Programmable UJT

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Programmable UJT The programmable unijunction transistor (PUT) is not a unijunction transistor at all. The fact that the V-I characteristics and applications of both are similar prompted the choice of labels. It is also a four-layer P-N-P-N solid-state device with a gate connected directly to the sandwiched N-type layer. The basic structure, schematic symbol and the basic biasing arrangement of PUT are shown in figures respectively. As the symbol indicates, it is essentially an SCR with a control mechanism that permits a duplication of the characteristics of the typical SCR. The term “programmable” is applied because the inter base resistance R BB , the intrinsic stand-off ratio Ƞ and peak-point voltage V P , as defined in UJT can be programmed to any desired values through external resistors R B and R B2 and the supply voltage V BB . From figure we see that by voltage divider rule when I G = 0, V G = (R B1 / R B1 + R B2 ) V BB = Ƞ V BB Consider figure The P-N-P-N device shown in figure has its gate connected to the junction of external resistors R B and R B . The four-layer construction shown in figure indicates that the anode-gate junction is forward biased when the anode becomes positive with respect to gate. When this occurs, the device is turned on. The anode-to-cathode voltage V AK then drops to a low level, and the device conducts heavily until the input voltage become too low to sustain conduction. It is seen that this action stimulates the performance of a UJT. The anode of the device acts as the emitter of UJT.

Transcript of Programmable UJT

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Programmable UJT

The programmable unijunction transistor (PUT) is not a unijunction transistor at all. The fact that the V-I characteristics and applications of both are similar prompted the choice of labels.

It is also a four-layer P-N-P-N solid-state device with a gate connected directly to the sandwiched N-type layer. The basic structure, schematic symbol and the basic biasing arrangement of PUT are shown in figures respectively. As the symbol indicates, it is essentially an SCR with a control mechanism that permits a duplication of the characteristics of the typical SCR. The term “programmable” is applied because the inter base resistance RBB, the intrinsic stand-off ratio Ƞ and peak-point voltage VP, as defined in UJT can be programmed to any desired values through external resistors RB and RB2 and the supply voltage VBB. From figure we see that by voltage divider rule when IG = 0,

VG =   (RB1 / RB1 + RB2 )  VBB =  Ƞ  VBB

Consider figure The P-N-P-N device shown in figure has its gate connected to the junction of external resistors RB and RB . The four-layer construction shown in figure indicates that the anode-gate junction is forward biased when the anode becomes positive with respect to gate. When this occurs, the device is turned on. The anode-to-cathode voltage VAK then drops to a low level, and the device conducts heavily until the input voltage become too low to sustain conduction. It is seen that this action stimulates the performance of a UJT. The anode of the device acts as the emitter of UJT.

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Characteristics of Programmable UJT

The typical characteristics of the device are shown in figure. The firing or peak-point potential is given as

VP = Ƞ VBB + VB  as defined for the UJT.

However VP represents the voltage drop VAK in figure [ the forward voltage drop across the conduct ing diode]. For silicon VB is typically 0.7 V.

In PUT RB1 and RB2 are the external resistors to the device permitting the adjustment of Ƞ and hence VG while in the UJT both RB1 and RB2 represent the bulk resistance and ohmic base contacts of the device (both inaccessible).  Although the characteristics of the PUT and UJT are similar, the peak and valley currents of the PUT are typically lower than those of a UJT of a similar rating. In addition, the minimum operating voltage of PUT is also lower than that of UJT.

Application of PUT

PUT RElaxation Oscillator

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PUT, because of its superiority over UJT, replaces UJT. One popular application of PUT is in the relaxation oscillator shown in figure. The instant the supply VBB is switched on, the capacitor starts charging toward VBB volts, since there is no anode current at this point. The instant the voltage across the capacitor equals VP, the device fires and anode current IA = IP is established through the PUT. As soon as the device fires, the capacitor starts discharging rapidly through the low on-resistance of the PUT and RK. Consequently, a voltage spike is produced across RK during the discharge. As soon as the capacitor C gets discharged, the PUT turns off and the charging cycle starts all over again as narrated above.

