ECE3283 (Chapt 10)
Diode circuit problems
Vers 4.9
Defaults: All resistances in k and currents in mA unless otherwise specified
10-1. For the circuits shown, find the values of the voltages and currents indicated using the constantvoltage-drop (VD = 0.7V) model.
10-2. For the diode circuit shown find the values of voltage and
current indicated using
(a) the ideal rectifier model (VD = 0.0).
(b) the constant-voltage drop (CVD) model (VD = 0.7).
Hint: Apply nodal analysis at nodes V1 and V2.
10-3. For the diode circuit shown find the values of voltage and
current indicated using
(a) the ideal rectifier model (VD = 0.0).
(b) the constant-voltage drop (CVD) model (VD = 0.7).
Hint: Use nodal analysis.
10-4. For the diode circuit shown find the values of voltage and
current indicated using the constant-voltage drop (CVD) model (VD
= 0.7).
And find the power dissipated in the diode string
Hint: Apply nodal analysis at nodes V3 and V4.
10-5. (a) Identify all possible active operating states for the diode circuit strings as shown by their ‘state
tables’. Use “1” to indicate that a diode is conducting and “0” to indicate that a diode is non-conducting.
(b) Solve for ID1, ID2, ID3, ID4 assuming that all diodes = "on", (one of the possible options) and R1 = 50k,
R2 = 25k, R3 = 10k, R4 = 5k. Use the CVD model (VD = 0.7V ).
10-6. (a) Identify all possible active operating states for the diode circuit string as shown by their ‘state
table’when the resistances R1, R2, R3, and R4 are not specified. Use “1” to indicate that a diode is conducting
and “0” to indicate that a diode is non-conducting. (b) Solve for ID1, ID2, ID3, ID4 assuming that all diodes =
"on" except D4, (one of the possible options) and R1 = 50k, R2 = 40k, R3 = 20k, R4 = 40k. Use the
CVD model (VD = 0.7V ).
10-7. (a) Identify all possible active operating states for the diode circuit strings as shown by their ‘state
tables’. Use “1” to indicate that a diode is conducting and “0” to indicate that a diode is non-conducting.
(b) Solve for ID1, ID2, ID3, ID4 assuming that all diodes = "on", (one of the possible options) and R1 = 100k,
R2 = 50k, R3 = 20k, R4 = 50k. Use the CVD model (VD = 0.7V ).
10-8. 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 rZ = 10
(a) Determine the current IZ and the power dissispated in (1) the Zener diode and (2) the resistance,
for the intended power supply of 9V.
(b) Determine the current IZ 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 }
10-9. Assume a zener diode for which VZ = 6.2V, rZ = 100
(a) for RL = 6 k and IZ = 0.5 mA, find VO and the value of R1
necessary to achieve these levels.
(b) Assume R1 = 3 k and find VO and IZ when RL = 10 k.
(HINT: Make use of nodal analysis at VO ).
10-10. The circuit shown represents a simple voltagereduction circuit designed for the 4.0-V voice persona of the
2001 Volvo. Automobile batteries of this generation range
from 12.0 to 13.6V, depending on their condition and on other
loads. The persona requires a maximum of 1.2W power at full
volume. The maximum power that can be dissipated by the
4.0V Zener diode is 2W.
(a) Choose resistance RS such that the Zener diode always remains in reverse breakdown (maintains
regulation) with current minimum of 20.0 mA.
(b) Determine the power dissipated in the Zener under worst-case conditions and the minimum power rating
of resistance RS.
Answers: { 25, 1.54W, 3.62W}
10-11. A simple half-wave AC-DC converter, as shown, is
used to provide an equivalent DC source for a 400 Hz, 10Vrms
aircraft system. If it is desired that the ripple VR be less than
5%, determine
(a) VP and VL(avg)
(b) The minimum value of C that will be required.
(c) PIV rating for the diode (Max reverse voltage + 50%)
Neglect the voltage drop across the diode.
10-12. An AC-DC converter power supply knob uses a full-wave bridge (FWB) and is required to provide
an average DC voltage of +12V to a PL = 300mW application at maximum ripple 0.5V. The converter is
supplied by a 120VAC line source through a transformer (= most of the knob), with turns ratio to be
determined. Assume diodes to be ideal rectifiers.
(a)
(b)
(b)
(c)
What AC (rms) voltage is needed across the rectifier bridge
Turns ratio of the transformer?
What minimum size filter capacitance C is needed?
What is the required PIV rating of the diodes (Max reverse voltage + 50%)?
Answers: { (a) VS = 8.66VAC, n1/n2 = 13.9, (b) C = 408F, (c) PIV = 36.8V
10-13. The alternator of an automobile can be
represented by the circuit shown. The alternator
consists of three coils, energized sequentially at phase
angles that are 120o with respect to each other by the
rotating electromagnet . The three phases are
rectified by diodes D1, D2, and D3. The charging
voltage applied to the battery is sampled by a control
circuit which adjusts the strength of the electromagnet
rotor by means of a dependent source.
The resistance RB = battery resistance = 0.1
(a) For VB = 12V determine VN at charging
current 50 A and the percentage ripple in VN that results.
(b) Determine mechanical power in HP necessary to provide this current. (note: 746W = 1 hp)
(c) What is the drop-down resistance RS if we insert an application (shown as RL) that uses 0.45W of
power at 9.0V, and what voltage ripple VR would occur across the application?
Answers: { VN = 17.8, VR/VN = 13.4%, Pmech = 1.11HP }
10-14. 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 rM = 100
and requires 0.4 mA for full-scale reading. Using time
average of the rectified input signal, determine the value of
R necessary for full-scale reading to correspond to an input
of 20Vrms at Vin. What maximum VA will occur at the
output of the opamp?
10-15 For the FWB (full-wave bridge) `knob'
shown, we desire to choose component values
that will support a regulated 450mW, 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.
(a) If VC(min) = 12V with the load
connected, what value of R1 and of C1 is
required, assuming that the current through the zener diode approaches zero when VC approaches VC(min).
(Note that Vripple = VR = VP - VC(min) ).
(b) What average power must the zener diode dissipate when the load is not connected?
Answers: (60, 208F, 675mW)
(c) What values for (a) and (b) result if the transformer turns ratio were changed so that VP = 21V and there
is a ripple of 4V at VC..
10-16 (a) For the 2-diode level-shifter circuit shown we
know that when equilibrium is reached the charge that flows
onto the capacitance on the positive swing and flows off on
the negative swing, i.e. QC (+) = QC (-). If the input is a
square-wave form then this implies that I1 = I2 for which
V1
V2

