6. Thyristor and Other Devices EET 105/4 ANALOGUE SYSTEMS II EET105/4 – ANALOGUE SYSTEMS 2 6. Thyristor and Other Devices Syllabus IDENTIFY and EXPLAIN Shockley diode, SCR and its applications, SCS, Diac, Triac, UJT, Phototransistor, LASCR, Optocoupler.

6. THYRISTOR AND OTHER DEVICES Unijunction Transistor (UJT) EET105/4 – ANALOGUE SYSTEMS 2 Schematic Symbol Basic Structure

6. THYRISTOR AND OTHER DEVICES Unijunction Transistor (UJT) EET105/4 – ANALOGUE SYSTEMS 2 RB1 = dynamic resistance of the silicon bar between the emitter and base 1. RB1 varies inversely with emitter current IE , and therefore it is shown as a variable resistor. Range of RB1 : several thousand ohms down to tens of ohms Equivalent Circuit

6. THYRISTOR AND OTHER DEVICES Unijunction Transistor (UJT) EET105/4 – ANALOGUE SYSTEMS 2 RB2 = dynamic resistance between the emitter and base 2 Equivalent Circuit

6. THYRISTOR AND OTHER DEVICES Unijunction Transistor (UJT) EET105/4 – ANALOGUE SYSTEMS 2 The total resistance betweem the base terminals is known as interbase resistance RBBO , i.e. Equivalent Circuit

6. THYRISTOR AND OTHER DEVICES Unijunction Transistor (UJT) EET105/4 – ANALOGUE SYSTEMS 2 The ratio of RB1 to the interbase resistance is called the standoff ratio, . Equivalent Circuit

6. THYRISTOR AND OTHER DEVICES Unijunction Transistor (UJT) EET105/4 – ANALOGUE SYSTEMS 2 If the device is biased as shown in the following figure, the voltage across RB1 is;

6. THYRISTOR AND OTHER DEVICES Unijunction Transistor (UJT) EET105/4 – ANALOGUE SYSTEMS 2

6. THYRISTOR AND OTHER DEVICES Unijunction Transistor (UJT) EET105/4 – ANALOGUE SYSTEMS 2 As long as VE is less than VBB +VD, there is no emitter current (except a very small leakage current in A known as IEO) because the pn junction is not forward-biased. The level of emitter voltage that causes the pn junction to become forward-biased is called VP (peak-point-voltage):

6. THYRISTOR AND OTHER DEVICES Unijunction Transistor (UJT) EET105/4 – ANALOGUE SYSTEMS 2 Once VE reaches VP , IE begins to flow (UJT turns on) – conductivity increases and RB1 decreases. After turn-on, the UJT operates in a negative resistance region (VE decreases as IE increases) up to a certain level of IE . Beyond this level, VE begin to increase again. The minimum level of VE is known as valley point (VV , IV). Beyond this point, the device enters its saturation region.

6. THYRISTOR AND OTHER DEVICES Unijunction Transistor (UJT) EET105/4 – ANALOGUE SYSTEMS 2 For fixed values of  and VD, the magnitude of VP will vary as VBB i.e. Fixed

6. THYRISTOR AND OTHER DEVICES Unijunction Transistor (UJT) EET105/4 – ANALOGUE SYSTEMS 2 Application Trigger device for SCR’s and triac Nonsinusoidal (relaxation) oscillators, Sawtooth generators Phase control Timing circuits

6. THYRISTOR AND OTHER DEVICES Unijunction Transistor (UJT) EET105/4 – ANALOGUE SYSTEMS 2 Application – relaxation oscillator When the dc voltage +VBB is applied, C charges through R1 and VE increases exponentially towards +VBB. During this period, the UJT is off.

6. THYRISTOR AND OTHER DEVICES Unijunction Transistor (UJT) EET105/4 – ANALOGUE SYSTEMS 2 Application – relaxation oscillator When VE reaches VP, the UJT turns on and provides a discharge path for the capacitor C. The discharge is rapid since R2 is small – causing a voltage spike across R2.

6. THYRISTOR AND OTHER DEVICES Unijunction Transistor (UJT) EET105/4 – ANALOGUE SYSTEMS 2 Application – relaxation oscillator Towards the end of the discharge process, the current IE decreases to a level that causes the UJT to turn off and hence the charging process restarts. The charge and discharge cycle produces the waveforms shown in the following figure. The output VR2 can be used to trigger an SCR or a triac in the AC power control circuit.

