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PREPARED BY V.SANDHIYA LECT/ ECE Zener diode is a voltage regulator device because it is able to fix the output voltage at a constant value (DC voltage). RS is to limit the zener current, IZ so that it is less than the maximum current, IZM (to avoid the zener diode from broken). RS RS + + VS + VZ VS - - - A simple regulator circuit + VZ - A regulator circuit with load resistance RL How to determine whether the zener acts as a regulator or not?? ◦ Use Thevenin Theorem ◦ See example If VTH<VZ, regulation does not occur. Referring to zener I-V charateristic curve, if the voltage across the zener diode zener is between 0VZ, the zener diode is operating in the reverse bias region, thus it DOES NOT functioned as a regulator. VTH must at least the same value as VZ (VTH VZ) so that the diode CAN function as a voltage regulator because it is operating in reverse breakdown region. Reverse breakdown region Reverse biased region In this simple illustration of zener regulation circuit, the zener diode will “adjust” its impedance based on varying input voltages and loads (RL) to be able to maintain its designated zener voltage. Zener current will increase or decrease directly with voltage input changes. The zener current will increase or decrease inversely with varying loads. Again, the zener has a finite range of operation. Three types of Zener analysis ◦ Fixed VS and RL ◦ Fixed VS and variable RL ◦ Variable VS and fixed RL - Fixed VS and RL The applied dc voltage is fixed, as the load resistor. The analysis : 1. Determine the state of the Zener diode by removing it from the network and calculating the voltage across the resulting open circuit. 2. Substitute the appropriate equivalent circuit and solve for the desired unknowns. - For the on state diode, the voltages across parallel elements must be the same. VL=VZ The Zener diode current is determined by KCL: IZ = IR – IL The power dissipated by the Zener diode is determined by: PZ = VZ IZ - For the off state diode, the equivalent circuit is open-circuit. - Fixed VS and Variable RL Rs + Vs - Step 1- get the RLmin so that zener is on. VL RLVs RS RL RL min RSVZ VS VZ - if RL ≥ RLmin, zener diode ‘on’, so that VL=VZ Step 2: Calculate the IZ using KCL: 2 condition 1. If RLmin , then ILmax and IZmin because of constant I1 2. If RLmax, then ILmin and IZmax I Z min or max I1constant I L max or min Where V VZ I1 S RS and I L max ; Izmax taken from data sheet Izmin = 0, if not given VZ RL min or RL max VZ I L min - Variable VS and fixed RL Step 1- get the VSmin so that zener is on. Rs + Vs - VL RLVs RS RL VS min ( RL RS )VZ RL if VS ≥ VSmin, zener diode will ‘on’, so that VL=VZ Step 2: Calculate the IZ using KCL: 2 condition 1. if VSmin , then I1min and IZmin because of constant IL 2. if VSmax, thenI1max and IZmax I Z min or max I1min or max I Lconstant where V IL L RL ; Izmax= Pzmax/Vz VS min VZ or and I 1min RS VS max I1max RS VZ The 78xx (also sometimes known as LM78xx) series of devices is a family of self-contained fixed linear voltage regulator IC. The 78xx family is a very popular choice for many electronic circuits which require a regulated power supply, due to: i. ease of use and ii. Low cost. 17 LED for emitter Air as barrier for isolation Phototransistor for detector Transformer is similar, but only for AC Optocoupler can be used for DC 18 Incandescent lamp ◦ Much slower response time than LED ◦ Can filter out high frequency noise ◦ Lower lifespan than LED however 19 Optocoupler’s bandwidth - determines the highest signal frequency that can be transferred through it Typical opto-couplers with a single output phototransistor may have a bandwidth of 200 - 300kHz, while those with a Darlington pair are usually about 10 times lower, at around 20 - 30kHz. 20 Darlington Pair 21 The other main type of optocoupler is the type having an output Diac or bilateral switch, and intended for use in driving a Triac or SCR. Examples of these are the MOC3020 and MOC3021. Here the output side of the opto-coupler is designed to be connected directly into the triggering circuit of the Triac where it’s operating from and floating at full 120/240 VAC 22 Vcc A simple circuit with an opto-isolator. When switch S1 is open, LED D1 is off, so Q1 is off and no current flows through R2, so Vout = Vcc. When switch S1 is closed, LED D1 lights. Phototransistor Q1 is now triggered, so current flows through R2 Vout is then pulled down to low state. This circuit, thus, acts as a NOT gate. 23