Diodes (4.1.1.1) CASP Part 66 PDF

Diodes (4.1.1.1) Learning Objectives 4.1.1.1.1 Identify symbols for the following semiconductor types; Diodes, Silicon Controlled Rectifiers (Thyristors), Light Emitting Diodes (LEDs), Photo Conductive Diodes and Varistors (Level 2). 4.1.1.1.2 Describe the characteristics and prope...

Diodes (4.1.1.1) Learning Objectives 4.1.1.1.1 Identify symbols for the following semiconductor types; Diodes, Silicon Controlled Rectifiers (Thyristors), Light Emitting Diodes (LEDs), Photo Conductive Diodes and Varistors (Level 2). 4.1.1.1.2 Describe the characteristics and properties of a diode (Level 2). 4.1.1.1.3 Describe the effects of using diodes connected together in series and parallel (Level 2). 4.1.1.1.4 Describe the main characteristics and use of silicon controlled rectifiers (thyristors), light emitting diodes, photoconductive diodes, varistors and rectifier diodes (Level 2). 4.1.1.1.5 Describe the functional testing of diodes (Level 2). 2022-11-10 B1-04 Electronic Fundamentals Page 22 of 163 CASA Part 66 - Training Materials Only  Diodes The Diode A diode (sometimes called a rectifier diode) is a device that allows current to flow in one direction but will oppose, or stop, current flow in the opposite direction. An ideal diode will allow the current flow in forward bias (act as a conductor) and block current flow entirely in reverse bias (act as in insulator). © Aviation Australia Diode markings, orientation and symbol There are several types of solid state diodes currently in use. Solid state is the term used to refer to devices that use solid materials to control electrical current flow through the manipulation of electrons, rather than a vacuum tube. 2022-11-10 B1-04 Electronic Fundamentals Page 23 of 163 CASA Part 66 - Training Materials Only  Diode Analogy - Ball in a Funnel If liquid pressure is applied to a funnel with a ball in the funnel throat, it pushes the ball into the funnel throat, sealing it and stopping flow. If electrical potential is applied in reverse polarity to a diode the conduction band gap increases, stopping electron flow. Aviation Australia Diode reverse bias analogy If liquid pressure is applied to the other side of the funnel the ball it is pushed away from the funnel throat allowing the liquid to flow. If electrical potential is applied in forward polarity to a diode the conduction band gap decreases and and electron flow commences. Aviation Australia Diode forward bias analogy This analogy is used to show that when electrons are pushed in the direction of the funnel (arrow / anode) current is blocked, and when electrons are pushed from the narrow end of the funnel ( arrow/ cathode) current can flow. 2022-11-10 B1-04 Electronic Fundamentals Page 24 of 163 CASA Part 66 - Training Materials Only  Diode Symbols There are a number of variations of diodes, each with unique practical uses. The following diagram shows some of the most common diode circuit symbols. The main characteristic and use of each will be discussed throughout this topic. © Aviation Australia Common diode circuit symbols It is worth noting that a Varistor is technically not a diode, but uses similar semiconductor principles. The term Varistor is only used for non-ohmic varying resistors. Variable resistors, such as the potentiometer and the rheostat, have ohmic characteristics and operate differently. 2022-11-10 B1-04 Electronic Fundamentals Page 25 of 163 CASA Part 66 - Training Materials Only  Diode Configurations Several common physical configurations of diodes are illustrated. The anode and cathode are indicated on a diode in several ways, depending on the type of package. The cathode is usually marked by a band, a tab, or some other feature. In some cases the anode/cathode is identified by one lead being connected to the case. However, always check the data sheet. Diode physical components 2022-11-10 B1-04 Electronic Fundamentals Page 26 of 163 CASA Part 66 - Training Materials Only  Basic Diode Characteristics The PN Junction It is understood that P-type and N-type doping is used to create semiconductors. A diode created by joining two equivalently doped P-type and N-type semiconductors. When they are joined an interesting phenomenon occurs. The P-type semiconductor (which will be labelled as the p-region) contains many holes (majority carriers) and is of a net positive charge. The N-type semiconductor has excess electrons (negative charge) and at the point of contact of the p-region and the n-region, the holes in the P-type material attract electrons in the N-type material. This point of contact is known as the PN junction (illustrated below). The p-region contains a few thermally generated free electrons (minority carries) and the n-region contains a few thermally generated holes as well. © Aviation Australia PN Junction 2022-11-10 B1-04 Electronic Fundamentals Page 27 of 163 CASA Part 66 - Training Materials Only  Formation of the Depletion Region The free electrons in the n-region are moving about randomly in all directions. At the instant of the PN junction formation, the free electrons near the junction in the n-region begin to diffuse across the junction into the p-region, where they combine with holes in the covalent bond near the junction. When the PN junction is formed, the n-region loses it's free electrons as they diffuse across the junction. This creates a layer of positive charges (as the doping atom becomes an ions) near the junction. As the electrons move across the junction, into the p-region completing the covalent bond making the P impurity an Ion. This creates a layer of negative charges near the junction. These two layers of