Each piece of ceramic is bonded by a gold film to each end of the n type Si bar

Each piece of ceramic is bonded by a gold film to

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Each piece of ceramic is bonded by a gold film to each end of the n- type Si bar, which forms a very low resistance contact. Each end of the Si bar is called a base.
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Unijunction Transistors Figure 28 Figure 28 (a) shows the basic construction of a UJT, and Figure 28 (b) shows the schematic symbol. The SCR has three external leads: the base1, emitter, and base2. (a) (b)
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Unijunction Transistors Figure 29 Figure 29 UJT equivalent internal resistance and equivalent diode
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Unijunction Transistors Figure 30 Negative resistance is illustrated in the emitter characteristic curve shown in Figure 30 . Once V P is reached, the emitter voltage, V E , decreases as I E increases.
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Basic UJT Operation When the voltage between emitter and base 1, V E , is less than a certain value called the peak voltage V P , the UJT is turned OFF, and no current can flow from E to B 1 (I E = 0). When V E exceeds V P by a small amount, the UJT fires ON. When this happens, the E to B 1 circuit becomes almost a short circuit, and current can surge from one terminal to the other. In virtually all UJT circuits, the burst of current from E to B 1 is short-lived, and the UJT quickly reverts back to the OFF condition.
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Unijunction Transistors Figure 31 Figure 31 shows how a UJT can be used as a relaxation oscillator . Because the voltage waveform, V B1 is a sharp pulse of short duration, it is the ideal gate triggering source for either an SCR or triac.
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UJT Relaxation Oscillator
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UJT Relaxation Oscillator Conditions for turn ON and OFF
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UJT Relaxation Oscillator Example: Given the relaxation oscillator of the figure. a) Determine R B1 and R B2 at I E = 0A. b) Calculate V P , the voltage necessary to turn on the UJT. c) Determine whether R 1 is within the permissible range of values to ensure firing of UJT. d) Determine the frequency of oscillation if R B1 = 100 during the discharge phase. e) Sketch the waveform of v C and v R2 for a full cycle
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Unijunction Transistors Figure 32 Figure 32 shows how the firing of an SCR can be controlled by a UJT.
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PROGRAMMABLE UJT (PUT or PUJT) PUT is a four-layer pnpn device with a gate connected directly to the sandwiched n type layer. Unlike in UJT, R BB , , and V P can be controlled through resistors R B1 , and R B2 (external to the device).
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PROGRAMMABLE UJT (PUT or PUJT) Figure 33 Schematic symbol of PUT with external resistor controller Figure 33
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PROGRAMMABLE UJT (PUT or PUJT) Figure 34 Waveform characteristics of PUT. Figure 34
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PROGRAMMABLE UJT (PUT or PUJT) Important Equations
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PUT as Relaxation Oscillator
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PUT as Relaxation Oscillator Conditions for turn ON and OFF :
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PUT Characteristics Example If V BB = 12V,R = 20k , C = 1 F, R K = 100 , R B1 = 10k , R B2 = 5k , I P = 100 A, V V = 1V, and I V = 5.5mA. Determine: a) V P b) R min and R max c) T and frequency of oscillation d) The waveforms of v A ,v G ,and v K .
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