CHAPTER 2 Diode Applications By Syahrul Ashikin Azmi

CHAPTER 2 Diode Applications By: Syahrul Ashikin Azmi School of Electrical System Engineering 1

Objectives Ø Explain and analyze the operation of both half and full wave rectifiers Ø Explain and analyze filters and regulators and their characteristics Ø Explain and analyze the operation of diode limiting and clamping circuits Ø Explain and analyze the operation of diode voltage multipliers Ø Interpret and use a diode data sheet Ø Troubleshoot simple diode circuits 2

Lecture’s contents Ø 2 -1 Half-wave rectifier Ø 2 -2 Full-wave rectifier Ø 2 -3 Power supply filters and regulators Ø 2 -4 Diode limiting and clamping circuits Ø 2 -5 Voltage multipliers Ø 2 -6 Diode data sheet Ø 2 -7 Troubleshooting Ø Summary 3

2 -1 Half-Wave Rectifiers Diode – ability to conduct current in one direction and block current in other direction used in circuit called RECTIFIER (ac dc) Objective: Discuss the operation of half-wave rectifiers Describe a basic dc power supply & half-wave rectifications Determine the average value, VAVG of half-waves rectified voltage Discuss the effect of barrier potential, VP on a half-wave rectifier output Ø Define Peak Inverse Voltage (PIV) Ø Describe Transformer-couple half-wave rectifier Ø Ø 4

2 -1 Half-Wave Rectifiers (cont. ) (The basic DC power supply) The basic function of a DC power supply is to convert an AC voltage to a constant DC voltage (AC DC) Either half or full-wave rectifier convert ac input voltage to a pulsating dc voltage. Maintains a constant dc voltage Eliminates the fluctuations - produce smooth dc voltage 5

2 -1 Half-Wave Rectifiers (cont. ) (The Half-Wave Rectifier) • A half wave rectifier(ideal) allows conduction for only 180° or half of a complete cycle. • During first one cycle: -Vin goes positive – diode FB – conduct current ac source load resistor -Vin goes negative – diode RB – no current- 0 V • The output frequency is the same as the input (same shape). The average value VDC or VAVG : Ideal diode model -Measure on dc voltmeter 6

2 -1 Half-Wave Rectifiers (cont. ) (Effect of the Barrier Potential on the Half-Wave Rectifier Output) Practical Diode – barrier potential of 0. 7 V (Si) taken into account. Ø During +ve half-cycle – Vin must overcome Vpotential forward bias. Ø Ø Example 1: Calculate the peak o/p voltage, Vp(out)? The peak o/p voltage: 7

2 -1 Half-Wave Rectifiers (cont. ) (Effect of the Barrier Potential on the Half-Wave Rectifier Output) Example 2: Sketch the output V 0 and determine the output level voltage for the network in above figure. 8

2 -1 Half-Wave Rectifiers (cont. ) [Peak Inverse Voltage (PIV)] - Peak inverse voltage (PIV) is the maximum voltage across the diode when it is in reverse bias. The diode must be capable of withstanding this amount of voltage. 9

2 -1 Half-Wave Rectifiers (cont. ) (Half-Wave Rectifier with Transformer-Coupled Input Voltage) ØTransformers are often used for voltage change and isolation. ØThe turns ratio, n of the primary to secondary determines the output versus the input. ØThe advantages of transformer coupling: 1) allows the source voltage to be stepped up or down 2) the ac source is electrically isolated from the rectifier, thus prevents shock hazards in the secondary circuit. to couple ac input to the rectifier If n>1, Vsec is greater than Vpri. If n<1, Vsec is less than Vpri. If n=1, Vsec= Vpri. 10

Example 3: Determine the peak value of output voltage as shown in below Figure. 11

2 -2 Full-Wave Rectifiers (Introduction) Objective: Ø Explain & Analyze the operation of Full-Wave Rectifier. Ø Discuss how full wave rectifier differs from half-wave rectifier Ø Determine the average value Ø Describe the operation of center-tapped & bridge. Ø Explain effects of the transformers turns ratio Ø PIV Ø Comparison between center-tapped & bridge. 12