R1

R2
If the diodes are ideal rectifiers. And since it is always true that the peak-to-peak amplitude =
2VP  V1  V2 then
V1  2V P 1  1 /  
and
Show that the level shift at output Vo is
V2  2V P 1   
V  V1  VP  
 1
 VP
 1
(b) Assume that R2 = 20k and VP = 5V. Determine the value of resistance R1 that will:
(1) Shift Vo by V = +2.0V
(2) Shift Vo by V = -1.0V
10-17 (a) For the 2-diode level-shifter circuit and squarewave form the charge and discharge currents are equal.
So if the CVD model of the diode is used then I1 = I2 and
V1  VD
V2  V D

R1

R2
where VD = 0.7V.
It is always true that the peak-to-peak amplitude = 2VP  V1  V2
Show that the level shift at output Vo is
V 
 1
 VP  VD 
 1
(b) Assume that R2 = 20k and VP = 5V. Determine the value of resistance R1 that will:
(1) Shift Vo by V = +2.0V
(2) Shift Vo by V = -1.0V
10-18. For sine-wave input applied to the level shifter it can
shown that the equilibrium state is identified by
V1  VD
V2  V D
and
V 
R 
  1 
 R2 
be
23

 1
 VP  VD 
 1
For a sinusoidal input with amplitude Vp = 4.0V, and the assumption that R2 = 20k and that the diodes are
ideal rectifiers (VD = 0) determine the value of R1 for which the following level shifts occur:
(a) Shift Vo by V = +2.0V
(b) Shift Vo by V = -1.0V
10-19. The circuit shown is to be used to ignite a combustible vapor in a resonant sonic chamber. The
electrodes are 1.25mm apart and the breakdown field of the vapor is 10,000 V/cm. If the signal generator
provides signal of amplitude VP = 1.5V. Assume ideal rectifier model for the diodes.
(a) What is the voltage across capacitance CQ?
VN = __________
(b) How many stages are needed to ignite the mixture?
Nstages = __________
(c) How much energy is stored in each stage?
wea = __________
(d) What is the energy released by CQ on discharge?
wQ = __________
(e) If discharge takes place in an spark of 10s duration, to what power (in watts) does (d)
correspond?
PQ = __________
(f) If the signal frequency is 50MHz and it takes one cycle to charge up one stage, how long will it
take to charge up the capacitance CQ to its desired voltage?
Time = __________
10-20. For the following circuit construct the transfer curve (plot Vo vs Vin)
for 0 < Vin < 5V indicating break points and slopes.
10-21. For the following circuit construct the transfer curve
(plot Vo vs Vin) for 0 < Vin < 5V indicating break points and
slopes.
10-22. For the circuit shown construct the transfer curve (plot VO vs Vin )
for -4.0< Vin < +4.0 . Assume CVD model. Indicate break points and
slopes.
Hint: Identify which diodes remain ON when Vin > 2.65 V
10-23. Assume that each Zener diode has VZ = 3.3V and internal
resistance rZ = 50. And assume that each junction diode has
internal resistance 50 and forward (CVD) biasVD = 0.7V.
Construct (plot VO vs Vin) the transfer curve for -15V < Vin < +15V.
Indicate break points and slopes.
Answers: slope(1) = 1.0, V(break) = 8.0V, slope(2) = .091
10-24. Assume ideal rectifier model, and construct the transfer
curve for -10V < Vin < +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.