Page 63 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

Equivalent circuit of UJT
The electrical equivalent circuit of UJT is shown in fig 1d. The resistance between the B1 and B2 terminals is
called the inter-base resistance RBB. The N-type silicon bar serves as a resistance divided into two parts RB1
and RB2 by the PN junction. The total of the internal RB1 and RB2 is the interbase resistance RBB. Value of RBB
is typically in the range of 4 to 10 K ohms. Also rB1 usually a little greater than rB2 because the emitter is a little
closer to B2.
The interbase resistance RBB is measured with the emitter open.
R BB = R + R at I =0.
B2
E
B1
Operation of UJT
The DC supply polarities for a UJT to function is shown in Fig 3. As can be seen from fig 3, B is connected to
2
+ve and B1 to ground. As a result current(conventional) flows from B to B . This conduction results in a voltage
1
2
gradient along the N-type silicon bar. Therefore there is a voltage in the region of the emitter junction(V ) which
E
is positive with respect to ground. The magnitude of this voltage is given by the simple voltage divider action
between R and R .
B1 B2

The Greek letter h (eta) is called the intrinsic stand-off ratio. This is an important data of any UJT and is invariably
mentioned in all UJT data sheets. From the above equation, intrinsic stand-off ration h (eta) is given by,

....[2]

UJT and its applications of triggering circuits

UJTs are employed in a wide variety of circuits involving electronic switching and voltage or current sensing
applications. These include
– triggers for thyristors
– as oscillators
– as pulse and saw tooth generators
– timing circuits
– regulated power supplies
– bistable circuits and so on.

Let us analyse the waveform generated across the capacitor and R1 with respect to the relaxation oscillator or
free running oscillator as in Fig 3.
The negative - resistance portion of the UJT characteristic is used in the circuit shown in Fig 3 to develop a
relaxation oscillator.
The wave form developed across the capacitor is shown in Fig 3 as VE, whereas the waveform produced across
the resistor RB1 is shown as a pulse VB1.

Fig 3

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CITS : Power - Electrician & Wireman - Lesson 60-69