, 2016 Llorera, Jeshua B. DATE SUBMITTED: SEPTEMBER 21, 2016 BSME-4D Submitted To: ENGR. EDWIN C. ESPINAS INSTRUCTOR RATING
- 2. ZENER DIODE Applications: Zener Diode as a voltage In a DC circuit, Zener diode can be used as a voltage regulator or to provide voltage reference. The main use of zener diode lies in the fact that the voltage across a Zener diode remains constant for a larger change in current. This makes it possible to use a Zener diode as a constant voltage device or a voltage regulator. In any power supply circuit, a regulator is used to provide a constant output (load) voltage irrespective of variation in input voltage or variation in load current. The variation in input voltage is called line regulation, whereas the variation in load current is called load regulation. Zener Diode as voltage regulator A simple circuit involving Zener diode as a regulator requires a resistor of low value connected in series with the input voltage source. The low value is required so as to allow the maximum flow of current through the diode, connected in parallel. However, the only constraint being, the current through zener diode should not be less than minimum zener diode current. Simply put, for a minimum input voltage and a maximum load current, the Zener diode current should always be Izmin. While designing a voltage regulator using zener diode, the latter is chosen with respect to its maximum power rating. In other words, the maximum current through the device should be:- Imax = Power/Zener Voltage Since the input voltage and the required output voltage is known, it is easier to choose a zener diode with a voltage approximately equal to the load voltage, i.e. Vz ~=Vo. The value of the series resistor is chosen to be R = (Vin – Vz)/ (Izmin + IL), where IL = Load Voltage/Load resistance.
- 3. Note that for load voltages up to 8V, a single zener diode can be used. However for load voltages beyond 8V, requiring Zener voltages of higher voltage value, it is advisable to use a forward biased diode in series with the Zener diode. This is because the Zener diode at higher voltage follows the avalanche breakdown principle, having a positive temperature of coefficient. Hence a negative temperature coefficient diode is used for compensation. Of course, these days, practical temperature compensated Zener diodes are used. Zener Diode as a voltage reference Zener diode as voltage reference In power supplies and many other circuits, Zener diode finds its application as a constant voltage provider or a voltage reference. The only conditions are that the input voltage should be greater than zener voltage and the series resistor should have a minimum value such that the maximum current flows through the device. Zener Diode as a voltage clamper In a circuit involving AC input source, different from the normal PN diode clamping circuit, a Zener diode can also be used. The diode can be used to limit the peak of the output voltage to zener voltage at one side and to about 0V at other side of the sinusoidal waveform.
- 4. zener diode as voltage clamper In the above circuit, during positive half cycle, once the input voltage is such that the zener diode is reverse biased, the output voltage is constant for a certain amount of time till the voltage starts decreasing. Now during the negative half cycle, the zener diode is in forward biased connection. As the negative voltage increases till forward threshold voltage, the diode starts conducting and the negative side of the output voltage is limited to the threshold voltage. Note that to get an output voltage in positive range only, use two oppositely biased Zener diodes in series. Construction: The Zener diode's operation depends on the heavy doping of its p-n junction. The depletion region formed in the diode is very thin (<1 µm) and the electric field is consequently very high (about 500 kV/m) even for a small reverse bias voltage of about 5 V, allowing electrons to tunnel from the valence band of the p-type material to the conduction band of the n-type material. At the atomic scale, this tunnelling corresponds to the transport of valence band electrons into the empty conduction band states; as a result of the reduced barrier between these bands and high electric fields that are induced due to the relatively high levels of doping on both sides. The breakdown voltage can be controlled quite accurately in the doping process. While tolerances within 0.07% are available, the most widely used tolerances are 5% and 10%. Breakdown voltage for commonly available Zener diodes can vary widely from 1.2 volts to 200 volts. For diodes that are lightly doped the breakdown is dominated by the avalanche effect rather than the Zener effect. Consequently, the breakdown voltage is higher (over 5.6 V) for these devices.