2 -2 Full-Wave Rectifiers (cont. ) (Introduction) ØA full-wave rectifier allows current to flow during both the positive and negative half cycles or the full 360º whereas half-wave rectifier allows only during one-half of the cycle. ØThe no. of +ve alternations is twice the half wave for the same time interval ØThe output frequency is twice the input frequency. ØThe average value – the value measured on a dc voltmeter Twice output 63. 7% of Vp 13

2 -2 Full-Wave Rectifiers (i - The Center-Tapped Full-Wave Rectifier) • This method of rectification employs two diodes connected to a secondary center-tapped transformer. • The i/p voltage is coupled through the transformer to the center-tapped secondary. Coupled input voltage 14

2 -2 Full-Wave Rectifiers (cont. ) (i - The Center-Tapped Full-Wave Rectifier) • +ve half-cycle input voltage (forward-bias D 1 & reverse-bias D 2)-the current patch through the D 1 and RL • -ve half-cycle input voltage (reverse-bias D 1 & forward-bias D 2)-the current patch through D 2 and RL • The output current on both portions of the input cycle – same direction through the load. • The o/p voltage across the load resistors – full-wave rectifiers 15

2 -2 Full-Wave Rectifiers (cont. ) (i - The Center-Tapped Full-Wave Rectifier) -Effect of the Turns Ratio on the Output Voltage. If n=1, Vp(out)=Vp(pri) - 0. 7 V 2 v Vp(sec)=Vp(pri) If n=2, • In any case, the o/p voltage is always one-half of the total secondary voltage minus the diode drop (barrier potential), no matter what the turns ratio. 16

2 -2 Full-Wave Rectifiers (cont. ) (i - The Center-Tapped Full-Wave Rectifier) -Peak Inverse Voltage (PIV)Ø Maximum anode voltage: D 1: forward-bias – its cathode is at the same voltage of its anode minus diode drop; This is also the voltage on the cathode of D 2. n PIV across D 2 : n Ø We know that Ø Thus; 17

2 -2 Full-Wave Rectifiers (cont. ) (ii - The Bridge Full-Wave Rectifier) § It employs four diodes arranged such that current flows in the direction through the load during each half of the cycle. §When Vin +ve, D 1 and D 2 FB and conduct current. A voltage across RL looks like +ve half of the input cycle. During this time, D 3 and D 4 are RB. §When Vin –ve, D 3 and D 4 are FB and conduct current. D 1 and D 2 are RB. Used 4 diode: 2 diode in forward 2 diode in reverse Without diode drop (ideal diode): 2 diode always in series with load resistor during +ve and –ve half cycle. With diode drop (practical diode): 18

2 -2 Full-Wave Rectifiers (cont. ) (ii - The Bridge Full-Wave Rectifier) For ideal diode, PIV = Vp(out) For each diode, 0 V (ideal diode) Note that in most cases we take the diode drop into account. 19

2 -3 Power Supply Filters And Regulators (introduction) Objective: Ø Ø Ø Explain & Analyze the operation & characteristic of power supply filters & Regulators Explain the purpose of a filter Describe the capacitor-input filter Define ripple voltage & calculate the ripple voltage Discuss surge current in capacitor-input filter Discuss voltage regulation & integrated circuit regulator 20

2 -3 Power Supply Filters And Regulators (cont. ) (introduction) Power Supply Filters To reduce the fluctuations in the output voltage of half / full-wave rectifier – produces constant-level dc voltage. Ø It is necessary – electronic circuits require a constant source to provide power & biasing for properation. Ø Filters are implemented with capacitors. Ø Regulators Voltage regulation in power supply done using integrated circuit voltage regulators. Ø To prevent changes in the filtered dc voltage/ to fix output dc voltage due to variations in input voltage or load. Ø 21

