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ECE3424 (Ch 9/Ch3 ) Diode circuit problems Vers 3.2 Defaults: All resistances in k
ECE3424 (Ch 9/Ch3 )
Diode circuit problems
Vers 3.2
Defaults:
All resistances in k and currents in mA unless otherwise specied
3-1.
Find the values of the voltages and currents indicated using the constant-voltage-drop (0.7V) model.
V
2
I
2
10
V
1
I
1
20
V
3
40
10
V
4
40
10
+2.0V
+4V
GND
(A)
(B)
(C)
(D)
I
3
I
4
10
V
1
20
10
+10V
GND
20
3-2.
For the diode circuit shown nd the values of voltage and
current indicated for
(a) the ideal rectier model and
(b) the constant-voltage-drop (0.7V) model
(HINT: Use nodal analysis)
I
1
V
2
40
20
+4V
GND
50
3-3.
For the diode circuit shown nd the values of voltage and
current indicated for
(a) the ideal rectier model and
(b) the constant-voltage-drop (CVD) (0.7V) model.
(c) for part (b) nd the power dissipated in the diode.
(HINT: Use nodal analysis)
50
I
2 3-6
(a) Identify all permissible options for the diode circuit shown when the resistances R
1
, R
2
, R
3
, R
4
are not specied. Use
1 to indicate that the diode is conducting and 0 to indicate that the diode is non-conducting. (b) Solve for I
D1
, I
D2
, I
D3,
I
D4
assuming that all diodes = on (one of the possible options) and R
1
= 100k , R
2
= 50 k , R
3
= 25 k , R
4
= 50 k . Use
the CVD model (V
D
= 0.7V ).
V
2
40
20
+20V
GND
50
3-4.
For the diode circuit shown nd the values of voltage and
current indicated for
(a) the ideal rectier model and
(b) the constant-voltage-drop (CVD) (0.7V) model.
(c) for part (b) nd the power dissipated in the diode.
(HINT: Use nodal analysis)
50
I
2
V
4
40
10
+15V
GND
20
3-5.
For the diode circuit shown nd the values of voltage and
current indicated for the constant-voltage-drop (CVD)) (0.7V)
model. And nd the power dissipated in the diode string.
(HINT: Apply nodal analysis at nodes V
3
and V
4
.
Answers: {10.62V, 9.22V, .172mA}
20
V
3
I
2
R
1
R
3
D
1
D
2
D
3
D
4
R
2
R
4
D
4
D
3
D
2
D
1
+6.4
-4.3 3-7
(a) Identify all permissible options for the diode circuit shown when the resistances R
1
, R
2
, R
3
, R
4
are not specied. Use
1 to indicate that the diode is conducting and 0 to indicate that the diode is non-conducting. (b) Solve for I
D1
, I
D2
, I
D3,
I
D4
assuming that all diodes = on except D4 , (one of the possible options) and R
1
= 40k , R
2
= 50 k , R
3
= 20 k , R
4
= 40
k . Use the CVD model (V
D
= 0.7V ).
3-8.
A 1.0A power diode (i.e. 1.0A at 0.7V) is used in series with a resistance of value 20 and a voltage source of value
10.68 VDC. The current through the diode is 0.5A. What is the effect on the power dissipated in the diode if an identical
diode is placed in parallel with this diode? Assume the ideal diode model with emission coefcient n = 1.
3-9.
A voltage regulator which uses a 6.8V Zener in series with a 100 resistance, intended for operation with a 9.0V supply
is accidentally connected to a 15V supply. Assume that the Zener resistance r
Z
= 10 (a) Determine the current I
Z
and the power dissispated in (1) the Zener diode and (2) the resistance, for the intended
power supply of 9V.
(b) Determine the current I
Z
and the power dissispated in (1) the Zener diode and (2) the resistance, for the unintended
power supply of 15V.
Answers: {(a) 20mA, 0.14W, .04W (b) 74mA, 0.562W, 0.555W }
R
4
R
2
D
4
D
3
D
2
D
1
R
3
R
1
D
4
D
3
D
2
D
1
+3.6
-7.1
V
O
R
1
+12V
GND
R
L
3-10.
Assume a zener diode for which V
Z
= 6.2V, r
Z
= 100 (a) for R
L
= 6 k and I
Z
= 0.5 mA, nd V
O
, and the value
of R
1
necessary to achieve these levels.
(b) Assume R
1
= 3 k and nd V
O
and I
Z
when
R
L
= 10 k
.
(HINT: Make use of nodal analysis at V
O
). Answers: { 30 , 0.81W, 90 }
3-13.
Design an AC-DC converter using a full-wave bridge that provides an average DC outout voltage of +15V with maxi-
mum ripple 0.6V when supplying a 50 load. The converter is supplied by a 120VAC (rms) line source through a trans-
former, with turns ratio to be determined. Assume that the diodes are approximately ideal rectiers.
(a) What is the maximum AC (rms) voltage that must be applied across the rectier bridge, and the turns ratio necessary
for the transformer?
(b) What is the mimimum size of lter capacitance needed?
