AIN SHAMS UNIVERSITY
FACUTY OF ENGINEERING
Electronics and Communication Engineering Department
Electrical Testing (2)
3rd Year Communications
Electronic Circuits Lab
Experiment (4)
REGULATED POWER SUPPLIES
Student Name: ………………………………………………………..
Section: ………………………………………………...
Date: ……………………………………………...
Pre-Lab y/n
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EXPERIMENT (4)
REGULATED POWER SUPPLY
0.0 Pre-Lab:
Read the Lab notes carefully, and follow the same steps in Part 4. (Using experiment
simulation files)
- In 4.1 Change the resistor by applying parametric sweep on RLoad (change resistor
values from 400 Ohm to 1.5 kOhm.
- In 4.2, Change the supply voltage from 11v to 16v by applying DC sweep on V1,
with RLoad = 400 Ohm.
- In 4.3, adjust the output voltage by changing p1_x and p1_y, taking into your
consideration that p1_x+p1_y = 1k.
- In 4.3, Change the values of RLoad to the same values of R5,R6 and R7.
- Get the Results and document them in your report.
1.0 Objectives
1- To get acquainted with regulated power supplies.
2- To investigate the effect of loading on regulated power supplies, and to suggest some
methods for improving their performance.
2.0 Theory
An unregulated power supply consists of a transformer, a rectifier, and a filter. There are
three reasons why such a simple system is not good enough for some applications. The first
reason is its poor regulation; the output voltage is far from constant as the load varies. The
second one is that the dc output voltage varies directly with the ac input. The third one is that the
dc output varies with the temperature, particularly is semiconductor devices are used. An
electronic feedback or control circuit is used in conjunction with an unregulated power supply to
overcome the above three shortcomings and also to reduce the ripple voltage.
Such a system is called a regulated power supply.
2.1 Performance Metrics:
Since the output dc voltage V0 depends on the input unregulated dc voltage Vi, load
current IL, and temperature T, we have to define some performance metrics to quantify a
regulator’s performance.
2.1.1 Line Regulation
∆𝑉
𝑆𝑣 = ∆𝑉0 |
𝑖
∆𝐼𝐿 =0,∆𝑇=0
(1)
2.1.2 Load Regulation
𝑅0 =
∆𝑉0
|
(2)
|
(3)
∆𝐼𝐿 ∆𝑉 =0,∆𝑇=0
𝑖
2.1.3 Temperature Stability
𝑆𝑇 =
∆𝑉0
∆𝑇 ∆𝑉𝑖 =0,∆𝐼𝐿 =0
The change ΔV0 in output voltage of a power supply can be expressed as follows:
∆𝑉0 =
𝛿𝑉0
𝛿𝑉𝑖
∆𝑉𝑖 +
𝛿𝑉0
𝛿𝐼𝐿
∆𝐼𝐿 +
𝛿𝑉0
𝛿𝑇
∆𝑇
(4)
Or
∆𝑉0 = 𝑆𝑣 ∆𝑉𝑖 + 𝑅0 ∆𝐼𝐿 + 𝑆𝑇 ∆𝑇
(5)
The smaller the value of the three coefficients, the better the regulation of the power
supply. The input-voltage change ∆𝑉𝑖 may be due to a change in ac line voltage or may be ripple
due to inadequate filtering. In our experiment we assume constant temperature; and thus, the
third term in Eqs. (4) and (5) is zero.
2.1.4 Transient Line Regulation
Time needed for the output to settle to its final value when a step voltage is applied at the
input.
2.1.5 Transient Load Regulation
Time needed for the output to settle to its final value when a step current is applied at the
output.
2.2 The Zener Regulator
Zener diode can be used as a voltage reference by biasing it with the input and a resistor. The
first regulator is to connect the load directly to theZener. Refer to Fig. 1. Unfortunately the Zener diode
has internal impedance which may be large, this may result in large output and bad line regulation
𝑅0 = 𝑅𝑧 // 𝑅
(6)
Where𝑅𝑧 represents the dynamic resistance of the zener diode D.
