UJT Relaxation Oscillator,EDWinXP Tutorials, Projects, EDWinXP Training Theory. UJT is an uni-junction device. This single pn junction device consists of a. The UJT is often used as a trigger device for SCR's and TRIAC's. Other applications include non-sinusoidal oscillators; saw tooth generators, phase control, and timing circuits. The below Fig is called the relaxation oscillator. The resistor and capacitor connected to the emitter form an RC timing circuit. The pulse signal required to drive the digital circuits can be obtained from a single stage oscillator circuits using a particular device like unijunction transistor.
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The popular 's comparator design permits accurate operation with any supply from 5 to 15 volts or even wider.
Other, non-comparator oscillators may have unwanted timing changes if the supply voltage changes. Comparator—based relaxation oscillator[ edit ] Alternatively, when the capacitor reaches each threshold, the charging source can be ujt relaxation oscillator theory from the positive power supply to the negative power supply or vice versa.
This case is shown in the comparator -based implementation here.
A comparator-based hysteretic oscillator. This relaxation oscillator is a hysteretic oscillator, named this way because of the hysteresis created by the positive feedback loop implemented with the comparator similar to an operational amplifier.
When the voltage across capacitor becomes equal to peak point voltage, the emitter diode is forward biased and UJT conducts.
The discharging time constant of the capacitor is very small as compared to charging time constant.
When the voltage across capacitor becomes less than the valley point voltage during discharging of the capacitor, the UJT comes in to non conducting state. As a result, the region between emitter E and B1 terminal starts saturating by holes and the conductivity of this region starts to increase.
This ujt relaxation oscillator theory of increasing conductivity by the insertion of holes is called conductivity modulation.
This results in a condition where emitter current Ie increases and the emitter voltage Ve decreases. This situation is similar to a negative resistance scenario.
In the graph Fig: This negative resistance region in the UJT characteristics is employed in relaxation oscillators. At last the emitter current Ie will be increased to a point that no more increase in conductivity is possible.
UJT relaxation oscillator, circuit diagram, UJT theory and working
The emitter current corresponding to valley point is denoted as Iv and the corresponding emitter voltage is denoted as Vv. Hence emitter current is zero.
If a progressively rising positive voltage is applied to the emitter the diode will become forward biased when input voltage exceeds h Vbb ujt relaxation oscillator theory Vd, the forward voltage drop across the silicon diode. Now the emitter current increases regeneratively until it is limited by the emitter power supply.
Here we can define the peak point voltage of the UJT.