EE100B Labs

PAGE 14

EE1o0B Electronic Circuits IIDepartment of Electrical Engineering

University of California Riverside

Instructor: Mihri

Laboratory 5EE 100 BWinter 2015

LABORATORY # 5M A N U A LFeedback Based Voltage RegulatorsTHEORY. Design and Characterization of Voltage RegulatorsPART 1. Characterization of Voltage RegulatorsPART 2. Characterization of Adjustable Voltage Regulator LM317T NOTES:

1. WARNING: When replacing load resistors accidental shorting is possible which will send a tremendous amount of current through fingers, easily 0.5 0.8 A by conservative estimates. The circuit schematic in Figure P1-1 contains the procedure on how to deal with resistors. Do not be fooled by the simplicity of the circuits, both of them are very powerful, and dangerous if mishandled;

2. Lowest value resistors may get very hot if hold in the circuit for too long. Again, follow the procedure mentioned on the schematic;

3. If students want to, they may tap the transistor to feel how hot it is when the load resistance is 100 Ohm. DO NOT TAP THE LOAD RESISTOR 100 Ohm until it is disconnected from ground to prevent accidental shorting. It should not stay in the circuit for too long;

4. Usual warning about polarized electrolytic capacitors.

ObjectivesLab 5 contains two parts and objectives are to get familiar with:

1. Design of Adjustable Voltage Regulators2. Characterization of Voltage Regulators3. Characterization of an Adjustable Voltage Regulator LM317TEquipment PC or compatible

DMM (digital multimeter) Digital Oscilloscope

Function Generator Split Power supply (+20V)

Wires to interconnect parts on solderless breadboard

Solderless breadboard (you need to bring one)

Parts transistor: 2n3866

IC (voltage regulator): LM317T diode:

1n5234 (zener)

capacitors: 1 uF, 100 uF (polarized), 0.1 uF (ceramic) resistors (all 1/4W, 5%): 2x 100, 2x 510, 1k, 2x 5.1k, 2x 10k, 2x 100k potentiometer: 10k

BEWARE In contrast to a popular belief, electrical engineers know better:

High Voltage doesnt kill, HIGH CURRENT DOESExample: 100 mA = 0.1 A is a very high current. Remember current sources?

WARNINGPolarized Capacitors must be handled with great caution:

1. Polarized capacitors must be properly connected in a circuit: + terminal of a cap to + terminal in the circuit, - to -. Otherwise the cap can be permanently damaged;

2. Never touch both legs of a polarized capacitor with your fingers after usage since it may contain a substantial amount of charge that can electrocute you or at best burn the skin;

3. Never discharge a polarized capacitor by short-circuiting the legs. Such a discharge will create a great amount of current that can overheat the cap and cause an explosion (no kidding). Since polarized capacitors contain liquid chemical acid matter such an event may cause permanent damage to your eyes. It is a good habit to wear safety glasses while handling polarized capacitors;

4. Never discharge a capacitor while still in circuit;

5. In order to safely discharge a capacitor after handling, use a high wattage low value resistor (say, 100 Ohm, 1W), connect the resistor to the cap legs and wait for a couple of seconds to fully discharge the cap (10-20 seconds may be enough but it may vary, easy to compute though by RC circuit analysis);

6. Never store (long-term storage) used polarized capacitors without properly discharging them.

SPECIFICATIONTHEORETICAL BACKGROUND INTRODUCTIONNearly all electronic circuits require sources of stable DC voltage. The simple transformer - rectifier bridge capacitor unregulated power supplies are not generally adequate because their output voltages change with load current and line voltage and because they have a significant amount of 120 Hz ripple. It is however easy to construct stable power sources using either negative or positive feedback requiring only a source of unregulated DC input. a) b)Figure 1. Power Supply a) non-ideal; b) idealIn the design of power supplies it is frequently required to attain source impedances less then just a few ohms. SHAPE \* MERGEFORMAT

Figure 2. Conventional Voltage Transformation FlowThere are two types of voltage regulators linear (Vout < Vin) and switching or pulsed (also known as DC-DC converters). While linear regulators do have a poor efficiency in terms of power consumption, they are simple, inexpensive and provide electrically quiet and pure DC output. Switchmode regulators on the other hand are more complex but very efficient (>90%), provide wide ranges of output voltages for wide ranges of input voltages, and are extremely noisy.In this laboratory we will deal with a design and characterization of linear, adjustable voltage regulators.CHARACTERIZATION OF VOLTAGE REGULATORSThere are two fundamental characteristics of voltage regulators:1. Line regulation which shows how much the output voltage changes with the change in the in the input voltage