positive and negative charges form the depletion region. The term depletion refers to the fact that the region near the PN junction is depleted of charge carriers (electrons and holes) due to diffusion across the junction. Keep in mind that the depletion region is formed very quickly and is very thin compared to the n-region and the p-region. The diagram is designed to illustrate the depletion region and is not to scale. © Aviation Australia Depletion region formation 2022-11-10 B1-04 Electronic Fundamentals Page 28 of 163 CASA Part 66 - Training Materials Only  Barrier Potential Any time there is a positive charge and a negative charge near each other, there is a force acting on the charges as described by Coulomb’s law. In the depletion region there are many positive charges and many negative charges on opposite sides of the PN junction. The forces between the opposite charges form a field of forces called an electric field, as illustrated in the diagram by the arrows between the positive charges and the negative charges. This electric field is a barrier to the free electrons in the n-region, and energy must be expended to move an electron through the electric field. That is, external energy must be applied to get the electrons to move across the barrier of the electric field in the depletion region. © Aviation Australia Barrier Potential The potential difference of the electric field across the depletion region is the amount of voltage required to move electrons through the electric field. This potential difference is called the barrier potential and is expressed in volts. Stated another way, a certain amount of voltage equal to the barrier potential and with the proper polarity must be applied across a PN junction before electrons will begin to flow across the junction. 2022-11-10 B1-04 Electronic Fundamentals Page 29 of 163 CASA Part 66 - Training Materials Only  Barrier Potential Variables The barrier potential of a PN junction depends on several factors, including the type of semi- conductive material, the amount of doping and the temperature. The typical barrier potential is approximately 0.7 V for silicon and 0.3 V for germanium at 25°C. Barrier potential is inversely proportional to the temperature. An increase in temperature is an increase in the velocity of holes and electrons and results in an increase in conductance. An increase in conductance causes the barrier potential to decrease and conductivity of the diode increases. Conversely, as the temperature lowers, the kinetic energy of charge carriers decreases and therefore the potential barrier will increase. Diode Forward Bias To bias a diode, you apply a DC voltage across it. Forward bias is the condition that allows current through the PN junction. The diagram shows a DC voltage source connected by conductive material (contacts and wire) across a diode in the direction to produce forward bias (top circuit in diagram below). This external bias voltage is designated as VBIAS. The resistor limits the current to a value that will not damage the PN structure of the diode. © Aviation Australia Forward and reverse bias In forward bias, the negative side of VBIAS is connected to the n-region (cathode) of the diode and the positive side is connected to the p-region (anode). This is one requirement for forward bias. A second requirement is that the bias voltage, VBIAS, must be greater than the barrier potential. 2022-11-10 B1-04 Electronic Fundamentals Page 30 of 163 CASA Part 66 - Training Materials Only  Forward Bias Operation When there is 0 V across the diode, there is no forward current. © Aviation Australia Diode circuit with no input voltage As you gradually increase the forward-bias voltage, there will be no current until the voltage is greater than the barrier voltage, after which, the forward current through the diode will gradually increase. A portion of the forward-bias voltage is dropped across the limiting resistor (Ohms law). © Aviation Australia Diode circuit with voltage across diode 0.7 V for silicon 2022-11-10 B1-04 Electronic Fundamentals Page 31 of 163 CASA Part 66 - Training Materials Only  When the forward-bias voltage is increased to a value where the voltage across the diode reaches approximately 0.7 V (barrier potential), the forward current across the diode begins to increase rapidly and the depletion region narrows. © Aviation Australia Diode circuit with voltage across diode surpassing 0.7 V As you continue to increase the forward-bias voltage, the current continues to increase very rapidly, but the voltage across the diode increases only gradually above 0.7 V, as illustrated in the diagram. This small increase in the diode voltage above the barrier potential is due to the voltage drop across the internal dynamic resistance of the semi-conductive material. 2022-11-10 B1-04 Electronic Fundamentals Page 32 of 163 CASA Part 66 - Training Materials Only  Ideal Diode Model The ideal model of a diode is a simple switch. When the diode is forward biased, it acts like a closed (on) switch, as shown in the illustration below. When the diode is reverse biased, it acts like an open (off) switch. In this ideal model, the barrier potential, the forward dynamic resistance, and the reverse current are all neglected. © Aviation Australia An ideal diode acts as a switch In the diagram, the ideal V-I characteristic curve graphically depicts the ideal diode operation. Since the barrier potential and the forward dynamic resistance are neglected, the diode is assumed to have a zero voltage across it when forward biased, as indicated by the portion of the curve on the positive vertical axis. © Aviation Australia Ideal