by Matthew H. Williams
Back to the British Amateur Electronics Club.
The basic structure of a unijunction transistor (UJT) is shown in Fig.1. It is essentially a bar of N type semiconductor material into which P type material has been diffused somewhere along its length. Contacts are then made to the device as shown; these are referred to as the emitter, base 1 and base 2 respectively. Fig.2 shows the schematic symbol used to denote a UJT in circuit diagrams. For ease of manufacture alternative methods of making contact with the bar have been developed, giving rise to the two types of structure - bar and cube - shown in Fig.3
The equivalent circuit shown in Fig.4 ...view middle of the document...
Further increase in Ve causes the emitter current to increase which in turn reduces RB1 and this causes a further increase in current. This runaway effect is termed regeneration. The value of emitter voltage at which this occurs is known as the peak voltage VP and is given by: VP = AVVBB + VD (4)
The characteristics of the UJT are illustrated by the graph of emitter voltage against emitter current (Fig.6).
As the emitter voltage is increased, the current is very small - just a few microamps. When the peak point is reached, the current rises rapidly, until at the valley point the device runs into saturation. At this point RB1 is at its lowest value, which is known as the saturation resistance.
The simplest application of a UJT is as a relaxation oscillator, which is defined as one in which a capacitor is charged gradually and then discharged rapidly. The basic circuit is shown in Fig.7; in the practical circuit of Fig.8 R3 limits the emitter current and provides a voltage pulse, while R2 provides a measure of temperature compensation. Fig. 9 shows the waveforms occurring at the emitter and base 1; the first is an approximation to a sawtooth and the second is a pulse of short duration.
The operation of the circuit is as follows: C1 charges through R1 until the voltage across it reaches the peak point. The emitter current then rises rapidly, discharging C1 through the base 1 region and R3. The sudden rise of current through R3 produces the voltage pulse. When the current falls to IV the UJT switches off and the cycle is repeated.
It can be shown that the time t between successive pulses is given by:
VBB - VV
t + R1C ln secs (5) N.B. R measured in Megaohms. C in µF.
VBB - VP
Design for a lKHz relaxation oscillator
The oscillator uses a 2N2646 UJT, which is the most readily available device, and is to operate from a 10V D.C. power supply.
From the relevant data sheet the specifications for the 2N2646 are:
VEB2O IE(peak) PTOT(max) IP(max) IV(max) Case style TO18
30V 2A 300mw 5µA 4ma 0.56 - 0.75
It is important that the value of R1 is small enough to allow the emitter current to reach IP when the capacitor voltage reaches VP and large enough so that the emitter current is less than IV when the capacitor discharges to VV. The limiting values for R1 are given by:
VBB - VP VBB - VV
R1(max) = and R2(min) =
From the specifications for the 2N2646 the average value of is 0.56 + 0.75/2 = 0.655. Substituting this value in equation (4) and assuming VD = 0/7V: VP = 0.655 x 10 + 0.7 = 7.25V.
So R1(max) = 10 - 7.25/5µA = 550K, and if VV = approx VBB/10,
R1(min) = 10 - 1/4mA = 2.25K.