(1)

[mV/V]2. Load regulation which shows how much the output voltage changes with the change in the output current resulting from a change in the out put impedance(2)

[mV/mA]DESIGN OF VOLTAGE REGULATORS

STEP 1. REFERENCE VOLTAGEAt the heart of linear voltage regulators is zener reference. The diode operates in its breakdown region where the dynamic resistance rz is nearly constant:(3)

Figure 3. The i-v characteristic of zener diode and its equivalent model

Figure 4. The simplest voltage regulator (a) circuit, (b) modelThe voltage regulator shown in Figure 4 will serve the purpose in many applications where non-linearity of the reverse breakdown i-v characteristic does not affect the performance, e.g., in cases when there is little load, input voltage or temperature variation. The DC operating conditions of the zener diode are established by utilizing the load line analysis as usual.STEP 2. REDUCTION OF OUTPUT IMPEDANCEAs we already know, emmiter followers provide a unity gain and a very low output impedance:

(4)

a) b)Figure 5. Emitter Follower stage a) and its small signal model and parameters b)

Figure 6. Emitter Followers stiff DC biasing a) b)Figure 7. Emitter Follower DC biasing with diodes a) and equivalent model b)STEP 3. ADJUSTABLE OUTPUT VOLTAGEObserve that the voltage regulator of Figure 7 is not flexible in terms of output voltages it can provide since the zener diodes breakdown voltage is pretty much fixed. One way to overcome this problem is to use feedback to change the voltage level at the base of the transistor. For this purpose we will use the positive DC feedback.By adjusting R1 in the voltage divider R1-R2 we can shift quite substantially the voltage level both at the input and the output of the emmiter follower which also serves as the output of the voltage regulator, see Figure 8.

Figure 8. Positive Feedback is used for voltage adjustmentSTEP 4. INPUT NOISE REMOVAL (LOW-PASS FILTER DESIGN)

Figure 9. LPF characterization and designIn the previous labs we have already dealt with low-pass filtering of incoming noise (spikes) by using large value bypass capacitors. It comes as no surprise since the bypass capacitors in series with the output impedance of power supplies form low-pass filters as shown in Figure 9.

3dB-bandwidth of a simple passive LPF can be estimated as

(5)

Note that it is impossible in general to remove completely the input noise (interference) of very low frequency.PART 1. Characterization of Voltage RegulatorsIn this laboratory experiment it is required to implement and characterize an adjustable voltage regulator which provides regulated output voltage 5.5V 8V and nominal 6V output with the input, assumingly unregulated, 9V power supply.

Component ratings

Resistors

Maximum power dissipation PMAX = 250 mW WARNING: do not keep 100 load resistor RL in the circuit for too long. It will operate at a power level it is not designed for, and it will get very hot. Use precaution.Zener diode 1n5234 Breakdown voltage VZ = -6.2VMaximum power dissipation PMAX = 500 mW @ 75 C

Impedance rz = 1k @ ID = 0.25 mAnpn Transistor 2n3866

Maximum power dissipation PMAX = 500 mWWARNING: with attached 100 load resistor RL in the circuit it will get hot.

Figure P1-1. Adjustable Voltage Regulator

Report:

1. Assemble the circuit as shown in Figure P1-1 without the load resistor;

2. Set the input power supply voltage to about 9V;3. Set R4POT at about 100 Ohm and adjust with a set-screw the output voltage to about 6V;4. By varying the input voltage VDC = VIN from 0V to about 15V record the output voltage using a multimeter;12345N

VIN, V

VOUT, V

5. Plot the characteristic VOUT vs VIN and determine the line regulation characteristic according to formula (1);

6. Based on your results, at which input power supply voltages will the circuit provide a reasonable voltage regulation? What is the largest swing of the input voltage at nominal 6V output the voltage regulator will tolerate to within 10% of its nominal output;

7. Set the input voltage back to about 9V;8. Using a set of load resistors 100k, 10k, 5.1k, 1k, 510, 100 measure the output voltage VOUT and the load current IL vs the load impedance RL.WARNING: follow the procedure outlined in the schematic. Not following the procedure current in the short may cause personal injury, send you straight to the hospital, and permanently damage circuit components and/or equipment. If you smell burning odor, turn off the power of the power supply unit, disconnect load resistor from the ground and double-check pin connections. Lowest value load resistors may get very hot.RL, VOUT, VIL, mAPRL, mWlog(RL)

open

100000

10000

5100

1000

510

100

9. Plot the characteristic VOUT vs IL and determine the load regulation according to (2);

10. Compute the power consumed by the load resistor of each nominal value. Which of the resistors would not suite as a load?

