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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