- 5. Characteristics: The Zener Diode goes through a number of different regions or stages, of which are explained below. The right half side of the characteristics curve is the part in which the zener diode receives forward voltage, which is the positive voltage across its anode to cathode terminals. The diode in this region is in forward biased. During this period, the current is small for a while until it spikes exponentially up once the voltage reaches a certain point, called the threshold voltage. The left half side of the characteristics curve is the more important part, when considering zener diodes. This is the part in which the zener diode receives positive voltage across its cathode to anode terminals. The diode in this region is in reverse biased. At first, when receiving reverse voltage, the current is very small. There is only a small current, called the leakage current, flowing through the diode. Once it hits the breakdown voltage, the current drastically increases. This current is the called the avalanche current, because it spikes so drastically up. The breakdown voltage point is also important, not just because of the avalanche current, but more importantly because once the voltage of the zener diode has reached this point, it remains constant at this voltage, even though the current across it may increase largely. This makes the zener diode useful in applications such as voltage regulation. The zener diode has the important characteristic in that once the voltage across a zener has reached this breakdown voltage, also called a zener diode's zener voltage, VZ, the voltage that a zener drops across itself will not continue to increase. For example, if a zener diode has a zener voltage of 5.1V, and the voltage feeding the diode is approximately around 5.1V, the zener will drop the 5.1V across its terminals. Now if the voltage supplying it continues to increase, to say 12V, the zener diode will maintain its zener voltage, 5.1V, even though the voltage (and current) supplying it continues to increase. This is the single most important characteristic of a zener diode, which as stated before, allows it it to act as a voltage regulator in a circuit. The voltage that is dropped across a zener will not exceed its breakdown or zener voltage, even if the voltage or current in the circuit increases, which is what the I-V characteristics curve above depicts.
- 6. LED LED APPLICATION: LEDs are utilized in many industries for many things. The applications of this efficient and environmentally-friendly light source are endless, however, there are 5 main fields of application: automotive use, backlight sources for consumer electronics, display screens for advertising, electronic equipment, and generallighting. Automotive: Uses for the interior of automobiles include indicator lights on dashboard gauges, audio status lights, security status lights and warning signals. Automobile exterior usage includes third brake lights, the left and right rear lamps, turn signals, etc. Currently many European series vehicles including Audi and BMW integrate high power LEDs. Backlight Sources: The mobile phone is the largest market of integration for SMD (surface mount diode) LEDs. An estimated 400 million mobile phones are sold annually. One mobile phone takes two LED backlight sources and six SMD LED key lights. As a result, mobile phones create a demand for 3.2 billion LEDs per year. Display Screen: The LED screen has become the new display medium for advertising and information. It is commonly used in concert, arena and trade show venues. Its popularity has prompted the rapid technological development of large-scale integrated circuits. Compared to plasma- and LCD- based displays and traditional media such as neon lights, LED displays offer advantages such as high brightness, dynamic visual display, high reliability, low energy consumption, long service life, display content diversity, high durability, and low maintenance costs. LED display screens have been widely adapted in various fields. Electronic Equipment: Due to attributes such as low power consumption, small size, and long life, LEDs have become the preferred light source on various electronic equipment. Today, LEDs have been integrated as warning lights and indicators on most electronics. General Lighting: LEDs are being used in advertising billboards, illumination of commercial building exteriors, landmark buildings, bridges, roads, town centers and landscape lighting because of the numerous advantages they offer. Their long life, rich color, and easily-controlled features with integrated electronics offer a scalable lighting solution. As technology continues to bring rapid improvements in luminous efficiency and as cost compression persists, applications expand rapidly. Many airports, subways, Hotels, shopping centers,and individual homes feature LEDs now. The integration of LED-based solutions into traffic lighting has been successfuland is also growing rapidly.
- 7. LED Construction: One of the methods used to construct LED is to deposit three semiconductor layers on the substrate. The three semiconductor layers deposited on the substrate are n-type semiconductor, p-type semiconductor and active region. Active region is present in between the n-type and p-type semiconductor layers. When LED is forward biased, free electrons from n-type semiconductor and holes from p-type semiconductor are pushed towards the active region. When free electrons from n-side and holes from p-side recombine with the opposite charge carriers (free electrons with holes or holes with free electrons) in active region, an invisible or visible light is emitted. In LED, most of the charge carriers recombine at active region. Therefore, most of the light is emitted by the active region. The active region is also called as depletion region. LED characteristics: Individual LEDs are low voltage devices. Single indicator LEDs require 2 to 4 volts of direct current, with current in the range from 1 to 50 milliamperes. An illumination-grade LED containing a single semiconducting element requires the same voltage, but operating currents are much higher, typically several hundred milliamperes. A device containing multiple elements connected in series will require higher voltage corresponding to the larger number of individual elements in the device.
