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Unijunction transistor: Although a unijunction transistor is not a thyristor, this device can trigger larger thyristors with a pulse at base B1. A unijunction transistor is composed of a bar of N-type silicon having a P-type connection in the middle. See Figure a. The connections at the ends of the bar are known as bases B1 and B2; the P-type mid-point is the emitter. It varies from 0. The schematic symbol is Figure c.
The Unijunction emitter current vs voltage characteristic curve Figure a below shows that as V E increases, current I E increases up I P at the peak point. Beyond the peak point, current increases as voltage decreases in the negative resistance region. The voltage reaches a minimum at the valley point. The resistance of R B1 , the saturation resistance is lowest at the valley point. The relaxation oscillator is an application of the unijunction oscillator. R E charges C E until the peak point.
The unijunction emitter terminal has no effect on the capacitor until this point is reached. Once the capacitor voltage, V E , reaches the peak voltage point V P , the lower emitter-base1 E-B1 resistance quickly discharges the capacitor.
Once the capacitor discharges below the valley point V V , the E-RB1 resistance reverts back to high resistance, and the capacitor is free to charge again. During the capacitor discharge through the E-B1 saturation resistance, a pulse may be seen on the external B1 and B2 load resistors, Figure above.
The load resistor at B1 needs to be low to not affect the discharge time. The external resistor at B2 is optional. It may be replaced by a short circuit. A more accurate expression for frequency is given in the Figure above. The charging resistor R E must fall within certain limits. Programmable Unijunction Transistor PUT : Although the unijunction transistor is listed as obsolete read expensive if obtainable , the programmable unijunction transistor is alive and well.
It is inexpensive and in production. Though it serves a function similar to the unijunction transistor, the PUT is a three terminal thyristor. The PUT shares the four-layer structure typical of thyristors shown in Figure below. Moreover, the gate lead on the schematic symbol is attached to the anode end of the symbol. Programmable unijunction transistor: Characteristic curve, internal construction, schematic symbol. The characteristic curve for the programmable unijunction transistor in Figure above is similar to that of the unijunction transistor.
This is a plot of anode current I A versus anode voltage V A. The gate lead voltage sets, programs, the peak anode voltage V P. As anode current increases, voltage increases up to the peak point. Thereafter, increasing current results in decreasing voltage, down to the valley point. The PUT equivalent of the unijunction transistor is shown in Figure below.
Figure below shows the PUT version of the unijunction relaxation oscillator. Resistor R charges the capacitor until the peak point, then heavy conduction moves the operating point down the negative resistance slope to the valley point. A current spike flows through the cathode during capacitor discharge, developing a voltage spike across the cathode resistors.
After capacitor discharge, the operating point resets back to the slope up to the peak point. Problem: What is the range of suitable values for R in Figure above, a relaxation oscillator? The charging resistor must be small enough to supply enough current to raise the anode to V P the peak point while charging the capacitor. Once V P is reached, anode voltage decreases as current increases negative resistance , which moves the operating point to the valley. It is the job of the capacitor to supply the valley current I V.
Once it is discharged, the operating point resets back to the upward slope to the peak point. The resistor must be large enough so that it will never supply the high valley current I P. If the charging resistor ever could supply that much current, the resistor would supply the valley current after the capacitor was discharged and the operating point would never reset back to the high resistance condition to the left of the peak point. We still need V V , the valley voltage.
How much less? This will raise the lower limit on the resistor range a little. Figure below shows the PUT relaxation oscillator with the final resistor values. This circuit needs a V BB unfiltered supply not shown divided down from the bridge rectifier to reset the relaxation oscillator after each power zero crossing.
The variable resistor should have a minimum resistor in series with it to prevent a low pot setting from hanging at the valley point. PUT timing circuits are said to be usable to 10kHz. If a linear ramp is required instead of an exponential ramp, replace the charging resistor with a constant current source such as a FET based constant current diode.
Don't have an AAC account? Create one now. Forgot your password? Click here. Latest Projects Education. Home Textbook Vol. Unijunction transistor: a emitter characteristic curve, b model for VP. Unijunction transistor relaxation oscillator and waveforms. Oscillator drives SCR. Selected 2n PUT parameters, adapted from 2n datasheet.
PUT relaxation oscillator with component values. The E-B1 junction has negative resistance properties; it can switch between high and low resistance. A PUT programmable unijunction transistor is a 3-terminal 4-layer thyristor acting like a unijunction transistor. Unijunction transistors and programmable unijunction transistors are applied to oscillators, timing circuits, and thyristor triggering.
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2N6027 - Programmable Unijunction Transistor Programmable UJT
2N6027 TRANSISTOR. Datasheet pdf. Equivalent