11. Plot the characteristic log(IL) vs log(RL) and determine what current your skin would need to tolerate if you accidentally shorted the output. Speaking of carelessness.PART 2. ADJUSTABLE VOLTAGE REGULATOR LM317T

Figure P2-1. Commercial Adjustable Voltage Regulator LM317TFrom specifications: This monolithic integrated circuit is an adjustable 3-terminal positive voltage regulator designed to supply more than 1.5A of load current with an output voltage adjustable over a 1.2V to 37V. It employs internal current limiting, thermal shut-down and safe area compensation.In this lab experiment we will determine the voltage regulation characteristics of LM317T, an inexpensive but very robust component of choice in multiple electronic circuits.

Figure P2-2. Schematic of the circuit with the adjustable voltage regulator LM317TFollowing the same precautions outlined in Part 1 of the lab we will repeat the procedures to determine the characteristics of this voltage regulator.Report:

1. Assemble the circuit as shown in Figure P2-2 without the load resistor;

2. Set the input power supply voltage to about 9V;

3. Set R2POT at about 380 Ohm and adjust with a set-screw the output voltage to about 6V;

4. By varying the input voltage VDC = VIN from 0V to about 15V record the output voltage using a multimeter;

12345N

VIN, V

VOUT, V

5. Plot the characteristic VOUT vs VIN and determine the line regulation characteristic according to formula (1);

6. Based on your results, at which input power supply voltages will the circuit provide a reasonable voltage regulation? What is the largest swing of the input voltage at nominal 6V output the voltage regulator will tolerate to within 10% of its nominal output;

7. Set the input voltage back to about 9V;

8. Using a set of load resistors 100k, 10k, 5.1k, 1k, 510, 100 measure the output voltage VOUT and the load current IL vs the load impedance RL.

WARNING: follow the procedure outlined in the schematic. Not following the procedure current in the short may cause personal injury, send you straight to the hospital, and permanently damage circuit components and/or equipment. If you smell burning odor, turn off the power of the power supply unit, disconnect load resistor from the ground and double-check pin connections. Lowest value load resistors may get very hot.RL, VOUT, VIL, mAPRL, mWlog(RL)

open

100000

10000

5100

1000

510

100

9. Plot the characteristic VOUT vs IL and determine the load regulation according to (2);

10. Compute the power consumed by the load resistor of each nominal value. Which of the resistors would not suite as a load?

11. Plot the characteristic log(IL) vs log(RL) and determine what current would flow through the shorted output;12. Compare the characteristics obtained in Part 1 and Part 2.

Presentation and Report

Must be presented according to the general EE 100 lab guidelines. Prelab

1. Review EE100A material related to properties of diodes, and zener diodes in particular;

2. Review Lectures 9, 11 and provided brief theoretical discussion in this manual;3. Preassemble components on the breadboard

TRANSFORMER

POWER

LINE

Vout

Voltage

Regulator

1. Adj

2. Output

3. Input

LM317T

TO-220 (case)

1 2 3

LINEAR REGULATOR

LOW-PASS

FILTER (LPF)

DIODE BRIDGE

RECTIFIER

This part is optional. At any rate, review how adjustable voltage regulator ICs function, LM317N in our case.

Part 2 is optional

TIP29 or TIP31 npn power transistors would be way more preferable if we had them in the lab kit

They will be briefly discussed later in the lecture course

more specifically, of this labs voltage regulator. The procedure is however general.

Example: For 1n5225 1n5267 series of zener diodes the breakdown voltage ranges from 3V 74V.

Example: for a DC biasing collector current IC = 2 mA, Rout 25.6 mV / 2 mA = 12

Just for info, besides its direct purpose of providing the DC bias, diode biasing provides indirectly very robust temperature stability (temperature compensation).

Positive Feedback will be discussed in more detail later in the course. For now, just note that this will slightly amplify the incoming voltage noise thus worsening a bit the line regulation. However, the advantage of having an adjustable voltage regulator justifies the choice.

Remember that there are two limiting factors: 1) diode must be in the breakdown region and 2) the voltage of the power supply VCC.

EMBED Equation.DSMT4

In actual applications it is required to use power transistors either TIP29 or TIP31 packaged in TO-220 cases. We dont have them in the lab kit, so 2n3866 was selected due to its relatively large TO-39 case.

Use +20V output of the split power supply unit

Lab 5 Feedback Based Voltage Regulators Manual

EE100B Electronic Circuits II

University of California, Riverside

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