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- 1. 9EE306.79 1 DEPARTMENT OFTECHNICAL EDUCATION ANDHRA PRADESH NAME : P.KRISHNA Designation : Lecturer Branch : ECE Institute : Govt.Polytechnic, Srikakulam. Year/Semester : III Semester Subject : Electronics Engg Subject code : EE 306 Topic : Oscillators Duration : 50 Minutes Sub Topic : Relaxation oscillator Teaching Aids : PPT, Scans, Animations
- 2. 9EE306.79 2 Recap In thelast class, we have learnt • Crystal oscillator • Working of Crystal oscillator • Expression for the frequency of oscillations • Advantages, disadvantages and applications
- 3. 9EE306.79 3 Objectives Objectives Upon thecompletion of this topic the student will be able to understand • Relaxation oscillator • Working of Relaxation oscillator • Expression for the frequency of oscillations • Applications
- 4. 9EE306.79 4 • Multivibrators and timers generate square/rectangular waveforms • Colpitt’s, Hartley, RC-phase shift, Crystal Oscillators generate sinusoidal oscillations • UJT relation oscillator generate saw tooth wave forms
- 5. 9EE306.79 5 Relaxation oscillator Relaxationoscillator UJT Fig .1
- 6. 9EE306.79 6 Time mplitude Output waveform Sawtooth wave
- 7. 9EE306.79 7 Relaxation oscillator Majorparts of Oscillator are • Uni -junction transistor • Variable resistor R1 • Charging / discharging capacitor C
- 8. 9EE306.79 8 • R R1 1and R and R2 2 are resistors are resistors • C is capacitor C is capacitor • UJT is UJT is Uni-junction transistor • V VBB BB is Power supply is Power supply Components
- 9. 9EE306.79 9 Circuit analysis Circuitanalysis • It is a non-sinusoidal oscillator • UJT exhibits negative resistance characteristics • Due to this, it can be used as a relaxation oscillator • R1 is a variable resistor, which controls the time constant R1C. • The discharging of capacitor C through UJT develops saw-tooth wave form
- 10. 9EE306.79 10 Circuit operation •When the circuit is switched ON, the capacitor C charges through resistor R1. • During the charging period, the voltage across the capacitor rises in an exponential manner until it reaches the peak voltage. • At this instant of time, the UJT switches to its low resistance conducting mode and the capacitor is discharged between E and B1. Relaxation oscillator
- 11. 9EE306.79 11 Operation • Asthe capacitor voltage flies back to zero, the emitter ceases to conduct and the UJT is switched off. • When the UJT is switched off, the capacitor again starts charging through resister R1. • The frequency of the output saw-tooth wave can be varied by changing the value of R1.
- 12. 9EE306.79 12 • Thevalue of R1 controls the time constant R1 C. • The frequency of a relaxation oscillator is given by • Therefore time period t = • Where ‘ŋ’ = Intrinsic standoff ratio t 1 f = 2.3R1C log10 1 1 ŋ
- 13. 9EE306.79 13 Applications • Forgeneration of saw tooth waveforms in CRO. • To drive SCR, DIAC and TRIAC. in timer circuits
- 14. 9EE306.79 14 Summary Summary We havediscussed about • Circuit diagram • Working of Relaxation oscillator • Frequency of oscillations • Applications
- 15. 9EE306.79 15 Quiz 1) Relaxationoscillators are used in a) Digital clocks b) Function generators c) Temperature controlled ovens d) Timer circuits
- 16. 9EE306.79 16 Quiz (Contd.) 2)Which one of the oscillator is used for generation of saw-tooth wave a) colpitt’s oscillators b) hartley oscillators c) crystal oscillators d) relaxation oscillator
- 17. 9EE306.79 17 Quiz (Contd.) 3)..…… is the negative resistance oscillator. a) colpitt’s oscillator b) hartley oscillator c) relaxation oscillator d) crystal oscillator
- 18. 9EE306.79 18 Frequently AskedQuestions • Draw and explain the working of Relaxation oscillator ? • List the applications of Relaxation oscillators ?