Testing and trouble shooting of multivibrators

List Of Topics
 Active Testing
 Troubleshooting Of Multivibrator
 Phase shift oscillator
 Transistorized sweep generator
 Clipping and clamping circuits

CENT-113 Digital Electronics 1

Active testing involves :-
 Voltage analysis
 Resistance analysis
 Signal analysis


• After carrying out the visible testing, the next
step is to check the DC voltage at different points
of the given circuits this is called as the voltage
testing.
 The instrument used for voltage testing is DMM
 In voltage testing, the DC power supply output
voltages are first checked then the voltages at the
supply points of various sub circuits are checked.
 Voltage testing can tell us about waveform
distortion.

•IN resistance testing, a multimeter is used to
measure the resistance between selected pairs of
points in the given circuit.

• Resistance testing is used for carrying out the
continuity testing. If there is a connection(continuity)
between the selected points, then the meter will
show a zero resistance.

• Similarly we can use the resistance testing for
testing of components such as diodes, capacitors
and transistors

• It is also possible to check the status of a fuse links
using the resistance testing.
4

 Waveforms can be observed with the help of a
cathode ray oscilloscope (CRO)
 By observing the waveform at the output we can
make the estimation .
 In the circuits like amplifiers, we have to apply a
sinusoidal signal from a signal generator at the
input of the circuit and observe corresponding
output should be observed on CRO.
 The output of a multivibrator can be observed on
CRO and the pulse width in case of a monostable
or frequency in case of an astable can be measured

 MONOSTABLE MULTIVIBRATOR
 PHASE SHIFT OSCILLATOR
 SWEEP GENERATOR
 CLIPPER AND CLAMPER CIRCUITS


-VCC

R1 R2 R4
R3

Q1 C1

R5 Q2
C2
0
+VBB
INPUT
-

 If MMV is not working properly then follow the
procedure given below for fault finding:-
 Check the Vce of both transistor. One of them
should be Vcc and other Vce(sat).
 Check the voltages at the base of transistor , one
of them should be small indicating the transistor
is ON.
 Check the collector and base resistor if Vc and Vb
is not normal.
 Check all the capacitors and replace faulty ones.


 Isolate RC feedback network from the
amplifier.
 Connect the signal generator and adjust the
frequency of the sine wave equal to
oscillator frequency .
 Observe the waveform at output on a dual
trace CRO and measure phase shift
between them .
 This the phase shift introduced by first RC
phase shift network. It should be exactly 60

Check the DC supply voltages and transistor
biasing
If DC voltage are abnormal then check transistor
and resistor.
If dc voltages are Ok then observe the output
waveform on CRO and measure the following
characteristics of the sweep waveform
 Linearity
• Peak magnitude of the sweep
 Frequency
 Waveform Distortion • Frequency stability
11

Apply a sine wave of proper amplitude and
frequency at the input of the clipper circuit by
connecting signal generator
Observe the waveform on CRO
Depending on the type of clipper one or both the
half cycle OF CLIPPERS completely clipped as
TYPE
should be CYCLE CLIPPED OFF
POSITIVE CLIPPERS POSITIVE/FULL
shown in CLIPPER
NEGATIVE
the table NEGATIVE/FULL
POSITIVE BIASED POSITIVE PARTIALLY
NEGATIVE BIASED NEGATIVE PARTIALLY
COMBINED CLIPPER POSITIVE AND
NEGATIVE PARTIALLY

 Check the input and output waveforms on
CRO by connecting a signal generator at
the input.
 If the expected output waveform is not
observed then check the diode and
resistors.
 If the capacitor becomes leaky, then the
output DC shift does not remains constant.
Under such circumstances replace the
leaky capacitor.

 Check for response.
 Have someone call 911.
 Clear airway.
 Look, listen and feel for breathing.
 Give 2 full breaths.
 15 compressions (1 and 2 and 3)
 Continue till medical help arrives, you are
relieved or are too tired to continue.


 Q1. Who is responsible for safety?
 A1. Everybody is responsible for their safety.
 Q2. What protects electronic circuits from ESD?
 A2. ESD packaging & wrist straps.
 Q3. What is the worst electrical shock you have
heard of or experienced?
 A3. Various.


Prefix Symbol Decimal Power of Ten
tera T 1,000,000,000,000 1012
giga G 1,000,000,000 109
mega M 1,000,000 106
kilo k 1,000 103
basic unit 1

milli m .001 103‫־‬
micro μ .000001 106‫־‬
nano n .000000001 109‫־‬
pico p .000000000001 1012‫־‬


Given:

E
E = Voltage
I = Current
R = Resistance

I R E=IR
I = E/R
R = E/I


Multivibrators
Monostable (One Shot) Multivibrator
-VCC

R1 R2 R4
R3

C1 OUTPUT
Q1

0 R5 Q2
C2
0
- +VBB
INPUT
-


Monostable Multivibrator
1 .) Uses
a .) Used for pulse stretching
b .) Used in computer logic systems and Communication /
Navigation systems.
2 .) Operational Characteristics
a .) +VBB is connected to the base of Q1 which places Q1 in
cutoff.
b .) Q2 is saturated by -VCC applied to its base through R2.
c .) C1 is fully charged maintaining approximately -VCC on
the base of Q2.
d .) A negative gate signal is applied to the base of
transistor Q1 which turns Q1 on and drives it into
saturation.
e .) The voltage at the collector of Q1 is then attached to the
base of Q2 which turns Q2 off.
f .) C1 is discharged to attempt to keep VC at Q2 constant.
This maintains Q2 off.