2 -3 Power Supply Filters And Regulators (cont. ) (introduction) • In most power supply – 60 Hz ac power line voltage is converted to constant dc voltage. • 60 Hz pulsating dc output must be filtered to reduce the large voltage variation. • Small amount of fluctuation in the filter o/p voltage - ripple 22

2 -3 Power Supply Filters And Regulators (cont. ) (Capacitor-Input Filter) For half-wave rectifier: • +ve first quarter cycle –diode is FBcapacitor is charging within 0. 7 V of i/p peak load capacitor • I/p decreased below its peak-capacitor retains its charge-diode become RB (cathode is more +ve than the anode) • Capacitor can discharge through load resistance – at rate by the RLC time constant (>> time constant, << capacitor will discharge) • Next cycle-diode is FB when i/p voltage exceeds the Vc by 0. 7 V • A capacitor-input filter will charge and discharge such that it fills in the “gaps” between each peak. This reduces variations of voltage. This voltage variation is called ripple voltage. 23

2 -3 Power Supply Filters And Regulators (cont. ) (Capacitor-Input Filter) Ripple Voltage: the variation in capacitor voltage due to the charging and discharging. The advantage of a full-wave rectifier over a half-wave is quite clear. The capacitor can more effectively reduce the ripple when the time between peaks is shorter. Easier to filter -shorted time between peaks. -smaller ripple. 24

2 -3 Power Supply Filters And Regulators (cont. ) (Capacitor-Input Filter) Ripple factor: indication of the effectiveness of the filter [half-wave rectifier] Vr(pp) = peak to peak ripple voltage; VDC = VAVG = average value of filter’s output voltage. • Lower ripple factor better filter [can be lowered by increasing the value of filter capacitor or increasing the load resistance] • For the full-wave rectifier: Vp(rect) = unfiltered peak. 25

2 -3 Power Supply Filters And Regulators (cont. ) (Capacitor-Input Filter) Surge Current in the Capacitor-Input Filter: Being that the capacitor appears as a short during the initial charging, the current through the diodes can momentarily be quite high. To reduce risk of damaging the diodes, a surge current limiting resistor is placed in series with the filter and load. The min. surge Resistor values: IFSM = forward surge current rating specified on diode data sheet. 26

2 -3 Power Supply Filters And Regulators (cont. ) (IC Regulators) v Connected to the output of a filtered & maintains a constant output voltage (or current) despite changes in the input, load current or temperature. v Combination of a large capacitor & an IC regulator – inexpensive & produce excellent small power supply v Popular IC regulators have 3 terminals: (i) input terminal 1. (ii) output terminal 2. (iii) reference (or adjust) terminal v Type number: 78 xx (xx –refer to output voltage) i. e 7805 (output voltage +5. 0 V); 7824 (output voltage +24 V) 27

2 -3 Power Supply Filters And Regulators (cont. ) (IC Regulators) Regulation is the last step in eliminating the remaining ripple and maintaining the output voltage to a specific value. Typically this regulation is performed by an integrated circuit regulator. There are many different types used based on the voltage and current requirements. Connected to the output of filtered rectifier output Bridge-full wave rectifier Gnd filter regulators Fig. 2 -23 : A basic +5. 0 V regulated power supply 28

2 -3 Power Supply Filters And Regulators (cont. ) (IC Regulators) Keeps a constant 1. 25 V between the o/p and adjust terminals const. current in R 1 Adjustable resistor 0 – 1. 0 k. Ohm FIGURE 2 -34 A basic power supply with a variable output voltage (from 1. 25 V to 6. 5 V). Adjustable regulators min. 1. 25 V , max 6. 5 V 29

2 -3 Power Supply Filters And Regulators (cont. ) (Percent Regulations) How well the regulation is performed by a regulator is measured by it’s regulation percentage. There are two types of regulation, line and load. Line and load regulation percentage is simply a ratio of change in voltage (line) or current (load) stated as a percentage. VNL : out put voltage with no load VFL : output voltage with full load 30