The time period required to attain the firing potential VP is given approximately by the expression

T = RC loge = VBB / VBB – VP = Ƞ VBB

At the point of firing of PUTIP R = VBB – VP

If R is too large, the current IP cannot be established, and the device will not fire

So RMAX = VBB – VP / IP

Similarly  RMIN = VBB – VV / IV

VBB – VP / IP > R > VBB – VV / IV

SCS-Silicon Controlled Switch

Silicon controlled switch (SCS), like the SCR, is a unilateral, four layer three junction P-N-P-N silicon device with four electrodes namely cathode C, cathode gate Gx, anode gate G2 and the anode A, as shown in figure. Infact, the SCS is a low power device compared with the SCR. It handles currents in milli amperes rather than amperes. SCS differs from an SCR in the following aspects. It has an additional gate—the anode gate.It is physically smaller than SCR.It has smaller leakage and

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holding currents than SCR.It needs small triggering signals. It gives more uniform triggering characteristics from sample to sample.

The basic structure and schematic symbol of SCS are shown in the figures. It may be fabricated by using either the grown junction technique or the planar technique.

SCS Symbol

Operation of a Silicon Controlled Switch

The easiest way to understand how it operates is to realize it to be formed of two transistors Q1 and Q2 placed back-to-back, as shown in figure.b

In a two-transistor equivalent circuit shown in figure.c, it is seen that a negative pulse at the anode gate G2 causes transistor Q1 to switch on. Transistor Q1 supplies base current to transistor Q2, and both transistors switch-on. Similarly, a positive pulse at the cath ode gate G1 can switch the device on. Since only small currents are involved, the SCS may be switched off by an appropriate polarity pulse at one of the gates. At the cathode gate a negative pulse is required for switching-off while at the anode gate a positive pulse is needed.

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Silicon Controlled Switch Characteristics

Volt-Ampere Characteristic of SCS

The volt-ampere characteristic of an SCS is similar to that of an SCR and is shown in figure. With the increase in applied voltage, the current first increases slowly upto point A and then rapidly in the region AB, as shown in the figure. At point B, the product β1β2 exceeds unity and the device is suddenly switched on. In the on-state, the current increases enormously and is limited by the external series resistor. SCS also exhibits negative differential resistance in the on region similar to SCR. SCS gets switched on accidentally if the anode voltage gets applied suddenly. This is known as rate effect, which is caused by inter-electrode capacitance between elec trodes G1 and G2, known as transition capacitance.

Advantages and Applications of SCS

An advantage of SCS over an SCR is the reduced turn-off time, typically within the range of 1 to 10 micro seconds for the SCS and 5 to 30 micro seconds for the SCR. Other advantages of the SCS over SCR are increased control and triggering sensitivity and a more predictable firing situation. However, the SCS is limited to low power, current, and voltage ratings (typical maximum anode currents range from 100 mA to 300 mA with dissipation rating of 100 to 500 mW).

A few of the more common areas of application of SCS include a variety of computer circuits (such as counters, registers, and timing circuits) voltage sensors, pulse generators, oscillators etc.

Silicon Bilateral Switch

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Silicon bilateral switch (SBS) is another breakover device which is capable of triggering triacs, and it is popular in low-voltage trigger control circuits. They have breakover voltages lower than those for diacs, ± 8 V being the most popular rating. V-I characteristic curve of an SBS is similar to that of the diac, but it has more pronounced “negative resistance” region i.e., its decline in voltage is more drastic after it enters the conducting state. The schematic symbol and V-I characteristic of an SBS are shown in figure a and b respectively. When SBS switches into conducting state the voltage across its anode termi nals drops almost to zero (to about 1 V). The SBS is said to have a breakback voltage of 7 V when it turns on.

