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INDUSTRIAL ELECTRONICS DDPE 3103 TOPIC 5 RELAXATION OSCILLATOR PROF MADYA DR MORINA ABDULLAH | ENCIK KAMARUDDIN TAWI UTM KUALA LUMPUR CAMPUS RELAXATION OSCILLTOR • A relaxation oscillator is a circuit that produces output which repeatedly alternates between two states with a period that depends on the charging of a capacitor. • Two devices commonly used as relaxation oscillator is the Unijuntion transistor (UJT) and Programmble Unijunction transistor (PUT). • UJT and PUT circuit generates pulses. 2 UNIJUNTION TRANSISTOR SYMBOL AND CONSTRUCTION N P Three terminal : Base1 (B1), Base2(B2) and Emitter (E) 3 Characteristic of a UJT • At cut off region, voltage supply to the VE emitter terminal not enough to turn on VP the UJT therefore, IE = 0 and UJT is in the cut-off region. • As the voltage supply across emitter increases, reaching the peak voltage, VP the UJT will now turn on and I peak, (IP) will flow. VV IE IP IV 4 • Characteristic of a UJT Between VP to the valley point (VV) the voltage drops and the current increases. VE This region is called the negative VP resistance region. The voltage decreases until the value of VV. At the valley point the current produce is the valley current, IV. • Further increases of the emitter current, places UJT into saturation region. VV IE IP IV 5 Unijunction transistor as relaxation oscillator VBB VE VB2 VE VB1 6 Programmable Unijunction Transistor (PUT) o In a UJT, the parameters such as VP , h and etc are fixed and cannot be change. o However, for a PUT these parameters can be programmed with the help of two external resistor. 7 PUT AS RELAXATION OSCILLTOR VBB VC V V VG C p VK VK 8 References 1. Electronic Devices and Circuit Theory , Robert L. Boylestad & Louis Nashelsky , 9th Edition, 2006 2. Electronic Devices, Thomas L. Floyd, 5th Edition, 1999 3. Wikimedia Commons for images 9.

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Unit-1 Mphycc-7 Ujt
UNIT-1 MPHYCC-7 UJT The Unijunction Transistor or UJT for short, is another solid state three terminal device that can be used in gate pulse, timing circuits and trigger generator applications to switch and control either thyristors and triac’s for AC power control type applications. Like diodes, unijunction transistors are constructed from separate P-type and N-type semiconductor materials forming a single (hence its name Uni-Junction) PN-junction within the main conducting N-type channel of the device. Although the Unijunction Transistor has the name of a transistor, its switching characteristics are very different from those of a conventional bipolar or field effect transistor as it can not be used to amplify a signal but instead is used as a ON-OFF switching transistor. UJT’s have unidirectional conductivity and negative impedance characteristics acting more like a variable voltage divider during breakdown. Like N-channel FET’s, the UJT consists of a single solid piece of N-type semiconductor material forming the main current carrying channel with its two outer connections marked as Base 2 ( B2 ) and Base 1 ( B1 ). The third connection, confusingly marked as the Emitter ( E ) is located along the channel. The emitter terminal is represented by an arrow pointing from the P-type emitter to the N-type base. The Emitter rectifying p-n junction of the unijunction transistor is formed by fusing the P-type material into the N-type silicon channel. However, P-channel UJT’s with an N- type Emitter terminal are also available but these are little used.
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