2 -4 Diode Limiting & Clamping Circuits (Introduction) Objectives: ØAnalyze the operation of diode limiting, clamping circuit, voltage multipliers and interpret and use diode data sheet. ØDetermine V of biased limiter & used voltage-divider bias to set limiting level. ØDiscuss voltage doublers, triplers & quadruples. ØIdentify V & current ratings. ØDetermine the electrical characteristics of a diode. ØAnalyze graphical data ØSelect an appropriate diode for a given set of specifications. 31

2 -4 Diode Limiting & Clamping Circuits (Diode Limiters) • Diode limiters/clippers – that limits/clips the portion of signal voltage above or below certain level. • Limiting circuits limit the positive or negative amount of an input voltage to a specific value. • When i/p is +ve – the diode becomes FB – limited to +0. 7 V because cathode is at ground. • When i/p << 0. 7 V-diode is RB – o/p voltage likes –ve part of i/p voltage • Turn the diode around-negative part of i/p voltage is clipped off. • When diode is FB-negative part of i/p voltage-diode drop -0. 7 V 32

Question 4: What would you expect to see displayed on an oscilloscope connected across RL in the limiter shown below. 33

Solution question 4 Ø The diode is forward biased and conducts when input voltage goes below -0. 7 V. So, for –ve limiter, the peak output voltage across RL is: Ø Ø The waveform is shown below: Ø 34

2 -4 Diode Limiting & Clamping Circuits (cont. ) (Diode Limiters) Biased Limiters : • The level to which an ac voltage is limited can be adjusted by adding bias voltage VBIAS in series with diode. • Voltage at point A : VA = VBIAS + 0. 7 V (forward-biased & conduct). So, all Vin > VA is clipped off. • For –ve level, then VA = -VBIAS - 0. 7 V to forward-biased. • Turning diode around, +ve limiter – modified to limit Vout to the portion of Vin waveform above VBIAS – 0. 7 V. • -ve limiter; below -VBIAS - 0. 7 V. • By tuning the diode around- the +ve limiter can modified to limit the o/p voltage to the portion of the i/p voltage waveform above VBIAS-0. 7 V • Negative limiter – limit the o/p voltage to the i/p voltage below –VBIAS+0. 7 V A positive limiter A negative limiter 35

2 -4 Diode Limiting & Clamping Circuits (cont. ) (Diode Limiters) Voltage-Divider Bias: • The bias voltage source – used to illustrate the basic operation of diode limiters can be replace by a resistive voltage divider that derives the desired bias voltage from dc Vsupply. • VBIAS – set by the resistor values according to the voltage-divider formula: • The desired amount of limitation can be attained by a power supply or voltage divider. The amount clipped can be adjusted with different levels of VBIAS. • The bias resistor << R 1 - the forward current through the diode will not effect VBIAS 36

Example 5: 1. Sketch the output voltage waveform as shown in the circuit combining a positive limiter with negative limiter in Figure 5 -1. +15 V 6 V -15 V 6 V Figure 5 -1 37

Example 5 (cont. ): 2. A student construct the circuit as shown in Figure 5 -2. Describe the output voltage waveform on oscilloscope CH 2. +15 V +20 V CH 2 -20 V Figure 5 -2 38

2 -4 Diode Limiting & Clamping Circuits (cont. ) (Diode Clampers) q A diode clamper adds a DC level to an AC voltage. The capacitor charges to the peak of the supply minus the diode drop. Once charged, the capacitor acts like a battery in series with the input voltage. The AC voltage will “ride” along with the DC voltage. The polarity arrangement of the diode determines whether the DC voltage is negative or positive. q For negative clamper, the diode is turn around. A negative dc voltage is added to the input voltage to produce the output. Also known as dc restorers. Positive clamper operation Negative clamper operation For a good clamping action RC time constant ~ 10 fin 39

2 -4 Diode Limiting & Clamping Circuits (cont. ) (Diode Clampers) A Clamper Application: ØA clamping circuit is often used in TV receivers as a dc restorer. ØThe incoming composite video signal is normally processed through capacitively coupled amplifiers that eliminate the dc component, thus losing black and white reference levels and the blanking level. Before applied to the picture tube, these reference level must be restored. 40

