testing and trouble shooting of multivibrators

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

CENT-113 Digital Electronics 1

List Of Topics

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.

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•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.

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


+VBB

-VCC

R1 R2R3

R4

R5

C1

C2Q2

Q1

0

-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 degree.

We can measure the phase shift between all other networks .The total phase shift should be 180 degree .


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 Frequency Waveform Distortion

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• Peak magnitude of the sweep

• Frequency stability

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 should be completely clipped as shown in the table


TYPE OF CLIPPERS

CYCLE CLIPPED OFF

POSITIVE CLIPPERS POSITIVE/FULL

NEGATIVE CLIPPER 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 10 3־

micro μ .000001 10 6־

nano n .000000001 10 9־

pico p .000000000001 10 12־


E

I R E=IR

Given:

E = Voltage

I = Current

R = Resistance

I = E/R

R = E/I

Monostable (One Shot) Multivibrator


0

- +VBB

-VCC

R1 R2R3

R4

R5

C1

C2Q2

Q1

0

-INPUT

OUTPUT

Multivibrators

1 .) Usesa .) Used for pulse stretchingb .) 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.


Monostable Multivibrator

b .) Bistable (Flip - Flop) Multivibrator


+VBB

-VCC

R5

R2

R3 R4

R1C1 C2

Q2Q1

0

0

-

-

INPUT

OUTPUT 2

OUTPUT 1 R6

-

0

C3 C4

Bistable Multivibrator

1 .) Physical Descriptiona.) Multivibrator that functions in one of two

stable states as synchronized by an input trigger pulse.

2 .) Operational Characteristicsa.) 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.



c .) Astable (Free - Running) Multivibrator


-VCC

R1

Q2Q1

0

-

OUTPUT 2

OUTPUT 1 R4

-

0C2

R2 R3

C1

Astable Multivibrator

1 .) Physical Descriptiona.) 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.

2 .) Operational Characteristicsa.) 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.




•This chart shows the readings for a good transistor.Test Lead

Connection( + / - )

NPNResistance Reading

(High / Low)

PNPResistance Reading

(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

Transistor Testing

(+ 5 V)


(Ground)

(Ground for TTL family)HIGH indicator

LOW indicator

555Timer IC

ProbeTip

HIGH indicator

LOW indicator

OutputsInput ProbeTip

TTLGround

BlackGround

Red+ 5 V

+ V

TTLGround

BlackGround

D1

D2

D3

D4

LED 1

LED 2

R1

R2

R3

C1

15

6

2

4 8

3 390Ω

390Ω

3.9 KΩ

0.01µF

testing and trouble shooting of multivibrators

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