FACULTY OF ENGINEERING LAB SHEET

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1 EEN1046 Electronics III Experiment ECT2 FACULTY OF ENGINEERING LA SHEET ELECTRONICS III EEE1046 TRIMESTER 3 (2015/2016) ECT2: Voltage Regulators

2 EEE1046 Electronics III Experiment ECT2: Voltage Regulators 1.0 Objective i. To study the major parts of a voltage regulator and how they work ii. To determine the load and line regulation of a voltage regulator iii. To study the operation of a voltage regulator with constant current limiting 2.0 Apparatus Equipment Required Components Required Adjustable DC Power Supply 1 2N2222A JT 3 [TO-92 plastic package] Digital Multimeter 1 Resistor 10 [ 1 / 4 W] 1 readboard 1 Resistor 47 [ 1 / 4 W] 1 Resistor 1k [ 1 / 4 W] 1 Resistor 3.9k [ 1 / 4 W] 2 Resistor 10k [ 1 / 4 W] 1 Resistor 220 [ 1 / 2 W] 1 Potentiometer 1k [0.5W or 1W] 1 Potentiometer 10k [0.5W or 1W] 1 Ceramic capacitor 0.1 F 1 Zener Diode 4.7V [0.5W] [ZX55C4V7] Introduction Voltage regulator is used to provide a predetermined dc voltage which is not affected by the amount of current drawn, temperature, nor the variation in the AC line voltage. A linear series voltage regulator contains a control element [usually a transistor] which always operates in the active region, hence the term linear. The control element is in series between the unregulated line voltage and the regulated output voltage. When the control element is a transistor, it is often referred to as the pass transistor as it passes the required current to maintain the predetermined amount of regulated output voltage. The main elements of a linear series voltage regulator include: a) A control element b) A reference voltage c) An error detector d) A sampling network 1

3 Figure 3.1 below depicts the interconnection between these elements. V in Control element V out Reference voltage Error detector Sampling network Figure 3.1 The major parts of a linear series voltage regulator The basic operation of the linear series voltage regulator is as follows: i. The error detector compares the reference voltage with a sample of the output voltage ii. The output of the error detector is fed to the control element iii. The control element causes the output voltage to increase or decrease until the sample voltage equals the reference voltage iv. When this occurs, the error voltage is zero and the control element is held in a stable state v. This will keep the output voltage relatively constant regardless of the load requirements [within specific limits] Figure 3.2 shows a linear series voltage regulator built with discrete components. A zener diode is used to provide the reference voltage (V Z ). The sampling network has a potentiometer that acts as a variable voltage divider. A single transistor error detector [error amplifier] amplifies the differential voltage between its inputs [V Z and V Q2 ] causing an immediate change in the base current of the pass transistor of the control element. When the output voltage decreases for some reason, V Q2 decreases. This reduces the differential voltage of the error amplifier [since V Z is fixed], causing I CQ2 to decrease. A smaller I CQ2 reduces the voltage across R 2 causing the base voltage of the pass transistor to increase. This brings the output voltage back to its original level, as the control element allows more current to pass through. On the other hand, if the output voltage increases for some reason, V Q2 increases. This increases the differential voltage of the error amplifier causing its collector current to increase. More collector current increases the voltage drop across R 2, causing a decrease in the base voltage of the pass transistor. This reduces the output voltage to its original value as the control element limits the amount of current that can pass through. 2

4 R 2 Q 1 control element R 1 R 3 V S Q 2 error detector R 4 R L + V Q2 reference votlage D 1 V Z C 1 R 5 sampling network Figure 3.2 Linear series voltage regulator with discrete components Percent load regulation is one of the methods used to determine the relative quality or effectiveness of a voltage regulator to maintain nominal or no-load regulation. The lower the percent load regulation, the better the regulator is in keeping the output voltage at its nominal value [the no-load voltage] for a particular load. VNL VFL % L. R. 100% Eqn (1) V where V NL = the no-load output voltage [the output voltage when the load is open] V FL = the full-load output voltage [the output voltage when the load current demand is at its maximum value] FL Another method of measurement that is commonly used to determine the relative quality or effectiveness of regulation is source or line regulation. Line regulation is the variation in output voltage that occurs when the unregulated input voltage increases or decreases by a specified amount. The lower the percent line regulation, the better the regulator is in keeping the output voltage constant when changes in line voltage occur. where = variation in output voltage V S = variation in input voltage VO % S. R. 100% Eqn (2) V S 3.1 Constant Current Limiting Constant current limiting is a protection scheme that prevents damage to the pass transistor if a short-circuit or large current demand occurs. Figure 3.3 shows a discrete series voltage regulator with constant current limiting. The value of R SC is chosen to limit the pass transistor current to a specified and safe level: 3

