Circuits, Devices, Networks and Microelectronics
ECE3424
Diode circuit problems
Vers 4.12
Defaults: All resistances in k and currents in mA unless otherwise specified
8-1. For the circuits shown, find the values of the voltages and currents indicated using the constant-voltagedrop model for a silicon junction (VD = 0.7V) .
8-2. For the diode balance circuit shown find values of voltage and
current (V1, V2, I1) using
(a) A Si diode (VD = 0.7).
(b) A SiC LED (Cree red/amber)
Hint: Apply nodal analysis to nodes V1 and V2.
8-3. For the diode circuit shown find the values of voltage and
current indicated using
(a) the ideal rectifier model (VD = 0.0).
(b) the Si constant-voltage drop (CVD) model (VD = 0.7).
Hint: Use nodal analysis.
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8-4. Determine the current through each
diode if diode D2 in each case is a SiC LED
(Cree red/amber) and the others are Si (VD =
0.7V) diodes.
8-5. For the diode circuit shown find the values of voltage and
current indicated using the Si constant-voltage drop (CVD) model
(VD = 0.7).
And find the currents, I2, I3 through each diode.
Hint: Apply nodal analysis to nodes V3 and V4.
Answers: V4 = 5.65V, V3 = 7.05V
8-6. (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 = 5k,
R2 = 2.5k, R3 = 1.0k, R4 = 0.5k. Use the CVD model (VD = 0.7V ).
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8-7. (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 ).
8-8. 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 }
8-9. 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 ).
8-10. The circuit shown represents a simple
voltage-reduction circuit designed for the voice
persona of the 2012 Lexus. Automobile batteries
of this generation range from 12.0 to 14.5V,
depending on their condition and on other loads.
The persona requires a maximum of 500mW
power at full volume. The zener diodes are
1n749 (look up its VZ and Pmax). The other
diode(s) are Si switching diodes with VD = 0.7V.
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(a) Choose resistance RS such that the Zener diode set always remains in reverse breakdown (maintains
regulation) with current minimum of 5.0 mA.
(b) Determine the power dissipated in the set of Zener diodes under worst-case conditions and the
minimum power rating of resistance RS.
(c) Determine the number of Zener diodes (+ Si diodes) needed.
(d) Determine the number of (parallel) ¼ W resistances needed and their value.
Answers: { (a) 67, (b) 630mW, 0.98W}
8-11. 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.
8-12. An AC-DC power supply brick uses a
full-wave bridge (FWB) and is required to
provide an average DC voltage of +12V to a
PL = 300mW application with maximum
percentage ripple 5%. The converter is
supplied by a 120VAC line source through a
transformer, turns ratio to be determined.
The diodes are generic Si with VD = 0.7V.
(a)
(b)
(c)
(d)
What AC (rms) voltage is needed across the rectifier bridge?
Turns ratio of the transformer?
What minimum size filter capacitance C1 is needed?
What is the required PIV rating of the diodes (Max reverse voltage + 50%)?
Answers: { (a) VS = 9.69VAC, (b) n1/n2 = 12.4, (c) C1 = 347F, (d) PIV = 37.5
8-13. 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
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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 }
8-14. 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?
8-15 For the FWB (full-wave bridge)
`knob' shown choose component values
that will support a Zener regulated
360mW, 9V application from a 120V
60Hz power tap. Transformer turns ratio
n12 = 10:1.
(a) Determine VC and VP assuming Si power diodes (VD = 0.8V)
(b) 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 =
VC(max) - VC(min) ).
(c) What average power must the zener diode dissipate when the load is not connected?
Answers: (75, 127F, 560mW)
(d) What values for (a) and (b) result if the transformer turns ratio is n21 = 0.125 ?
8-16 All diodes have a reverse-bias capacitance that is voltage
dependent with respect to reverse bias of the form
C  C J 0 1  VR / VJ 
MJ
where VR = reverse bias. For the circuit shown determine the time
constant for the circuit when V3 = 0.0 V and when V3 = 4.0V.
Capacitance C2 is of value 4pF. The diode is a 1n914 high speed
switching diode. The parameters CJO, VJ and MJ are device model
parameter and are found with the 1n914 pspice model.
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8-17 (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
V2  2V P 1   
V  V1  VP  
Show that the level shift at output Vo is
 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
8-18. For sinusoidal input applied to the level shifter it
can be shown that the equilibrium relates to
V1  VD
V2  V D
and
V 
R 
  1 
 R2 
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
 1
 VP  VD 
 1
For a sinusoidal input with amplitude Vp = 4.0V, and Si
diodes (VD = 0.7) along with the assumption that R2 = 6k 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
Answers: (a) R1 = 49.5k
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8-19. The circuit shown is a high-power audio device realized by the ignition of a fuel vapor in a sonic
chamber. The electrodes are 0.6 mm apart. The electric field strength (breakdown field) of the vapor is
assumed to be similar to that for air, which is 30kV/cm. The device is driven by a sinusoidal input with
V(peak) = VP = 5.5V. The pump capacitances CP are of measure 50pF and the storage capacitance CQ is of
measure .001uF. Assume Si diodes with (VD = V = 0.7V).
(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 the spark discharge is 5ns duration, to what power (in watts) does (d) correspond?
PQ = __________
(f) If it takes one cycle to charge up one stage and it is desired that the sonic output be a concert Bflat (440Hz) at what frequency must the sonic device be operated?
fQ = __________
(g) How much current flows through each stage for (f)?
IQ = __________
Answers: (a) 1800V, (b) 188 (c) 4.61nJ
8-20. For the following circuit construct the transfer curve (plot Vo vs Vin)
for 0 < Vin < 5V indicating break points and slopes.
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8-21. For the following circuit construct the transfer curve
(plot Vo vs Vin) for 0 < Vin < 5V indicating break points and
slopes.
8-22. 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
8-23. 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
8-24. 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.
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