The voltage stabilization ratio (line regulation) is approximately,
∆𝑉
𝑆𝑣 = ∆𝑉0 ≈ 𝑅
𝑖
𝑅𝑧
𝑧 +𝑅
(7)
2.3 The Emitter-follower Regulator
If a power supply has a poor regulation, it possesses a high internal impedance. This
difficulty may be avoided by using an emitter follower to convert from high to low internal
impedance. Refer to Fig. 2. If the output resistance of the unregulated supply is called𝑟0 , then the
output resistance 𝑅0 after the emitter follower has been added is approximately:
𝑅0 =
(𝑅𝑧 // 𝑅)+𝑟𝜋
1+ℎ𝑓𝑒
(8)
Where𝑅𝑧 represents the dynamic resistance of the zener diode D.
The voltage stabilization ratio (line regulation) is approximately,
∆𝑉
𝑆𝑣 = ∆𝑉0 ≈ 𝑅
𝑖
𝑅𝑧
𝑧 +𝑅
Figure 2. An emitter-follower regulator.
(9)
From Eq. (9) we see that improving 𝑆𝑣 requires increasing R, with attendant increase in VCE and
power dissipation in the transistor. Other disadvantages of this circuit are the following:
1) No provision exists for varying the output voltage.
2) Changes in VBE and VR due to temperature variations appear at the output.
2.4 The Series Voltage Regulator:
The physical reason for the improvement in voltage regulation with the circuit of Fig.2
lies in the fact that a large fraction of the increase in input voltage appears across the control
transistor, so that the output voltage tries to remain constant.
If the input increases, the output must also increase (but to a much smaller extent),
because it is this increase in output that acts to bias the control transistor toward less current.
This additional bias causes an increase in collector-to-emitter voltage which tends to compensate
for the increased input.
From the foregoing explanation it follows that if the change in output were amplified
before being applied to the control transistor, better stabilization would result. The improvement
is demonstrated with reference to Fig. 2. Here, a fraction of the output voltage bV0 is compared
with the reference voltage VR. The difference bV0 – VR is amplified by Q2. If the input voltage
increases by ΔVi, the V0 need increase only slightly, and yet Q2 may cause a large current change
in R3. Thus, it is possible for almost all of ΔVi to appear across R3 (and since the base-to-emitter
is small, also across Q1) and for V0 to remain essentially constant.
The output voltage is given by:
𝑉2 +𝑉𝐵𝐸2
2 ⁄(𝑅2 +𝑅1 )
𝑉0 = 𝑅
𝑅
= (𝑉2 + 𝑉𝐵𝐸2 )(1 + 𝑅1 )
It's clear that it can be varied by operating the 𝑅2 , 𝑅1 divider
2
(10)
The output resistance is approximately,
𝑅 +𝑟
𝑟𝑜 + 3 𝜋
𝑅0 = 1+𝐺
1+ℎ𝑓𝑒
𝑚 (𝑟𝑜 +𝑅3 )
(11)
It's clear that it can be varied by operating the 𝑅2 , 𝑅1 divider
The line regulation is given by,
𝑆𝑣 = 𝐺
1
𝑚 𝑅3
(12)
where,
𝐺𝑚 = ℎ𝑓𝑒 (𝑅
𝑅2
2 +𝑅1
1
)
)+𝑟
1 // 𝑅2
𝜋 +(1+ℎ𝑓𝑒 )𝑅𝑧
) ((𝑅
(12)
Where𝑅𝑧 represents the dynamic resistance of the zener diode D.
To improve 𝑆𝑣 it is seen from Eq12 that 𝑅3 should be increased.
Since 𝑅3 =
𝑉𝑖−𝑉𝑜
𝐼
we can increase 𝑅3 by decreasing I. The current I can be decreased by using
Darlington pair for Q1 as shown in Fig. 4
The maximum dc load current of the power supply shown in Fig. 3 is restricted by the
maximum allowable current of the series transistor. The difference between the output and the
input voltages of the regulator is applied across Q1, and thus the maximum allowable VCE for a
given Q1 and specific output voltage determines the maximum input voltage to the regulator.