Monostable Multivibrator
g .) When C1 is discharged, it can no longer keep Q2
off.
h .) Q2 turns on and saturates which causes its VC to
go to approximately 0V.
i .) This 0V is applied to the base of Q1 which turns
Q1 off.
j .) Q1’s VC goes to -VCC and C1 charges to -VCC.
k .) The multivibrator will remain in this original
state until another gate “triggering” pulse is
received.
l .) Output from the circuit is taken from Q2’s
collector.
m.) Only one trigger pulse is required to generate a
complete cycle of output.


Bistable Multivibrator
b .) Bistable (Flip - Flop) Multivibrator

-VCC

OUTPUT C3 C4
R5 R6 OUTPUT
1 2

0 0
R3 R4
- -
Q1 R1 R2 Q2
C1 C2
+VBB
0

-
INPUT


1 .) Physical Description
a .) Multivibrator that functions in one of two stable states
as synchronized by an input trigger pulse.
a .) Circuit is turned on.
b .) One of the two transistors will conduct harder and
thereby reach saturation first. (Assume Q2)
c .) The 0V at the collector of Q2 is coupled to the base of
Q1 which drives Q1 into cutoff.
d .) The -VCC at the collector of Q1 is coupled to the base of
Q2 holding Q2 in saturation.
e .) An input trigger pulse is applied to the bases of both Q1
and Q2 simultaneously. Since Q2 is already in saturation,
there is no effect on Q2.


f .) The trigger pulse turns on Q1 and drives the transistor
into saturation.
g .) The 0V on the collector of Q1 is coupled to the base of
Q2 driving Q2 into cutoff.
h .) The -VCC on the collector of Q2 is coupled to the base of
Q1 holding Q1 in saturation.
i .) This process will continue as long as there are trigger
pulses applied to the circuit.
j .) The output frequency of the waveforms will be
determined by the frequency of the input trigger pulses.


Astable Multivibrator
c .) Astable (Free - Running) Multivibrator

-VCC

OUTPUT OUTPUT
1 R1 R2 R3 R4
2

0 0
C1 C2
- -
Q1 Q2


1 .) Physical Description
a .) Circuit has two outputs but no inputs.
b .) R1 = R4, R2 = R3, C1 = C2, Q1 & Q2 are as close as is
possible in their operating characteristics.
a .) Circuit is turned on.
b .) Assume that Q2 conducts harder than Q1 and goes into
saturation first.
c .) The 0V at the collector of Q2 is coupled to the base of
Q1 which drives Q1 into cutoff.
d .) C2 begins to charge. C1 is at -VCC and this voltage is
applied to the base of Q2 to hold Q2 in saturation.


e .) After a finite period of time, (as set by the RC time
constant of C2 and R3), C2 reaches a voltage value
sufficient to snap Q1 on.
f .) Q1 quickly goes into saturation. The change in voltage
from -VCC to 0Vcauses C1 to discharge.
g .) This voltage is coupled to the base of Q2 Placing /
holding Q2 in cutoff.
h .) C1 begins to charge and will snap Q2 on when a
sufficient voltage value is reached.
i .) In Summary, whenever a transistor saturates, its VC will
change from -VCC to 0V. This voltage will then be coupled
to the base of the other transistor which will drive the
other transistor into cutoff. The frequency of the output
waveform will depend on the RC time constants
established at C1R2 and C2R3.


Transistor Testing
•This chart shows the readings for a good transistor.
Test Lead NPN PNP
Connection Resistance Reading Resistance Reading
(+/-) (High / Low) (High / Low)
Base - Emitter LOW HIGH
Emitter - Base HIGH LOW

Base - Collector LOW HIGH

Collector - Base HIGH LOW

Emitter - Collector HIGH HIGH

Collector - Emitter HIGH HIGH


(+ 5 V)
LOW indicator
Probe
(Ground)
Tip
(Ground for TTL family)
HIGH indicator

D1
+V Red
+5V
LED 1 HIGH indicator
4 8
D2
2 R2
Probe 555 3 390Ω
Input D3 6 Outputs
Tip Timer IC

R3
R1 5 1 390Ω
TTL TTL
Ground Ground
3.9 KΩ C1 LOW indicator
D4 LED 2
Black 0.01µF
Black Ground
Ground


Testing and trouble shooting of multivibrators

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