# ujt relaxation oscillators

Post on 15-Aug-2015

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1. 1. UNIJUNCTION TRANSISTOR (UJT)
2. 2. Physical structure UNIJUNCTION TRANSISTOR (UJT)
3. 3. Equivalent circuit The equivalent circuit comprised of two resistors, one fixed (RB2) and one variable (RB1) and a single diode (D). RB1 varies with IE. UNIJUNCTION TRANSISTOR (UJT)
4. 4. Equivalent circuit UNIJUNCTION TRANSISTOR (UJT)
5. 5. Equivalent circuit RBB is the interbase resistance when IE = 0 i.e. 021 EIBBBB RRR Typical range of RBB : 4 k - 10 k The position of the aluminum rod determine the relative values of RB1 and RB2. UNIJUNCTION TRANSISTOR (UJT)
6. 6. 021 1 1 E B I BBBB BB B R VV RR R V UNIJUNCTION TRANSISTOR (UJT)
7. 7. 021 1 EIBB B RR R UNIJUNCTION TRANSISTOR (UJT)
8. 8. For VE > VRB1 by VD (0.35 0.70 V), the diode will fire and IE will begin to flow through RB1. UNIJUNCTION TRANSISTOR (UJT)
9. 9. The emitter potential VP is given by: DBBP VVV UNIJUNCTION TRANSISTOR (UJT)
10. 10. Characteristics of representative UJT: UNIJUNCTION TRANSISTOR (UJT)
11. 11. Programmable unijunction Transistor (PUT) Although it has the same name as a UJT the programmable unijunction transistors structure is not the same. It is actually more similar to an SCR.
12. 12. Programmable unijunction Transistor The PUT can be programmed to turn on at a certain voltage by an external voltage divider. This yields a curve similar to a UJT.
13. 13. Programmable unijunction Transistor (PUT) External PUT resistors R1 and R2 replace unijunction transistor internal resistors RB1 and RB2, respectively. These resistors allow the calculation of the intrinsic standoff ratio .
14. 14. Programmable unijunction Transistor (PUT) VR is voltage divider (R1 and R2 can be specified) Vc capacitor voltage When Vc > VR the PUT will conduct
15. 15. UJT RELAXATION OSCILLATORS Basic UJT relaxation oscillator
16. 16. UJT RELAXATION OSCILLATORS Assume that the initial capacitor voltage, VC is zero. When the supply voltage VBB is first applied, the UJT is in the OFF state. IE is zero and C charges exponentially through R1 towards VBB. The operation
17. 17. UJT RELAXATION OSCILLATORS When the supply voltage VC (= VE) reaches the firing potential, VP, the UJT fires and C discharges exponentially through R2 until VE reaches the valley potential VV.
18. 18. UJT RELAXATION OSCILLATORS When VE reaches the valley potential VV the UJT turns OFF, IE goes to zero and the capacitor is recharged. This process repeats itself to produce the waveforms for vC and vR2 as shown below;
19. 19. UJT RELAXATION OSCILLATORS The waveform, vC
20. 20. UJT RELAXATION OSCILLATORS The waveform, vR2
21. 21. UJT RELAXATION OSCILLATORS
22. 22. UJT RELAXATION OSCILLATORS Condition for switching-ON To switch-on a UJT, the emitter current IE must be able to reach the peak current IP i.e. 11 RIV PIIR PE
23. 23. UJT RELAXATION OSCILLATORS Condition for switching-ON
24. 24. UJT RELAXATION OSCILLATORS In other words, R1 must be small enough such that IE is not limited to a value less than IP when VC = VP. Condition for switching-ON
25. 25. UJT RELAXATION OSCILLATORS Thus, to fire the UJT; PPBB VRIV 1 1RIVV PPBB P PBB I VV R 1 Condition for switching-ON
26. 26. UJT RELAXATION OSCILLATORS Condition for switching-OFF To switch-off a UJT, the emitter current IE must drop below IV when VC = VV. Hence; VVBB VRIV 1
27. 27. UJT RELAXATION OSCILLATORS Thus, to the UJT; 1RIVV VVBB V VBB I VV R 1 Condition for switching-OFF
28. 28. UJT RELAXATION OSCILLATORS Thus, to ensure the switching ON and OFF, the following condition must be met; V VBB P PBB I VV R I VV 1
29. 29. UJT RELAXATION OSCILLATORS
30. 30. UJT RELAXATION OSCILLATORS It can be shown that; PBB VBB VV VV CRt ln11 and; V P B V V CRRt ln212
31. 31. UJT RELAXATION OSCILLATORS The periodic time; 21 ttT In many cases, t1 >> t2, therefore; PBB VBB VV VV CRtT ln11
32. 32. UJT RELAXATION OSCILLATORS When VBB and VP are much greater than VV, then; PBB BB VV V CRT ln1 And if VBB >> Vpn i.e. VP VBB, then BBBB BB VV V CRT ln1
33. 33. UJT RELAXATION OSCILLATORS or; 1 1 ln1CRT The frequency; 1 1 ln 11 1CR T f