Figure 4. A regulated power supply with short circuit current protection
2.5 Short-circuit protection
A power supply must be protected further from the possibility of damage through
overload. In case of overload or short circuit, the circuit of Fig. 3 can provide protection. Here,
the diodes D1, D2, and D3 are non-conducting until the voltage drop across the sensing resistor
Rm exceeds their forward threshold voltage. Thus, in the case of a short circuit, the current Im
would only increase up to a limiting point determined by the threshold value Imth given by:
𝐼𝑚𝑡ℎ =
𝑉𝛾1 +𝑉𝛾2 +𝑉𝛾3 −2𝑉𝐵𝐸1
𝑅𝑚
(11)
Under short-circuit conditions, the load current would be approximately:
𝐼𝑠𝑐 = 𝐼 + 𝐼𝑚𝑡ℎ ≈
𝑉𝑖 −(𝑉𝛾1 +𝑉𝛾2 +𝑉𝛾3 )
𝑅3
+
𝑉𝛾1 +𝑉𝛾2 +𝑉𝛾3 −2𝑉𝐵𝐸1
𝑅𝑚
(12)
3.0 Required Components and Equipment:
1)
2)
3)
4)
Power supply (variable to 15V)
Digital voltmeter
Ammeter
The DL 2155 RTD Kit.
Figure 5. The emitter-follower regulator test circuit
4.0 Procedure
4.1 Measurement of the output characteristic V0 = f(I0) of the emitter
follower regulator:
1. Arrange the circuit as shown in Fig. 5 using the RTD 2 board and using R2and P1 from
the RTD1 board as the variable load.
Note: The capacitor C1 is used to reduce the possible ripple, and C2 is used to reduce the
output impedance of the H.T. and to improve the transient response.
2. Supply the circuit with 15 V.
3. With unconnected load, measure the no-load voltage (VOC) using the DVM.
4. Connect the load and fully counterclockwise turn the potentiometer. Measure the output
voltage and current.
5. Partially clockwise turn the potentiometer and measure the new values of V0 and I0.
6. Plot V0 versus I0.
7. For I0 = I0max, calculate the output resistance of the regulator from:
𝑉 −𝑉
𝑅0 = 𝑜𝑐𝐼 𝑜𝑚𝑎𝑥
(13)
𝑜𝑚𝑎𝑥
4.2 Measurement of the output characteristic V0 = f(Vin) of the emitter
follower regulator:
1. With the same setup as the previous experiment, set P1 for maximum load current (you
may disconnect R1).
2. Supply the circuit with an 11 V to 16 V variable voltage, and measure the corresponding
output voltage.
3. Plot V0 versus Vin.
4. Calculate the stability factor Sv for the regulator for ±10% variation of the input voltage.
Figure 6. The regulated power supply with adjustable output test circuit
4.3 Measurement of the output characteristic V0 = f(I0) of the series
voltage regulator:
1. Arrange the circuit as shown in Fig. 6 using the RTD 3 board.
2. Supply the circuit with 15 V.
3. With the DVM, measure the zener voltage, the base voltage of Q2 and the output voltage.
Check that when P1 is operated, the output is varied.
4. Set the output voltage to a mod value, for instance 9 V.
5. As the load, alternatively use R5, R6, and R7, each time measuring the output voltage.
6. Plot V0 versus I0.
Figure 7. A regulated power supply with short circuit current protection test circuit
7. Calculate R0 at I0max using Eq. (13)
4.4 Measurement of the output characteristic V0 = f(I0) of the series
voltage regulator with short-circuit protection, and the maximum output
current.
1. Repeat the previous experiment using the circuit shown in Fig. 6 using the RTD 4 board.
2. Connect R6. A large load current must flow and the diodes D1, D2 and D3 are switched
ON. Measure the output current, and compare with the theoretical value.
5.0 Report:
-
Using ORCAD, apply Short Circuit Test on Figures 6 and 7 and compare the results.
For Figure (6), using ORCAD calculate:
o The stability factor
o The output resistance
o The temperature coefficient
Attach the figures used to calculate these three parameters with the report.
6.0 References
J. Millan and C. Halkias, “Electronic Devices and Circuits”, McGraw-Hill book company.