2 -5 Voltage Multiplier (Introduction) • Use clamping action to increase peak rectified voltages without necessary to increase input transformer’s voltage rating. • Multiplication factors: two, three or four. • Three types of voltage multipliers: * Voltage doubler - Half – wave voltage doubler - Full – wave voltage doubler * Voltage tripler * Voltage Quadrupler • Voltage multipliers are used in high-voltage, low current applications, i. e TV receivers. 41

2 -5 Voltage Multiplier (cont. ) (Voltage Doubler) Half-wave voltage Doubler: Clamping action can be used to increase peak rectified voltage. Once C 1 and C 2 charges to the peak voltage they act like two batteries in series, effectively doubling the voltage output. The current capacity for voltage multipliers is low. PIV = 2 Vp By applying Kirchhoff’s Law at (b): ~ approximately 2 Vp (neglecting diode drop D 2) Half-wave voltage doubler operation. Vp is the peak secondary voltage. 42

2 -5 Voltage Multiplier (cont. ) (Voltage Doubler) Full-wave voltage doubler: Arrangement of diodes and capacitors takes advantage of both positive and negative peaks to charge the capacitors giving it more current capacity. forward-bias output charges forward-bias charges Secondary voltage positive Secondary voltage negative 43

2 -5 Voltage Multiplier (cont. ) (Voltage Tripler & Voltage Quadrupler) Ø Voltage triplers and quadruplers utilize three and four diode capacitor arrangements, respectively. Ø Voltage tripler and quadrupler gives output 3 Vp and 4 Vp, respectively. Ø Tripler output is taken across C 1 and C 3, thus Vout = 3 Vp Ø Quadrupler output is taken across C 2 and C 4 , thus Vout = 4 Vp Ø PIV for both cases: PIV = 2 Vp Voltage Triple Voltage Quadruple 44

2 -6 The Diode Data Sheet (Introduction) • The data sheet for diodes and other devices gives detailed information about specific characteristics such as the various maximum current and voltage ratings, temperature range, and voltage versus current curves (V-I characteristic). • It is sometimes a very valuable piece of information, even for a technician. There are cases when you might have to select a replacement diode when the type of diode needed may no longer be available. • These are the absolute max. values under which the diode can be operated without damage to the device. 45

2 -6 The Diode Data Sheet (cont. ) (Maximum Rating) 1 N 4001 1 N 4002 1 N 4003 UNIT Rating Symbol Peak repetitive reverse voltage Working peak reverse voltage DC blocking voltage VRRM VRWM VR 50 100 200 V Nonrepetitive peak reverse voltage VRSM 60 120 240 V rms reverse voltage VR(rms) 35 70 140 V Average rectified forward current (single-phase, resistive load, 60 Hz, TA = 75 o. C Io Nonrepetitive peak surge current (surge applied at rated load conditions) IFSM Operating and storage junction temperature range Tj, Tstg A 1 A 30 (for 1 cycle) -65 to +175 o. C 46

2 -6 The Diode Data Sheet (cont. ) (Maximum Rating) FIGURE 2 -56 A selection of rectifier diodes based on maximum ratings of IO, IFSM, and IRRM. 47

2 -7 Troubleshooting (introduction) Objective: Troubleshoot diode circuits using accepted techniques. Ø Discuss the relationship between symptom & cause, power check, sensory check, component replacement method and discuss the signal tracing technique in the three variations. Ø Fault analysis. Ø Our study of these devices and how they work leads more effective troubleshooting. Efficient troubleshooting requires us to take logical steps in sequence. Knowing how a device, circuit, or system works when operating properly must be known before any attempts are made to troubleshoot. The symptoms shown by a defective device often point directly to the point of failure. There are many different methods for troubleshooting. We will discuss a few. 48