SBS Characteristics

Characteristics of Silicon Bilateral Switch or SBS

The characteristic curve of an SBS shown in fig b is for the gate terminal of the device disconnected. The basic V-I characteristic of an SBS can be altered by using the gate terminal. However, the SBS is quite useful even without its gate terminal, just by virtue of the snap-action breakover from A2 to A1.  The reason for using an SBS in place of diac is its superiority over the later. Not only does the SBS show a more vigorous switching characteristic as indicated in figure b, but it also more temperature stable and more symmetrical and has less batch spread than a diac. A modern SBS has a temperature coefficient of about + 0.02 % per °C, that is, its VB0 increases by only 0.02 % per degree of temperature rise, which comes out to only 0.16 V per 100°C which is very temperature stable indeed.

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SBSs are symmetrical to within about 0.3 V i.e., the difference in magnitude between + VB0 and -VB0 is less than 0.3 V. This yields virtually identical firing delays for positive ‘ and negative half cycles.The batch spread of SBSs is less than 0.1 V i.e., the difference in V B0 among the different SBSs in a batch is less than 0.1 V. The batch spread among diacs is about 4 VA by contrast.The main advantage of using gated SBS for the trigger control of a triac is that the hysteresis or flash-on effect is eliminated.

SCR Applications

1. Power Control.

Because of the bistable characteristics of semiconductor devices, whereby they can be switched on and off, and the efficiency of gate control to trigger such devices, the SCRs are ideally suited for

many industrial applications. SCRs have got specific advantages over saturable core reactors and gas tubes owing to their compactness, reliability, low losses, and speedy turn-on and turn-off.

The bistable states (conducting and non-conducting) of the SCR and the property that enables fast transition from one state to the other are made use of in the control of power in both ac and dc

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circuits.

In ac circuits the SCR can be turned-on by the gate at any angle α with respect to applied voltage. This angle α is called the firing angle and power control is obtained by varying the firing angle. This is known as phase control. A simple half-wave circuit is shown in figure a. for illustrating the principle of phase control for an inductive load. The load current, load voltage and supply voltage waveforms are shown in figure b. The SCR will turn-off by natural commutation when the current becomes zero. Angle β is known as the conduction angle. By varying the firing angle a, the rms value of the load voltage can be varied. The power consumed by the load decreases with the increase in firing angle a. The reactive power input from the supply increases with the increase in firing angle. The load current wave-form can be improved by connecting a free-wheeling diode D1, as shown by the dotted line in fig-a. With this diode, SCR will be turned-off as soon as the input voltage polarity reverses. After that, the load current will free wheel through the diode and a reverse voltage will appear across the SCR. The main advantage of phase control is that the load current passes through a natural zero point during every half cycle. So, the device turns-off by itself at the end of every conducting period and no other commutating circuit is required.

Power control in dc circuits is obtained by varying the duration of on-time and off-time of the device and such a mode of operation is called on-off control or chopper control. Another important application of SCRs is in inverters, used for converting dc into ac. The input frequency is related to the triggering frequency of SCRs in the inverters. Thus, variable frequency supply can be easily obtained and used for speed control of ac motors, induction heating, electrolytic cleaning, fluorescent lighting and several other applications. Because of the large power-handling capacity of the SCRs, the SCR controlled inverter has more or less replaced motor-generator sets and magnetic frequency multipliers for gener ating high frequency at large power ratings.

Operation of Power Control in SCR

A commonly used circuit for controlling power in load RL using two SCRs is shown in figure. Potentiometer R controls the angle of conduction of the two SCRs. The greater the resistance of the pot, lesser will be the voltage across capacitors C1 and C2 and hence smaller will be the time duration of conduction of SCR1 and SCR2 during a cycle.

During positive half cycle capacitor C2 gets charged through diode D1, pot R, and diode D4. When the capacitor gets fully charged, (charge on the capacitor depending upon the value of R) it discharges through Zener diode Z. This gives a pulse to the primary and thereby secondary of the transformer T2. Thus SCR2, which is forward biased, is turned on and conducts through load RL. During negative

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half cycle similar action takes place due to charging of capacitor C1 and SCR1 is triggered. Thus power to a load is controlled by using SCRs.