(c) What is the required PIV rating of the diodes (Max reverse voltage + 50%)?
(d) What is the average current through the diodes during conduction and the power dissipated in the diodes?
Answers: { (a) V
S
= 10.82V, n
1
/n
2
= 11.09, (b) C = 4330
µ
F, (c) PIV = 45.9V, (d) I
D
(av) = 3.74A, P
D
= 2.67W }
R
S
3-11.
The circuit shown represents a simple voltage-reduction circuit
designed for a 9.0V module used by the 2007 Volvo Automobile batter-
ies for this vehicle range from 12.0 to 15V, depending on their age and on
other loads.
(a) Assume that the maximum power that can be dissipated by the 9V
Zener diode is 1.8W. What drop-down resistance R
S
will we need to
ensure that the zener will survive, under worst case?
(b) Using the R
S
of part (a) what power level will be ensured for the
module if we require that the zener diode keep a minimum current of
10mA?
(c) Assuming the R
S
of part (a) What minimum resistance level is
allowed for the module before the current in the zener = 0?.
Module
+
-
V
BAT
I
D
3-12.
A simple half-wave AC-DC converter, as shown, is used to
provide a rough DC source for a 400 Hz, 40Vrms aircraft system. If
itis desired that the ripple V
R
be less than 2.4V, determine
(a) V
P
and V
L
(avg)
(b) The minimum value of C that will be required.
(c) PIV rating for the diode (Max reverse voltage + 50%)
(d) I
D
(avg)
Neglect the voltage drop across the diode.
V
L
+
-
R
L
= 50 C
40Vrms 3-14.
The circuit shown is the basic form of an AC voltmeter. Note
that the diodes form a full-wave bridge. Assume that the meter has
internal resistance r
M
= 100 W and requires 0.4 mA for full-scale
reading. Using time average of the rectied input signal, determine
the value of R necessary for full-scale reading to correspond to an
input of 20Vrms at Vin. What maximum V
A
will occur at the output
of the opamp?
AC Voltmeter
r
M
V
A
meter
V
in
R
3-15.
Construct the transfer curve for -10V < V
I
< +10V. Indicate
plainly all corners and slopes.
V
O
V
I
4k 2k 4k +10V
3-16.
For the circuit shown determine the transfer curve ( V
O
vs V
I
)
for -5.0< V
I
< +5.0
Hint: Identify which diodes remain ON when V
I
> 4.65 V
+10V
-10V
10
10
10
V
I
V
O 3-17.
Assume that each Zener diode has r
Z
= 50 , V
Z
= 3.3V, and
that each junction diode has internal resistance 50 and V
D
= 0.7V.
Construct the transfer curve for -15V < V
I
< +15V. Indicate plainly
all corners and slopes.
Answer: {slope1 = 1.0, V
brk
= 8.0V, slope2 = .091}
V
O
2.0k V
I
+
-
+
-
3-18.
Assume ideal rectier model, and construct the transfer curve
for -10V < V
I
< +10V. Indicate plainly all corners and slopes.
Hint: Assume that the diode state is (ON, OFF) according to polarity
and apply nodal analysis. And make use of symmetry.
V
O
V
I
+
-
2k 2k 2k 3-19.
For a square-wave input V
I
of amplitude + 4.5V to -4.5 and frequency 5 kHz, determine the steady-state response of
each of the circuits shown. Assume C = 10
µ
F and R = 1 k .
V
I
R
C
V
O
GND
V
I
R
C
V
O
GND
V
I
R
C
V
O
GND
V
I
R
C
V
O
GND
V
I
R
C
V
O
GND
R
V
I
2R
C
V
O
GND
R
(A)
(B)
(C)
(D)
(E)
(F) Answers: { V
P
= 17.4, V
R
= 2.33, P
M
= 1.11HP }
240VAC
3-20.
For the FWB (full-wave bridge) knob shown,
we desire to choose component values that will support
a regulated 0.45W, 9V application from a 240V 50Hz
European power tap. Assume that transformer turns
ratio n is chosen so that the peak voltage across the
capacitance is 15V.
V
A
n
C
1
Application
50Hz
V
Z
R
1
9 VDC
(a) If V
C
(min) = 12V, when the load is connected, what value of R
1
is required and what value of C
1
is required, assuming
that the current through the zener diode approaches zero when V
C
approaches V
C
(min). (Note that V
C
(min) = V
P
- V
R
).
(b) Using the value of R
1
in part (a), what is the maximum zener current?
(c) If the application is disconnected what is average power must the zener diode dissipate?
Answers: (60 , 20.8
µ
F, 50mA, 0.675W)
(d) What values for (a) and (c) result if the transformer turns ratio were changed so that V
P
= 21V and V
C
(min) = 17V?
V
C
CTRL
V
B
+
-
R
1
gV
x
D
1
D
2
D
3
3-21.
The alternator of an automobile can be repre-
sented by the circuit shown. The alternator consists of
three coils, energized sequentially at phase angles that
are 120
o
with respect to each other by the rotating elec-
tromagnet . The three phases are rectied by d