Diode V-I Curve 2022-11-10 B1-04 Electronic Fundamentals Page 33 of 163 CASA Part 66 - Training Materials Only  Practical Diode Model The practical model adds the barrier potential to the ideal switch model and is more realistic (but still not completely accurate). When the diode is forward biased, it is equivalent to a closed switch in series with a small equivalent voltage source equal to the barrier potential (0.7V) with the positive side toward the anode, as indicated in the illustration. This equivalent voltage source represents the fixed voltage drop produced across the forward-biased PN junction of the diode and is not an active source of voltage. When the diode is reverse biased, it is equivalent to an open switch (no current flow) just as in the ideal model. The barrier potential does not affect reverse bias, so it is not a factor. Since the barrier potential is included and the dynamic resistance is neglected, the diode is assumed to have a voltage across it when forward biased, as indicated by the portion of the curve to the right of the origin. © Aviation Australia Practical Diode V-I curve (Silicon diode) Remember that this curve is for a silicon diode; a germanium diode would be offset by 0.3 V instead. 2022-11-10 B1-04 Electronic Fundamentals Page 34 of 163 CASA Part 66 - Training Materials Only  Forward Bias Characteristics The Real Diode Model The most realistic V-I characteristic curve for a forward-biased diode is shown in the diagram below. In reality a diode is not a perfect switch. The diode forward voltage (VF) increases to the right along the horizontal axis, and the forward current (IF) increases upward along the vertical axis. Graphing the V-I curve (forward bias) The forward current increases very little until the forward voltage across the PN junction reaches approximately 0.7 V at the knee of the curve. After this point, the forward voltage remains at approximately 0.7 V, but IF increases rapidly. As previously mentioned, there is a slight increase in VF above 0.7 V as the current increases, due mainly to the voltage drop across the static or dynamic resistance. Normal operation for a forward-biased diode is above the knee of the curve. The IF scale is typically in mA, as indicated. 2022-11-10 B1-04 Electronic Fundamentals Page 35 of 163 CASA Part 66 - Training Materials Only  Diode Resistance The diode resistance is the effective opposition offered by the diode to the flow of current through it. In an ideal diode, a forward biased diode would offer zero resistance and infinite resistance when reverse biased. However, no real device can be ideal. Thus, practically speaking, every diode is seen to offer a small resistance when forward biased, and a considerable resistance when reverse biased. Unlike a linear resistance, the resistance of the forward-biased diode is not constant over the entire curve. Because the resistance changes along the V-I curve, it is called dynamic resistance. ΔVF Rd = ΔIF The resistance is greatest below the knee of the curve because the current increases very little for a given change in voltage. The resistance begins to decrease in the region of the knee of the curve and becomes smallest above the knee where there is a large change in current for a given change in voltage. This characteristic is illustrated in the diagram for equal changes in VF (ΔVF) on a magnified segment of the V-I curve below and above the knee. Diode resistance 2022-11-10 B1-04 Electronic Fundamentals Page 36 of 163 CASA Part 66 - Training Materials Only  Diode Reverse Bias Reverse bias is the condition that essentially prevents current through the diode. The image shows a DC voltage source connected across a diode in the direction to produce reverse bias (bottom circuit in diagram below). This external bias voltage is designated as VBIAS just as it was for forward bias. Note that the positive side of VBIAS is connected to the n-region of the diode and the negative side is connected to the p-region. © Aviation Australia Forward and reverse bias The diagram shows a reverse biased PN Junction. In reverse bias a voltage is applied across the device and the electric field at the junction increases. The higher electric field in the depletion region decreases the probability that current carriers can move from one side of the junction to the other. This results in a widening of the depletion region and as the electrostatic field equals the Bias voltage the only current is thermally produced electron-hole pairs in the depletion region. Reverse bias (inside the diode) 2022-11-10 B1-04 Electronic Fundamentals Page 37 of 163 CASA Part 66 - Training Materials Only  Reverse Bias Characteristics When a reverse-bias voltage is applied across a diode, there is only an extremely small reverse current (IR) through the PN junction. With 0 V across the diode, there is no reverse current. As you gradually increase the reverse-bias voltage, there is a very small constant reverse current as the voltage across the diode increases. When the applied bias voltage is increased to a value where the reverse voltage across the diode (VR) reaches the breakdown value (VBR), the reverse current begins to increase rapidly. As you continue to increase the bias voltage, the current continues to increase very rapidly, but the voltage across the diode increases very little above VBR. Breakdown, with exceptions, is not a normal mode of operation for most PN junction devices and would usually destroy them. The breakdown voltage is equivalent to the voltage that would break down an insulator into conduction. © Aviation Australia Graphing the V-I Curve (reverse bias) If you plot the results of