5 R SC VE ( Q3) VE ( Q3) Eqn (3) I I PT (max) RSC(max) where I PT(max) is the maximum limited current through the pass transistor (I PT = I C(Q1) ) I RSC(max) is the maximum limited current through the current limiting resistor, R SC Q 1 R SC constant current limiting R 1 R 2 R 3 Q 3 + V S R L Q 2 R 4 V Q2 D1 V Z C 1 R 5 Figure 3.3 Series voltage regulator with constant current limiting When the pass transistor current reaches I PT(max), Q 3 turns ON and the base current (of Q 1 ) is diverted away from the pass transistor Q 1, limiting the current through it to I PT(max). Under short-circuit condition ( = 0V), the output current will be: I SC VS VE ( Q1) VE ( Q3) I PT (max) - Eqn (4) R 2 Since I PT(max) keeps Q 3 ON with a base-emitter voltage of 0.7V, the current through R SC and Q 1 remains relatively constant. The major disadvantage of constant current limiting is that a heatsink is usually required on the pass transistor to prevent overheating damage. When a short-circuit occurs, almost the entire line voltage is dropped across the pass transistor [V C(Q1) = V S, V E(Q1) = 0.7V]. Hence, the power dissipation [P D = V CE(Q1) I PT(max) ] of the pass transistor will be high. At large line voltage, even small I PT(max) may require a heatsink. Heatsinking usually increases the cost and board space of series voltage regulators. Figure 3.4 illustrates the relationship between the output voltage and current when constant current limiting is employed. Note I PT = I C(Q1). 4

6 I PT(max) (nom) Note: Normally the feedback current is negligible I SC Figure 3.4 vs. for constant-current limiting protection scheme 4.0 Experiments 1. Instructor s checks : Student is responsible to ask the instructor to check your experimental results before proceeding to the next experiment. On-The-Spot Evaluation will be carried out in the first exp. 2. JTs and Zener diode checks: perform go/no-go testing as mentioned in Appendix A. 3. Importance to check your JTs: You must check your JTs. The number of burned JT will be recorded by the lab staff and penalty will be applied for burning > 1 JT. 4. JT burned cautions: Any form of short circuit between the collector (C) pin and the base () pin will burn the JT emitter junction (J E ). 5. Setting up the DC Power Supply: (a) Set DC Power Supply to 15V (power supply output has not connected to the circuit) (b) Set the current scale switch to LO (if any) (c) Set the current adjustment knob to about ¼ turn from the min position (d) On the DC power supply unit, connect the output terminal to the GND terminal Experimental Circuits: A V S +15V R 1 1k D 1 V Z 4.7V R 2 3.9k C E Q 2 C Q 1 E V,Q1 Space reserved for Exp 4.2 and Exp 4.3 V C1 C 1 0.1uF R3 3.9k R 4 10k pot R 5 10k J R L1 220 X R L2 1k pot Y {V L } Note: Q 1, Q 2 and Q 3 are 2N2222A Figure 4.1 (for Exp 4.1) 5

7 R SIE R SC A V S +15V R 1 1k D 1 V Z 4.7V R 2 3.9k C E Q 2 C Q 1 E C Q 3 E C 1 0.1uF R 3 3.9k R 4 10k pot R 5 10k R L1 220 X R L2 1k pot Y Note: Q 1, Q 2 and Q 3 are 2N2222A Figure 4.2 (for Exp 4.2) 4.1 Load and Line Regulation 1. Construct the circuit as shown in Figure 4.1. PRECAUTIONS to prevent to burn JT Q 1 : The emitter junction (J E ) or the whole transistor Q 1 can be burned in a short duration (e.g. by a transient touching of connections) if any mistake occurs. (i) J E gets burned: if there is a short circuit between the collector leg (or metal case) and the base leg. [The metal case of JT is internally connected to the collector]. (ii) Whole JT gets burned: if there is a short circuit between the points A and or the emitter leg is shorted to the ground (0V). Prevention Steps: (i) Use connecting wires to connect component legs to the JT legs. (ii) Avoid too compact or messy circuit. (iii) Double-check the circuit connections and the resistors used. (iv) Precautions (e.g. measure V,Q1 at the R 2 leg which is connected to the base leg) 2. efore connect the power supply output to the circuit, set V S = 15V. 3. Analyze the circuit to predict the values of V,Q1 (respect to ground), V C1, I o(min) (when R L = R L1 +R L2 = 1220Ω) and I o(max) (R L = 220Ω) for = +9V. Show your analysis in the column provided in the Discussion part. These values are used for checking purposes. 4. Measure V D1 (the voltage across D 1 ). This value should be around 4.7V DC. 5. With jumper wire J removed, adjust R4 to get = +9V. Record as no-load voltage V NL. Measure and record V,Q1 and V C1. These values should be around those in Step Connect the jumper J. Turn R L2 to Y side (max resistance). Let this is 0 o position. 7. Measure and V RL1. Record value as the loaded voltage V L in Table 4.1(a). Check V S with multimeter so that it is the same at each set of V L and V RL1 measurement. 8. Turn R L2 about 60 o and record V L and V RL1 values. Repeat for every 60 o turn in the same direction. 9. Calculate and record = V RL1 /R L1 and R L = V L /. Note: at 0 o and (max) angle should be around those predicted values in Step 3. 6

