LABORTATORY ANALOG
ELECTRONICS
Experiment #12
Course code :PHYS-312
Class:BS-6th(M)
Spring -2020
Experiment No.12
UJT relaxation oscillator
Report on Project
Submitted by
Ahmad Bilal (35)
Saleha Ghaffar (36)
Arslan Akhtar (37)
Submitted to
Ms. Aatika Amer
Contents
Objective
Apparatus
Definition
Working
Diagram
Characteristics and circuit arrangement
Application
Firing circuit
R Triggering
Conti…
Advantages of Uni-junction Transistor (UJT)
Disadvantage of Uni-junction Transistor (UJT)
wave form of ujt
Setup
Precautions
Result
Reference
Objective

To obtain a saw tooth waveform using
UJT and its performance as an oscillator
Apparatus
CRO (Dual channel)
 function generator
Breadboard
Resistor (100 ohm ,47ohm)
Capacitor
Transistor (2N2646
Regulator (Dc power supply)
Definition
 UJT relaxation oscillator is a type of RC ( resistorcapacitor) oscillator
 where the active element is a UJT (uni-junction transistor)
 UJT is an excellent switch with switching times in the
order of nano seconds
 It has a negative resistance region in the characteristics and
can be easily employed in relaxation oscillators
Cont.….
 The UJT relaxation oscillator is called so because the
timing interval is set up by the charging of a capacitor
and the timing interval is ceased by the the rapid
discharge of the same capacitor.
 Before going into the details of UJT relaxation
oscillator let’s have a look at the uni junction
transistor (UJT).
Working
 UJT or uni junction transistor is a semiconductor device
that has only one junction
 The UJT has three terminals designated B1, B2 and E
 The base material for a UJT is a lightly doped N-Type
Silicon bar with ohmic contacts given at the lengthwise
ends
Cont.…
 These end terminals are called B1 and B2.
 Since the silicon bar is lightly doped, the resistance
between B1 and B2 is very high (typically 5 to 10
KΩ).
 A heavily doped P-type region is constructed on one
side of the bar close to the B2 region.
Cont.……



A heavily doped P-type region is constructed on one
side of the bar close to the B2 region
This heavily doped P region is called emitter and it
is designated as E.
Resistance between E & B1 is higher than the
resistance between E & B2 because E is constructed
close to B2.
Cont.…
 Current starts flowing into the emitter only when the
bias voltage Ve has exceeded the forward drop of the
internal diode (Vd) plus the voltage drop across RB1
(Vrb1).
Ve = Vd + Vrb1
 Considering the intrinsic stand off ratio η=
RB1/(RB1+RB2)
Diagram
Characteristics and circuit arrangement
Special characteristics of UJT
1-A stable triggering voltage (VP)— a fixed fraction of applied
inter base voltage VBB.
2-A very low value of triggering current.
3-A high pulse current capability.
4-A negative resistance characteristic.
5- Low cost.
Applications
 Relaxation oscillators.
 Switching Thyristors like SCR, TRIAC etc.
 Magnetic flux sensors
 Voltage or current limiting circuit
 Bistable oscillators
 Voltage or current regulators.
 Phase control circuits.
Firing circuit
1- In land operations, an electrical circuit and/or
pyrotechnic loop designed to detonate connected
charges from a firing point.
2- In naval mine warfare, that part of a mine circuit
which either completes the detonator circuit or
operates a ship counter
R Triggering
It includes one fixed resistor, variable resistor, diode,
SCR(Silicon Controlled Rectifier), Load resistor.
The circuit diagram of an R Triggering is shown below
(Figure 1)
Diagram
Advantages of Uni-junction Transistor (UJT)
1- low cost
2-negative resistance characteristics
3-Requires low value of triggering current.
4-A stable triggering voltage
5-Low power absorbing device
Disadvantage of Uni-junction Transistor(UJT)
The main disadvantage of Uni-junction
Transistor is its inability to provide appropriate
amplification.
Wave form of UJT
Setup
This kit supports
 VI Characteristics of SCR, SCR Triggering Circuit.
 VI Characteristics of UJT, relaxation oscillator
 VI Characteristics of BJT in CC/CE/CB configuration.
 VI Characteristics of Diode, Zener Diode, LED.
 Features
Easy to connect configuration is use
Diagram
Precautions
 Put the oscilloscope on balance floor
 Put all the accurate values
 There should be maximum accuracy
Result
The saw tooth wave form obtain by
using the technique of ujt and
performance is tested by using
oscillator
References
 Introduction to EIGHTH EDITION Charles
Kittel Pr($essor Enwritus L unitiersity of
Cal$c~nlia, Berkeley B Chapter 18,
Nanostructures, was written by I Professor Paul
McEuen of Cornell University.
 https://opentextbc.ca/chemistry/chapter/10-6lattice-structures-in-crystalline-solids/