reverse-bias measurements on a graph, you get the V-I characteristic curve for a reverse-biased diode. The diode reverse voltage (VR) increases to the left along the horizontal axis, and the reverse current (IR) increases downward along the vertical axis. 2022-11-10 B1-04 Electronic Fundamentals Page 38 of 163 CASA Part 66 - Training Materials Only  Reverse Leakage Current There is very small constant reverse current (usually uA or nA) until the reverse voltage across the diode reaches approximately the breakdown value (VBR) at the knee of the curve. After this point, the reverse voltage remains at approximately VBR, but IR increases very rapidly, resulting in overheating and possible damage. The breakdown voltage for a typical silicon diode can vary, but a minimum value of 50 V is not unusual. Full Diode Characteristic Model Combine the curves for both forward bias and reverse bias, and you have the complete V-I characteristic curve for a diode. Note that the IF scale is in mA compared to the IR scale in µA. V-I characteristic curve for a silicon diode The characteristic curve for the complete diode model is illustrated above. Since the barrier potential and the forward dynamic resistance are included, the diode is assumed to have a voltage across it when forward biased. This voltage (VF) consists of the barrier potential voltage plus the small voltage drop across the dynamic resistance, as indicated by the portion of the curve to the right of the origin. The curve slopes because the voltage drop due to dynamic resistance increases as the current increases. As with all electronic components, temperature impacts the performance of a component. The current / voltage graph shown here is a diode at room temperature (approximately 25 degrees celsius). 2022-11-10 B1-04 Electronic Fundamentals Page 39 of 163 CASA Part 66 - Training Materials Only  Connecting Diodes Simple Diode Circuit To evaluate the following circuit, determine the biasing state of the diode. For a diode to be forward biased: The applied voltage must be greater than the barrier potential of the diode. The diode must be oriented with the anode to the positive potential and the cathode to the negative potential. In the example below, diode CR1 is forward biased. © Aviation Australia Basic diode circuit 2022-11-10 B1-04 Electronic Fundamentals Page 40 of 163 CASA Part 66 - Training Materials Only  Voltage Evaluation To determine the voltage applied to the series resistor R1, we know that the voltage applied to the circuit is 10 V. We also know that a silicon diode requires 0.7 V to overcome the barrier potential, if the diode material is not specified, assume the diode semiconductor material is silicon. Therefore the voltage across resistor (R1) is: VR1 = VAP P LI ED − VF (Si) = 10 V − 0.7 V = 9.3 V Where Si means silicon. Current Evaluation To determine the current flowing through resistor R1 and diode CR1, it is a simple use of Ohms law: V = IR 9.3 V IR1 = 2.7 kΩ IR1 = 3.4 mA 2022-11-10 B1-04 Electronic Fundamentals Page 41 of 163 CASA Part 66 - Training Materials Only  Series Connected Diodes Practical Usage of Series Diodes In many high-voltage applications, one commercially available diode cannot meet the required voltage rating, and diodes are connected in series to increase the reverse blocking capabilities (peak inverse voltage of the diode). © Aviation Australia Diodes in series However, in practice this diode formation is not practical as the voltage does not distribute evenly between diodes. The reverse leakage current for diodes is not a carefully controlled parameter during the manufacturing process, and can vary substantially from diode to diode even within the same batch. Aviation Australia Reverse bias resistance difference 2022-11-10 B1-04 Electronic Fundamentals Page 42 of 163 CASA Part 66 - Training Materials Only  When the diodes are placed in series, the diode with the lowest leakage current and therefore the highest reverse bias resistance will have the highest voltage across it, which will cause it to fail, which in turn will apply excessive voltage to the remaining diode(s) and also cause them to fail. A simple solution is to connect a high-value resistor in parallel with each diode (parallel voltage- sharing resistors). Due to voltage sharing, and with the correct size resistors selected, the reverse voltage that appears across the diodes can be brought to the same value and the circuit will work as intended. If the resistances are equal, R1 = R2, and lower than the diode resistance, the two diode voltages will be approximately the same, as the value of resistor selected makes the difference between the two diodes irrelevant. © Aviation Australia Series diodes and series diodes with parallel voltage-sharing resistors Without parallel resistors, the reverse voltages across each individual diode could vary drastically dependent on the resistance characteristic of each diode. In the diagram below it can be seen that the voltage drop across D2 may not cause breakdown however, avalanche breakdown will occur in diode D1 due to most voltage being dropped across it. 2022-11-10 B1-04 Electronic Fundamentals Page 43 of 163 CASA Part 66 - Training Materials Only  Aviation Australia Reverse bias resistance difference 2022-11-10 B1-04 Electronic Fundamentals Page 44 of 163 CASA Part 66 - Training Materials Only  Parallel Connected Diodes Connecting diodes in parallel will increase the forward current rating. If it is possible to match the diodes so that approximately equal current sharing is achieved this should be done. If diodes are put in parallel with varying current capacities, similar to the series circuit, the