8 10. Using the measured values, calculate [with Eqn (1)] the percent load regulation (%L.R.) for each R L value in Table 4.1(a). 11. Turn R L2 to 0 o position and record the output voltage as (nom) (Nominal output voltage) and the diode D 1 voltage as V Z(nom) in Table 4.1(b). 12. Decrease the DC input voltage from +15V to +12V (a change of 20% in the line voltage). 13. Measure and record as (min) and the diode D 1 voltage as V Z(min) in Table 4.1(b). 14. Using the measured values, calculate [with Eqn (2)] the percent line regulation (%S.R.). 15. Ask the instructor to check your results. Show your last multimeter reading to the instructor. On-The-Spot Evaluation: to be part of the Lab Performance Evaluation. On-The-Spot Evaluation references: JT gets burned: -10% of ECT2 total mark per burned JT if burned > 1 JT Equip. setup: bad (> 1 wrong setup) / average (1 wrong setup) / good (no mistake setup) Values: bad (> 1 wrong value) / average (1 wrong value) / good (no wrong value) Equipment setup: any equipment (power supply, multimeter, breadboard) - can be evaluated at anytime along the whole lab session Setup: Any mistakes in equipment wiring-connections and settings Values: Any much different between theory & Exp, bad/wrong records, errors in calculations from exp results Note: Circuit is not working but experimental results are correct Cheating (0 marks) 4.2 Constant Current Limiting 1. Modify the circuit in Figure 4.1 to that of Figure 4.2. CAUTIONS: Use connecting wires to connect component legs to the JT legs. Doublecheck the circuit connections. Set V S = 15V before connect its outputs to the circuit. 2. Analyze the circuit to predict I o(min) and V,Q1(RLmax) (when R L = 1220 ) for V o = +9V. Assume I o I RSC. Show your analysis in the Discussion part. 3. Estimate I PT(max) based on Eqn (3). Assume V E(Q3) = 0.7V. Show your analysis in the Discussion part. 4. Analyze the circuit to predict V o(rlmin) (R L = 220 ). Note Q 3 has turned on causing V o < 9V. Assume I o I RSC. Show your analysis in the Discussion part. 5. Turn R L2 to Y side (max resistance). Adjust R 4 to get = +9V. Measure and record V D1 and V,Q1(RLmax). Check with the value in Step Measure and record, V C1, V RL1 and V RSIE in Table Calculate = V RL1 /R L1 and I E,Q1 = V RSIE /R SIE. Check with the value in Step To plot V o versus I o, I o or V o is changed in step of I o (2mA) or V o (2V), depending on the portion of the graph as shown in Figure 4.3 below. NO direct measurement is required, instead measure V RL1 change ( V RL1 ). Steps: (a) calculate V RL1 = I o R L1 and then V RL1(next) = V RL1(present) + V RL1. (b) measure V RL1 while adjusting R L2 to get about the reference V RL1(next) value. 9. egin to measure at I o(min) and end at V o(rlmin). Make sure there are, V C1, V RL1 and V RSIE measurements at the point when V o begins to drop significantly (at I PT(max) ). Adjust R L2 from Y to X side to change I o or V o. Check with the values in Step 3 and Step 4. 7

9 (nom) [egin] I PT(max) 8V Δ = 2mA [End] 6V 4V (RLmin) Δ = 2V (min) Figure 4.3 V o versus I o for constant-current limiting protection scheme I SC 10. Plot versus and I E,Q1 (share the same x-axis) on Graph Ask the instructor to check your results. Show your last multimeter reading to the instructor. Report Submission You are to submit your report immediately upon completion of the laboratory session. End of Lab Sheet 8

LABORATORY MODULE. Analog Electronics. Semester 2 (2005/2006)

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