2 -7 Troubleshooting (cont. ) (Troubleshooting Techniques) Here are some helpful troubleshooting techniques: Ø Power Check: Sometimes the obvious eludes the most proficient troubleshooters. Check for fuses blown, power cords plugged in, and correct battery placement. Ø Sensory Check: What you see or smell may lead you directly to the failure or to a symptom of a failure. Ø Component Replacement: Educated guesswork in replacing components is sometimes effective. Ø Signal Tracing: Look at the point in the circuit or system where you first lose the signal or incorrect signal. 49

2 -7 Troubleshooting (cont. ) (Troubleshooting Techniques) Signal tracing techniques: Input to output Output to input 50

2 -7 Troubleshooting (cont. ) (Fault Analysis) Can be applied when you measure an incorrect voltage at a test point using signal tracing and isolate the fault to a specific circuit. Example 1: Effect of an Open Diode in a Half. Wave Rectifier: - Zero o/p voltage - Open diode breaks the current path from transformer secondary winding to the filter and load resistor – no load current. Other faults: open transformer winding, open fuse, or no input voltage. 51

2 -7 Troubleshooting (cont. ) (Fault Analysis) Example 2: Effect of an Open Diode in a Full-Wave Rectifier: -The effect of either of two diodes is open diode, the o/p voltage will have large than normal ripple voltage at 60 Hz rather than at 120 Hz. - Another fault – open in one of the halves of the transformer secondary winding. - Open diode give same symptom to bridge full-wave rectifier. (See Figure 2 -63) 52

2 -7 Troubleshooting (cont. ) (Fault Analysis) Example 3: Effect of a Shorted Diode in a Full-Wave Rectifier: § Fuse should blow – cause by short circuit § D 1, D 4 will probably burn open. 53

2 -7 Troubleshooting (cont. ) (Fault Analysis) Example 4: Effect of a fault filter capacitor: § Open – o/p is full-wave rectified voltage § Shorted – the o/p is 0 V § Leaky – increase the ripple voltage on the o/p Example 5: Effect of a Faulty Transformer: § Open primary/secondary winding of a transformer – 0 V o/p 54

2 -7 Troubleshooting (cont. ) (The complete Troubleshooting Process) Ø The complete troubleshooting process: (i) Identify the symptom(s). (ii) Perform a power check (iii) Perform a sensory check (iv) Apply a signal tracing technique. (v) Apply fault analysis (vi) Use component replacement to fix the problem. 55

Summary Ø The basic function of a power supply to give us a smooth ripple free DC voltage from an AC voltage. ØHalf-wave rectifiers only utilize half of the cycle to produce a DC voltage. ØTransformer Coupling allows voltage manipulation through its windings ratio ØFull-Wave rectifiers efficiently make use of the whole cycle. This makes it easier to filter. ØThe full-wave bridge rectifier allows use of the full secondary winding output whereas the center-tapped full wave uses only half. ØFiltering and Regulating the output of a rectifier helps keep the DC voltage smooth and accurate 56

Summary ØLimiters are used to set the output peak(s) to a given value. ØClampers are used to add a DC voltage to an AC voltage. ØVoltage Multipliers allow a doubling, tripling, or quadrupling of rectified DC voltage for low current applications. ØThe Data Sheet gives us useful information and characteristics of device for use in replacement or designing circuits. ØTroubleshooting requires use of common sense along with proper troubleshooting techniques to effectively determine the point of failure in a defective circuit or system. 57

Solution 2: The peak output voltage for the circuit is: Vp(out) = Vp(in) – 0. 7 V = 5 – 0. 7 = 4. 30 V 58

Solution 3: Vp(pri) = Vp(in) = 156 V The peak secondary voltage is: Vp(sec) = n. Vp(pri) = 0. 5 (156 V) = 78 V The rectified peak output voltage is: Vp(out) = Vp(sec) – 0. 7 V = 78 – 0. 7 = 77. 3 V where Vp(sec) is the input to the rectifier. 59

Solution 5: 1. +6. 7 V -6. 7 V 60

Solution 5 (cont. ): 2. +10. 31 V -10. 31 V 61