diode with the lowest forward voltage drop will try to carry a larger current. This may cause damage to the diode and it may overheat. In the event that the exact characteristics are not known, sharing resistors (with associated losses) can be used. The following diagram shows exaggerated characteristics to highlight the variation in current through each diode. Again, a simple method of calculating resistance values can be used if all resistors are set equal. Parallel connected diodes 2022-11-10 B1-04 Electronic Fundamentals Page 45 of 163 CASA Part 66 - Training Materials Only  Worked Example There is a circuit with three forward biased diodes in parallel. If the current rating of each diode is 20mA and the calculated voltage drop across the diodes is 0.7 V, what value resistors should be used? If the resistors placed in series are of equal resistance, then the current through each diode should be equal. The maximum current for one diode is 20mA. Therefore, the resistor value should be set to make the current differences irrelevant. For simplicity, all resistors will be set to have the same value, however, if different types of diodes are being used, the resistors should be selected separately to reflect the diode’s respective current ratings. The values can be calculated as follows. V 0.7 V R1 = R2 = R3 = = I 20 mA ∴ R1 = R2 = R3 = 35 Ω Therefore the resistors should be at minimum 100 ohms each in order to make the diode resistance irrelevant. 2022-11-10 B1-04 Electronic Fundamentals Page 46 of 163 CASA Part 66 - Training Materials Only  Rectifier Diodes Rectifier Diodes The term diode and rectifier diode are often used interchangeably. A diode is a small signal device with current capacity typically in milliamp range. Whereas a rectifier is a power device, conducting from 1 to 1000 amps or even higher. © Aviation Australia Rectifier diode component and circuit symbol Rectifier diodes are simply diodes redesigned to serve the purpose of rectifying alternating current. The circuit symbol for diodes and rectifier diodes are the same. The Schottky diode is a variant of the rectifier diode and is particularly popular in the field of digital electronics. In standard diodes and rectifier diodes, the connections remain the same, it is just the applications that differ. The primary uses of a rectifier diode include: Half Wave Rectifiers Full Wave Rectifiers DC Blockers Rectifiers are also used to convert AC voltage to DC voltage. 2022-11-10 B1-04 Electronic Fundamentals Page 47 of 163 CASA Part 66 - Training Materials Only  Half Wave Rectifiers One of the most important uses of a diode is rectification. The normal PN junction diode is well-suited for this purpose as it conducts very heavily when forward biased and only slightly when reverse biased (high-resistance direction). If we place this diode in series with a source of ac power, the diode will be forward and reverse biased every cycle. Since in this situation current flows more easily in one direction than the other, rectification is accomplished. The simplest rectifier circuit is a half-wave rectifier which consists of a diode, an AC power source, and a load resistor shown below. Aviation Australia Half wave rectifier operation Half Wave Rectifier circuits are cheaper than full wave rectifiers, so they are used in some insensitive devices that can withstand the larger voltage variations. The output average voltage of half wave rectifier is less than the input voltage so they perform two main functions; step down of voltage and voltage rectification. An important uses of half wave rectifiers is Low power simple battery charger circuits. 2022-11-10 B1-04 Electronic Fundamentals Page 48 of 163 CASA Part 66 - Training Materials Only  Aviation Australia Half Wave Rectifier output 2022-11-10 B1-04 Electronic Fundamentals Page 49 of 163 CASA Part 66 - Training Materials Only  Full Wave Rectifiers A full-wave rectifier is a device that has two or more diodes arranged so that load current flows in the same direction during each half cycle of the AC supply (shown below). A transformer (on the left) supplies the source voltage for two rectifier diodes. The connections to the diodes are arranged so that the diodes conduct on alternate half cycles. During the first alternation, current flows through the top diode, while the bottom one is reversed biased. During the negative half of the cycle the bottom diode conduct and the top one is reversed biased. The voltage measure through the output (represented by a resistor) is always in the same direction and is a series of rectified AC pulses. Aviation Australia Full Wave Rectifier Since both alternations of the input voltage cycle are used, the circuit is called a full wave rectifier. 2022-11-10 B1-04 Electronic Fundamentals Page 50 of 163 CASA Part 66 - Training Materials Only  Aviation Australia Fully rectified output 2022-11-10 B1-04 Electronic Fundamentals Page 51 of 163 CASA Part 66 - Training Materials Only  Bridge Rectifiers When four diodes are connected as shown the circuit is called a bridge rectifier. The input to the circuit is applied to the diagonally opposite corners of the network, and the output is taken from the remaining two corners. A bridge rectifier circuit is a common part of electronic power supplies. Many electronic circuits require a rectified DC power supply for powering the various electronic basic components from available AC mains supply. © Aviation Australia The Bridge Rectifier A transformer (on the left) inputs AC voltage into the bridge rectifier. During the positive half of the cycle diodes D1 and D2 are forward biased and current flows through them from earth back to the positive potential of the applied EMF. During the negative half cycle, D1 and D2 are reverse biased, but current now flows from earth, up through the load and through D3 and D4 then back to the positive potential of the EMF. The current is always flowing in one direction through the load, so it no longer alternates, it just pulses. Since current flows through the load during both half cycles of the applied voltage, this bridge rectifier is a full-wave rectifier. One advantage of a bridge rectifier over a conventional full-wave rectifier is that with a given transformer the bridge rectifier produces a voltage output that is nearly twice that of the conventional full wave circuit. This may be shown by assigning values to some of the components. 