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.
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)
Unijunction Transistors Figure 29 Figure 29 UJT equivalent internal resistance and equivalent diode
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.
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.
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.
UJT Relaxation Oscillator
UJT Relaxation Oscillator Conditions for turn ON and OFF
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
Unijunction Transistors Figure 32 Figure 32 shows how the firing of an SCR can be controlled by a UJT.
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).
PROGRAMMABLE UJT (PUT or PUJT) Figure 33 Schematic symbol of PUT with external resistor controller Figure 33
PROGRAMMABLE UJT (PUT or PUJT) Figure 34 Waveform characteristics of PUT. Figure 34
PROGRAMMABLE UJT (PUT or PUJT) Important Equations
PUT as Relaxation Oscillator
PUT as Relaxation Oscillator Conditions for turn ON and OFF :
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 .