2. الأسبوع السابع

6. Definition and construction of UJT. Theory of operation & V-I chara.. Relaxation oscillator. V-I characteristics oscillator. Examples Definition and construction of UJT. Theory of operation & V-I chara.. Relaxation oscillator. V-I characteristics oscillator. Examples

9. V BB R B2 R B1 VdVd VEVE R B2

10. UJT characteristic { v-I curve } Cut off region R BB =R B1 +R B2 η = R B1 / R BB = R B1 /( R B1 +R B2 ) η:intirsic standoff ratio η is determined by the manufacturer and it lies between (0.5-0.75 ). I E =0 V E = η V BB When V E < η V BB V E =V p or above then transistor triggered and I E, V E And transistor exhibits negative resistance. Iv Vv Vp Ip Saturation region Negative resistance region

11. Vp= η V BB +V D

12. A UJT Application The UJT can be used as trigger device for SCR s and triac’s. other applications include non sinusoidal oscillators,saw tooth generators,phase control and timing circuits. Relaxation Oscillator : 1- Circuit diagram : V BB C UJT

13. 2- Operation : when d.c power is applied,the capacitor C charge exponentially through R1 until it reaches the peak point voltage Vp at this point the PN junction becomes forward biased and the emitter characteristics goes the negative resistance region (i.e.V E and I E ) the capacitor then quickly discharges through R 2 when the capacitor voltage decreases to the valley point Vv, the UJT turns off the capacitor to charge again and the cycle repeated. 3-wave forms for UJT Relaxation

16. R1 V BB C R2 UJT

17. Note : since Vp= η V BB +V D And because varies with temp. and V D is temperature dependent the stabilization of Vp is achieved by adding a small resistor in series with (B 2 )its value is given by an Approximate relationship

18. VBB R pot R1 Rs R2 C

21. 470Ω 100Ω R pot 100Ω R1=2.2k Ω 12V Vc η=0.65

23. VBB 5k Ω 1k Ω 37kΩ 780 Ω 0.5µF