2022-11-10 B1-04 Electronic Fundamentals Page 52 of 163 CASA Part 66 - Training Materials Only  Example Assume that the same transformer is used in both circuits. The peak voltage developed is 10 volts in both circuits. In the conventional full wave rectifier circuit shown in the previous section, the peak voltage from the centre tap to either X or Y is 5 volts. Since only one diode can conduct at any instant, the maximum voltage that can be rectified at any instant is 5 volts. Therefore, the maximum voltage that appears across the load resistor is nearly — but never exceeds — 5 volts, as a result of the small voltage drop across the diode. In the bridge rectifier shown in this section, the maximum voltage that can be rectified is the full secondary voltage, which is 10 volts. Therefore, the peak output voltage across the load resistor is nearly 10 volts. With both circuits using the same transformer, the bridge rectifier circuit produces a higher output voltage than the conventional full-wave rectifier circuit. 2022-11-10 B1-04 Electronic Fundamentals Page 53 of 163 CASA Part 66 - Training Materials Only  Diode Devices Silicon Controlled Rectifiers (Thyristors) Thyristors are in a group of semiconductor devices that act as open or closed switches. An SCR is one type of thyristor. An SCR is a four-layer semiconductor device, consisting of alternating p type and n type materials (pnpn). A thyristor usually has three electrodes: an anode, a cathode, and a gate (control electrode). Silicon controlled rectifiers (SCRs) When the cathode is negatively charged relative to the anode, no current flows until a pulse of current is applied to the gate. Then the SCR begins to conduct and continues to conduct until the voltage between the cathode and anode is reversed or the current is reduced below a certain value. A common method used to switch off SCRs is to short anode to cathode. This reduces the current through them below the minimum specified value and it switches off. Using this type of thyristor, large amounts of power can be switched or controlled using a small triggering current or voltage. SCRs are used in motor speed controls, light dimmers, pressure-control systems, and liquid-level regulators. For an over-temperature type circuit a bimetallic sensor could trigger an SCR energising a warning light which would then remain on until cancelled by a reset switch (remove conducted current) regardless of whether the overtemp remains or dissipates. The following circuit shows an SCR in a LED torch circuit. 2022-11-10 B1-04 Electronic Fundamentals Page 54 of 163 CASA Part 66 - Training Materials Only  Thyristor LED Torch Light Circuit Light Emitting Diodes (LEDs) The basic operation of the light-emitting diode (LED) is as follows. When the device is forward-biased, electrons cross the pn junction from the n-type material and recombine with co-valent holes in the p- type material. Light Emitting Diode (LED) 2022-11-10 B1-04 Electronic Fundamentals Page 55 of 163 CASA Part 66 - Training Materials Only  When the conduction electron becomes a valence electron in the P material, the recombining electrons release energy in the form of light and some heat. A large exposed surface area of the extremely thin P layer of the semi-conductive material permits the photons to be emitted as light. This process is called electroluminescence. Various Synthetic Semiconductors are used to establish the wavelength of the emitted light. The wavelength is the colour of the light and if it is visible or invisible (Ultra Violet - Infrared). LED physical appearance and circuit symbol LED Semiconductive Materials LEDs are made of gallium arsenide (GaAs), gallium arsenide phosphide (GaAsP), or gallium phosphide (GaP). Silicon and germanium are not used because they are essentially heat-producing materials and are very poor at producing light. GaAs LEDs emit infrared (IR) radiation, which is invisible. GaAsP produces either red or yellow visible light, and GaP emits red or green visible light. 2022-11-10 B1-04 Electronic Fundamentals Page 56 of 163 CASA Part 66 - Training Materials Only  LED Biasing The forward voltage across an LED is considerably greater than for a silicon diode. Typically the VF for LEDs is between 1.2 V and 3.2 V, depending on the device. Reverse breakdown for an LED is much less than for a silicon rectifier diode (3V to 10V typical). The LED emits light in response to a sufficient forward current, as shown in Figure 47. The amount of power output translated into light is directly proportional to the forward current, as indicated in the figure. An increase in IF corresponds proportionally to an increase in light output but a reduced life span. The LED emits light in response to a sufficient forward current 2022-11-10 B1-04 Electronic Fundamentals Page 57 of 163 CASA Part 66 - Training Materials Only  Organic Light Emitting Diode (OLED) An Organic