Testing and trouble shooting of multivibrators


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includes all the trouble-shooting methods of multi-vibrator also clipper and clamper circuits

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  • Topic Learning Objectives: Identify several characteristics of digital circuits as opposed to analog circuits. 2. Classify devices as using digital, analog, or a combination of the technologies. 3. Differentiate between digital and analog signals and identify the high and low portions of the waveform. 4. List 3 types of multivibrators and describe the general purpose of each type of circuit. 5. Analyze simple logic-level indicator circuits
  • Testing and trouble shooting of multivibrators

    1. 1. List Of Topics Active Testing Troubleshooting Of Multivibrator Phase shift oscillator Transistorized sweep generator Clipping and clamping circuits CENT-113 Digital Electronics 1
    2. 2. Active testing involves :- Voltage analysis Resistance analysis Signal analysis CENT-113 Digital Electronics 2
    3. 3. • 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.
    4. 4. •IN resistance testing, a multimeter is used tomeasure the resistance between selected pairs ofpoints in the given circuit.• Resistance testing is used for carrying out thecontinuity testing. If there is a connection(continuity)between the selected points, then the meter willshow a zero resistance.• Similarly we can use the resistance testing fortesting of components such as diodes, capacitorsand transistors• It is also possible to check the status of a fuse linksusing the resistance testing. 4
    5. 5.  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 CENT-113 Digital Electronics 5
    7. 7. -VCC R1 R2 R4 R3 Q1 C1 R5 Q2 C2 0 +VBBINPUT - CENT-113 Digital Electronics 7
    8. 8.  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. CENT-113 Digital Electronics 8
    9. 9. CENT-113 Digital Electronics 9
    10. 10.  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 CENT-113 Digital Electronics 10
    11. 11. Check the DC supply voltages and transistor biasingIf 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
    12. 12. Apply a sine wave of proper amplitude and frequency at the input of the clipper circuit by connecting signal generatorObserve the waveform on CRODepending 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 CENT-113 Digital Electronics 12
    13. 13.  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. CENT-113 Digital Electronics 13
    14. 14. CENT-113 Digital Electronics 14
    15. 15.  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. CENT-113 Digital Electronics 15
    16. 16.  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. CENT-113 Digital Electronics 16
    17. 17. 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 103basic unit 1 milli m .001 103‫־‬ micro μ .000001 106‫־‬ nano n .000000001 109‫־‬ pico p .000000000001 1012‫־‬ CENT-113 Digital Electronics 17
    18. 18. Given: E E = Voltage I = Current R = ResistanceI R E=IR I = E/R R = E/I CENT-113 Digital Electronics 18
    19. 19. Multivibrators Monostable (One Shot) Multivibrator -VCC R1 R2 R4 R3 C1 OUTPUT Q10 R5 Q2 C2 0- +VBB INPUT - CENT-113 Digital Electronics 19
    20. 20. Monostable Multivibrator1 .) 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. CENT-113 Digital Electronics 20
    21. 21. Monostable Multivibratorg .) 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. CENT-113 Digital Electronics 21
    22. 22. Bistable Multivibrator b .) Bistable (Flip - Flop) Multivibrator -VCC OUTPUT C3 C4 R5 R6 OUTPUT 1 20 0 R3 R4- - Q1 R1 R2 Q2 C1 C2 +VBB 0 - INPUT CENT-113 Digital Electronics 22
    23. 23. Bistable Multivibrator1 .) Physical Description a .) Multivibrator that functions in one of two stable states as synchronized by an input trigger pulse.2 .) Operational Characteristics 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. CENT-113 Digital Electronics 23
    24. 24. Bistable Multivibratorf .) 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. CENT-113 Digital Electronics 24
    25. 25. Astable Multivibrator c .) Astable (Free - Running) Multivibrator -VCC OUTPUT OUTPUT 1 R1 R2 R3 R4 20 0 C1 C2- - Q1 Q2 CENT-113 Digital Electronics 25
    26. 26. Astable Multivibrator1 .) 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.2 .) Operational 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. CENT-113 Digital Electronics 26
    27. 27. Astable Multivibratore .) 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. CENT-113 Digital Electronics 27
    28. 28. 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 CENT-113 Digital Electronics 28
    29. 29. (+ 5 V) LOW indicatorProbe (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 TTLGround Ground 3.9 KΩ C1 LOW indicator D4 LED 2 Black 0.01µF Black Ground Ground CENT-113 Digital Electronics 29