6. THYRISTOR AND OTHER DEVICES Unijunction Transistor (UJT) EET105/4 – ANALOGUE SYSTEMS 2 Relaxation oscillator – mathematical analysis Fixing the value of R1. The value of R1 is crucial in ensuring that the UJT can turn on and of.

6. THYRISTOR AND OTHER DEVICES Unijunction Transistor (UJT) EET105/4 – ANALOGUE SYSTEMS 2 At the peak point, in order to swith the UJT to conduction mode, VE and IE must increase to VP and IP respectively.

6. THYRISTOR AND OTHER DEVICES Unijunction Transistor (UJT) EET105/4 – ANALOGUE SYSTEMS 2 Hence; On the other hand, in order to switch off the UJT at the end of the discharging period, VE and IE must reduce to VV and IV respectively. Hence;

6. THYRISTOR AND OTHER DEVICES Unijunction Transistor (UJT) EET105/4 – ANALOGUE SYSTEMS 2 Thus, UJT will switch on and off if the value of R1 lies within the following range;

6. THYRISTOR AND OTHER DEVICES Unijunction Transistor (UJT) EET105/4 – ANALOGUE SYSTEMS 2 Determination of frequency of oscillation During the charging period t1 of C; where;

6. THYRISTOR AND OTHER DEVICES Unijunction Transistor (UJT) EET105/4 – ANALOGUE SYSTEMS 2 The time constant for the discharging period t2 is given by; RB1 is the internal resistance between B1 and E of the UJT. The equation for 2 is quite complicated due to the fact that the value of RB1 is dependent on IE.

6. THYRISTOR AND OTHER DEVICES Unijunction Transistor (UJT) EET105/4 – ANALOGUE SYSTEMS 2 However since both RB1 and R2 are much smaller than R1, the voltage VR2 is given by the approximate equation; The reduced equivalent circuit for the discharge phase is as shown in the following figure;

6. THYRISTOR AND OTHER DEVICES Unijunction Transistor (UJT) EET105/4 – ANALOGUE SYSTEMS 2

6. THYRISTOR AND OTHER DEVICES Unijunction Transistor (UJT) EET105/4 – ANALOGUE SYSTEMS 2 Using the equivalent circuit, the peak value of VR2 is given by the approximate equation;

6. THYRISTOR AND OTHER DEVICES Unijunction Transistor (UJT) EET105/4 – ANALOGUE SYSTEMS 2 The period t1 may be found as follows; When vC = VP, t = t1, and;

6. THYRISTOR AND OTHER DEVICES Unijunction Transistor (UJT) EET105/4 – ANALOGUE SYSTEMS 2 or; and;

6. THYRISTOR AND OTHER DEVICES Unijunction Transistor (UJT) EET105/4 – ANALOGUE SYSTEMS 2 Hence;

6. THYRISTOR AND OTHER DEVICES Unijunction Transistor (UJT) EET105/4 – ANALOGUE SYSTEMS 2 For the discharge phase; Setting t1 as t = 0; and;

6. THYRISTOR AND OTHER DEVICES Unijunction Transistor (UJT) EET105/4 – ANALOGUE SYSTEMS 2 For the discharge phase; Since

6. THYRISTOR AND OTHER DEVICES Unijunction Transistor (UJT) EET105/4 – ANALOGUE SYSTEMS 2 And since

6. THYRISTOR AND OTHER DEVICES Unijunction Transistor (UJT) EET105/4 – ANALOGUE SYSTEMS 2 If we neglect the effect of VD, then; and;

6. THYRISTOR AND OTHER DEVICES Unijunction Transistor (UJT) EET105/4 – ANALOGUE SYSTEMS 2 or;

6. THYRISTOR AND OTHER DEVICES Programmable Unijunction Transistor (PUT) EET105/4 – ANALOGUE SYSTEMS 2 In terms of physical structure, PUT is in the thyristor family. However it is closer to UJT in terms of characteristics and applications .

6. THYRISTOR AND OTHER DEVICES Programmable Unijunction Transistor (PUT) EET105/4 – ANALOGUE SYSTEMS 2 Basic structure Symbol