- 8. LEDs require a specific electrical polarity Applying voltage in reverse polarity can destroy them. Manufacturers provide specifications about the maximum reverse voltages acceptable for LED devices; 5 volts is a typical maximum rating. There is a wide variety of different LEDs available on the market. The different LED characteristics include colours light / radiation wavelength, light intensity,and a variety of other LED characteristics. The different LED characteristics have been brought about by a variety of factors, in the manufacture of the LED. The semiconductor make-up is a factor, but fabrication technology and encapsulation also play major part of the determination of the LED characteristics. LED colours One of the major characteristics of an LED is its colour. Initially LED colours were very restricted. For the first years only red LEDs were available. However as semiconductor processes were improved and new research was undertaken to investigate new materials for LEDs, different colours became available. The diagram below shows some typical approximate curves for the voltages that may be expected for different LED colours. Typical (approximate) LED voltage curves LED voltage drops The voltage drop across an LED is different to that of a normal silicon LED. Typically the LED voltage drop is between around 2 and 4 volts. The actual LED voltage that appears across the two terminals is dependent mainly upon the type of LED in question - the materials used. As would be expected the LED voltage curve broadly follows that which would be expected for the forward characteristic for a diode. However once the diode has turned on, the voltage is relatively flat for a variety of forward current levels. This means that in some cases designers have used them as very rough voltage stabilisers - zener diodes do not operate at voltages as low as LEDs. However their performance is obviously nowhere near as good.
- 9. Seven segment display SevenSegmentDisplayApplications: The applications of seven segments are mostly in digital calculators, electronic meters, digital clocks, odometers, digital clocks, clock radios, etc. Today most of the 7 segment applications are using LCDs,because of low current consumption. Seven Segment Display Construction: The individual segments of a seven-segment display.A seven segment display, as its name indicates, is composed of seven elements. Individually on or off, they can be combined to produce simplified representations of the arabic numerals. Often the seven segments are arranged in an oblique (slanted) arrangement, which aids readability. In most applications, the seven segments are of nearly uniform shape and size (usually elongated hexagons, though trapezoids and rectangles can also be used), though in the case of adding machines, the vertical segments are longer and more oddly shaped at the ends in an effort to further enhance readability. Each of the numbers 0, 6, 7 and 9 may be represented by two or more different glyphs on seven- segment displays. LED-based 7-segment display showing the 16 hex digits.The seven segments are arranged as a rectangle of two vertical segments on each side with one horizontal segment on the top, middle, and bottom. Additionally, the seventh segment bisects the rectangle horizontally. There are also fourteen-segment displays and sixteen-segment displays (for full alphanumerics); however, these have mostly been replaced by dot-matrix displays. The segments of a 7-segment display are referred to by the letters A to G, as shown to the right, where the optional DP decimal point (an "eighth segment") is used for the display of non-integer numbers. The animation to the left cycles through the common glyphs of the ten decimal numerals and the six hexadecimal "letter digits" (A–F). It is an image sequence of a "LED" display, which is described technology-wise in the following section. Notice the variation between uppercase and lowercase letters for A–F; this is done to obtain a unique, unambiguous shape for each letter (otherwise, a capital D would look identical to an 0 (or less likely O) and a capital B would look identical to an 8). Seven segments are,effectively, the fewest required to represent each of the ten Hindu-Arabic numerals with a distinct and recognizable glyph. Bloggers have experimented with six-segment and even five-segment displays with such novel shapes as curves,angular blocks and serifs for segments; however, these often require complicated and/or non-uniform shapes and sometimes create unrecognizable glyphs.
- 10. Seven Segment Display Characteristics: There are two types of LED 7-segment displays: common cathode (CC) and common anode (CA). The difference between the two displays is the common cathode has all the cathodes of the 7-segments connected directly together and the common anode has all the anodes of the 7- segments connected together. Shown below is a common anode seven segment. As shown above all the anode segments are connected together. When working with a CA seven segment display, power must be applied externally to the the anode connection that is common to all the segments. Then by applying a ground to a particular segment connection (a-g), the appropriate segment will light up. An additional resistor must be added to the circuit to limit the amount of current flowing thru each LED segment. The above diagram shows the instance when power is applied to the CA connection and segments b & c are grounded causing these two segments to light up. A typical pinout for a seven segment common anode display is shown below.
- 11. A common cathode seven segment is different from a common anode segment in that the cathodes of all the LEDs are connected together. For the use of this seven segment the common cathode connection must be grounded and power must be applied to appropriate segment in order to illuminate that segment.