Light-Emitting Diode (OLED) is a type of LED in which the light emitting layer is a film of organic (carbon-bearing) compound. The emissive layer becomes ionised and emits light in response to a DC electric current. It is possible to produce an OLED as a thin, flexible sheet. Several applications are predicted for OLEDs in aircraft interiors over the next few years. The expected applications include: Diffuse and variable cabin lighting from large, flexible OLED panels that conform to the curved shape of cabin trim panels. The large area OLED panels produce no concentrated heat loads. Smart cabin signs. Lightweight displays with excellent contrast and wide viewing angles. Typical input DC voltages for OLED lighting panels are 6V or 8.5V from a current-limited power supply (driver). Energy efficiency is in the range of 55-60 lumens per Watt (lm/W) which is similar to the efficiency of a conventional LED. This has two benefits; LED and OLED lighting systems consume less electrical power, and the air-conditioning system also consumes less power because efficient lighting components run cooler and put less heat load into the cabin. Modern OLEDs now have a service life of 100 000 hours which is similar to that of a conventional LED. OLED Structure 2022-11-10 B1-04 Electronic Fundamentals Page 58 of 163 CASA Part 66 - Training Materials Only  Substrate (clear plastic, glass, foil) – The substrate supports the OLED. Anode (transparent) - The anode removes electrons when a current flows through the device. Organic layers - These layers are made of organic molecules or polymers. Conducting layer - This layer is made of organic plastic molecules that transport "holes" from the anode. One conducting polymer used in OLEDs is polyaniline. Emissive layer - This layer is made of organic plastic molecules (different ones from the conducting layer) that transport electrons from the cathode; this is where light is made. One polymer used in the emissive layer is polyfluorene. Cathode (may or may not be transparent depending on the type of OLED) - The cathode injects electrons when a current flows through the device. Photoconductive Diodes The photoconductive diode (also referred to as a photodiode) is a device that operates in reverse bias, where Iλ is the reverse current. Photodiode operates in reverse bias The circuit symbol for the photodiode is as follows. Photodiode circuit symbol 2022-11-10 B1-04 Electronic Fundamentals Page 59 of 163 CASA Part 66 - Training Materials Only  The photodiode has a small transparent window that allows light to strike the PN junction. Some typical photodiodes are shown in the picture. Image by Hamamatsu Photonics Photodiodes A photodiode is a semiconductor PN junction device that allows a high reverse bias current when struck by light. When photons are absorbed a current is allowed to pass. Photodiodes can contain optical filters, built-in lenses, and may have either large or small surface areas. Recall that when reverse-biased, a rectifier diode has a very small reverse leakage current. The same is true for a photodiode. The reverse-biased current is produced by thermally generated electron- hole pairs in the depletion region, which are swept across the PN junction by the electric field created by the reverse voltage. In a rectifier diode, the reverse leakage current increases with temperature due to an increase in the number of electron-hole pairs, photodiodes use light instead. Due to the adjacent Electro Magnetic wavelength of heat and light, Photodiodes react to the infrared light more than other wavelengths. 2022-11-10 B1-04 Electronic Fundamentals Page 60 of 163 CASA Part 66 - Training Materials Only  Zener Diodes The Zener diode is a silicon PN junction device that differs from rectifier diodes because it is designed for operation in the reverse-breakdown region. When a Zener diode reaches reverse breakdown, the voltage across its terminals remains almost constant even though the current may change drastically. This is the Zener Voltage of the device. The reverse breakdown voltage of a Zener diode is set by carefully controlling the doping level during manufacture. The doping process can be adjusted to obtain Zener voltage values from 1.2 volts to approximately 200 volts. Zener diodes are commonly used in electronic circuits to provide a stable reference voltage. The symbol for a Zener diode is: © Aviation Australia Zener diode The following diagram shows the relationship between the voltage across a Zener diode and the resulting current through it. The shaded area shows the normal operating region of the Zener diode when reverse biased. A Zener diode behaves like a normal diode if it is forward biased. 2022-11-10 B1-04 Electronic Fundamentals Page 61 of 163 CASA Part 66 - Training Materials Only  Aviation Australia Zener diode V-I characteristics Varistors A Varistor is used as a surge protection device that is connected directly across the AC input or across a component to be protected. They have a very fast response time and low leakage current. They behave like back to back zener diodes and only conduct when the breakdown voltage is exceeded. The rest state has a high impedance (several meg ohms) in relation to the component to be protected and does not change the characteristics of the circuit. Varistors 2022-11-10 B1-04 Electronic Fundamentals Page 62 of 163 CASA Part 66 - Training Materials Only  When a voltage surge or voltage spike is sensed, the Varistor's resistance instantaneously decreases, creating an instant shunt path for the over-voltage, thereby saving the sensitive components. Because the shunt path creates a short circuit, the circuit