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UJT Relaxation Oscillator

2016-08-05 14:28  
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UJT Relaxation Oscillator Circuit

The relaxation oscillator shown in figure consists of UJT and a capacitor C which is charged through resistor REwhen inter base voltage VBBis switched on. During the charging period, the voltage across the capacitor increases exponentially until it attains the peak point voltage VP. When the capacitor voltage attains voltage VP, the UJT switches on and the capacitor C rapidly discharges through B1.The resulting current through the external resistor R develops a voltage spike, as illustrated in figure and the capacitor voltage drops to the value VV. The device then cuts off and the capacitor commences charging again. The cycle is repeated continually generating a saw-tooth waveform across capacitor C. The resulting waveforms of capacitor voltage VCand the voltage across resis-tor R (VR) are shown in figure. The frequency of the input saw-tooth wave can be varied by varying the value of resistor REas it controls the time constant (T = REC) of the capacitor charging circuit. The discharge time t2is difficult to calculate because the UJT is in its negative resistance region and its resistance is continually changing. However, t2is normally very much less than t1and can be neglected for approximation.

For satisfactory operation of the above oscillator the following two conditions for the turn-on and turn-off of the UJT must be met.

RE< VBB– VP/ IPand RE> VBB– VV/ IV

That is the range of resistor REshould be as given below

VBB– VP/ IP> RE>  VBB– VV/ IV

The time period and, therefore, frequency of oscillation can be derived as below. During charging of capacitor, the voltage across the capacitor is given as

Vc= VBB(l-e-t/ReC)

where REC is the time constant of the capacitor charging circuit andtis the time from the commencement of the charging.The discharge of the capacitor commences at the end of charging period t1when the voltage across the capacitor Vcbecomes equal to VP,that is, (? VBBVB)

VP= ? VBBVB= VBB(l-e-t/ReC)

Neglecting VBin comparison to ?VBBwe have

? VBB= VBB(l-e-t1/ReC)

or   e-t1/ReC= 1 – ?

So charging time period,t1=2.3 REC log101/1- ?

Since discharging time duration t2is negligibly small as compared to charging time duration t1so taking time period of the wave, T = t1

Time period of the saw-tooth wave,T = 2.3 REC log101/1- ?

and frequency of oscillationf = 1/T = 1/2.3REClog10(1-?)

Variable Frequency UJT Relaxation Oscillator

By including a small resistor in each base circuit, three useful outputs (saw-tooth waves, positive triggers, and negative triggers), as shown in   figure, can be obtained. When the UJT fires, the surge of current through Btcauses a voltage drop across R1and produces the positive going spikes. Also at the UJT firing time, the fall of VEBcauses IBto rise rapidly and generate the negative-go-ing spikes across R2, as shown in figure. R1and R2should be much smaller than RBBto avoid altering the firing volt-age of the UJT. A wide range of oscillation fre-quencies can be achieved by making REadjustable and including a switch to select different values of capacitance, as illustrated. As already mentioned in previous blog post there is upper and lower limits to the signal source resistance REfor the satisfactory operation of the UJT.


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