protection device usually operates in the process. The resetting of a circuit breaker or the replacement of a fuse is much cheaper than the replacement of sensitive components. Aviation Australia Varistor Operation Diode Symbols (Recap) The following diagram shows common diode circuit symbols. The Schottky and the tunnel diode were not addressed as part of this lesson, however may be taught in later lessons or subtopics. © Aviation Australia Common diode circuit symbols 2022-11-10 B1-04 Electronic Fundamentals Page 63 of 163 CASA Part 66 - Training Materials Only  Functional Testing of Diodes DMM Diode Test Position Many digital multimeters (DMMs) have a diode test position that provides a convenient way to test a diode. A typical DMM has a small diode symbol to mark the position of the function switch. When set to diode test, the meter provides an internal voltage sufficient to forward-bias and reverse-bias a diode. This internal voltage may vary among different makes of DMMs, but 2.5 V to 2.6 V is a typical range of values. The meter provides a voltage reading or other indication to show the condition of the diode under test. © Aviation Australia Digital multimeter (DMM) diode testing 2022-11-10 B1-04 Electronic Fundamentals Page 64 of 163 CASA Part 66 - Training Materials Only  When the Diode Is Working The red (positive) lead of the meter is connected to the anode and the black (negative) lead is connected to the cathode to forward-bias the diode. If the diode is good, you will get a reading of between approximately 0.5 V and 0.9 V, with 0.7 V being a nominal value for forward bias. The diode is turned around to reverse-bias the diode, as shown. If the diode is working properly, you will get a voltage reading based on the meter’s internal source. The 2.6 V represents a typical value and indicates that diode has an extremely high reverse resistance with essentially all of the internal voltage appearing across it. Some meters will show Out of Limit (OL) instead of the meter voltage. Forward Bias Leads connected to the cathode to forward-bias the diode 2022-11-10 B1-04 Electronic Fundamentals Page 65 of 163 CASA Part 66 - Training Materials Only  Reverse Bias Aviation Australia Leads connected to reverse bias the diode 2022-11-10 B1-04 Electronic Fundamentals Page 66 of 163 CASA Part 66 - Training Materials Only  When the Diode Is Defective When a diode has failed "open circuit, you get an open circuit voltage reading (2.6 V is typical) or an Out of Limit OL indication for both the forward-bias and the reverse-bias condition. If a diode is shorted, the meter reads 0 V in both forward and reverse bias tests, as indicated in the illustration. Sometimes, a failed diode may exhibit a small resistance for both bias conditions rather than a pure short. In this case, the meter will show a small voltage much less than the correct open voltage. For example, a resistive diode may result in a reading of 1.1 V in both directions rather than the correct readings of 0.7 V for forward bias and 2.6 V for reverse bias. Aviation Australia Defective diode testing 2022-11-10 B1-04 Electronic Fundamentals Page 67 of 163 CASA Part 66 - Training Materials Only  Checking a Diode with the OHMs Function DMMs that do not have a diode test position can be used to check a diode by setting the function switch on an OHMs range. For a forward-bias check of a good diode, you will get a resistance reading that can vary depending on the meter’s internal battery. Many meters do not have sufficient voltage on the OHMs setting to fully forward-bias a diode and you may get a reading of from several hundred to several thousand ohms. For the reverse-bias check of a good diode, you will get some type of out- of-range indication such as OL on most DMMs because the reverse resistance is too high for the meter to measure. Even though you may not get accurate forward- and reverse-resistance readings on a DMM, the relative readings indicate that a diode is functioning properly, and that is usually all you need to know. The out-of-range indication shows that the reverse resistance is extremely high, as you expect. The reading of a few hundred or less for forward bias is relatively small compared to the reverse resistance, indicating that the diode is working properly. The actual resistance of a forward-biased diode is typically less than 100 ohms. Checking a diode with the ohms function 2022-11-10 B1-04 Electronic Fundamentals Page 68 of 163 CASA Part 66 - Training Materials Only  Forward Bias Testing diode in forward bias Reversed Bias Testing diode in reverse bias 2022-11-10 B1-04 Electronic Fundamentals Page 69 of 163 CASA Part 66 - Training Materials Only  Basic Summary of Diode Bias Forward bias: Permits current to flow. Forward bias voltage connections: Positive to the p-region; negative to the n-region. The bias voltage must be greater than the barrier potential. The depletion region narrows. Barrier potential: 0.7 V for silicon Electrons easily pass through the depletion region and are carried to the positive terminal by the P type materials. Aviation Australia Forward bias diode (PN junction) Reverse bias: Prevents current flow, 2022-11-10 B1-04 Electronic Fundamentals Page 70 of 163 CASA Part 66 - Training Materials Only  Reverse bias voltage connections: Positive to n-region; negative to p-region. The bias voltage must be less than the breakdown voltage. There is no current after a small transition time that allows the depletion region to form fully The depletion region widens. Aviation Australia Reverse bias diode (PN junction) 2022-11-10 B1-04 Electronic Fundamentals Page 71 of 163 CASA Part 66 - Training Materials Only