2. Switching.

Thyristor, being bistable device is widely used for switching of power signals owing to their long life, high operation speed and freedom from other defects associated with mechanical and electro-

mechanical switches.

Figure shows a circuit in which two SCRs are used for making and breaking an ac circuit. The input voltage is alternating and the trigger pulses are applied to the gates of SCRs through the control switch S. Resistance R is provided in the gate circuit to limit the gate current while resistors R1 and R2 are to protect the diodes D1 and D2 respectively.

For starting the circuit, when switch S is closed, SCR1 will fire at the beginning of the positive half-cycle (the gate trigger current is assumed to be very small) because during positive half cycle SCR1 is forward biased. It will turn-off when the current goes through the zero value. As soon as SCR1 is turned-off, SCR2 will fire since the voltage polarity is already reversed and it gets the proper gate current. The circuit can be broken by opening the switch S. Opening of gate circuit poses no problem, as current through this switch is small. As no further gate signal will be applied to the SCRs when switch S is open, the SCRs will not be triggered and the load current will be zero. The maximum time delay for breaking the circuit is one half-cycle.

Thus several hundred amperes of load current can be switched on/off simply by han dling gate current of few mA by an ordinary switch. The above circuit is also called the static contactor because it does not have any moving part.

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DC circuit breaker

As shown in figure, Capacitor C pro vides the required commutation of the main SCR since the cur rent does not have a natural zero value in a dc circuit. When the SCR1 is in conducting state, the load voltage will be equal to the supply voltage and the capacitor C will be charged through resis tor R. The circuit is broken by turning-off SCR1. This is done by firing SCR2, called the auxiliary SCR. Capacitor C discharges through SCR2 and SCR1. This discharge current is in opposite direction to that flowing through SCR1 and when the two become equal SCR2 turns-off. Now capacitor C gets charged through the load and when the capacitor C gets fully charged, the SCR2 tums-off. Thus the circuit acts as a dc circuit breaker. The resistor R is taken of such a value that current through R is lower than that of holding current.

3. Zero Voltage Switching.

In some ac circuits it is necessary to apply the voltage to the load when the instantaneous value of this voltage is going through the zero value. This is to avoid a high rate of increase of current in case

of purely resistive loads such as lighting and furnace loads, and thereby reduce the generation of

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radio noise and hot-spot temperatures in the device carrying the load current. The circuit to achieve this is shown in figure. Only half-wave control is used here. The portion of the circuit shown by the

dotted lines relates to the negative half cycle. Whatever may be the instant of time when switch S is opened (either during the positive or the nega tive half cycle), only at the begin ning of the following positive half-cycle of the applied voltage SCR1 will be triggered. Similarly, when switch S is closed, SCR1 will stop conducting at the end of the present or previous positive half-cycle and will not get

triggered again. Resis tors R3 and R4 are designed on the basis of minimum base and gate currents required for transistor Q1 and SCR1. Resistors Rl and R2 govern rates of the charging and discharging of capacitor C1 Resis tor R5 is used for preventing large discharge currents when switch S is closed.

4. Over-Voltage Protection.

SCRs can be employed for protecting other equipment from over-voltages owing to their fast switching action. The SCR employed for protection is connected in parallel with the load. Whenever the voltage exceeds a specified limit, the gate of the SCR will get energized and trigger the SCR. A large current will be drawn from the sup ply mains and voltage across the load will be reduced. Two SCRs are used—one for the positive half-cycle and the other for negative half-cycle, as shown in figure. Resistor R1 limits the short-circuit current when the SCRs are fired. Zener diode D5 in series with resistors Rx and R2 consti tutes a voltage-sensing circuit.

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5. Pulse Circuits.

SCRs are used for producing high voltage/current pulses of desired waveform and duration. The capacitor C is charged during the positive half cycle of the input supply and the SCR is triggered during the nega tive half-cycle. The capacitor will discharge through the output circuit, and when the SCR forward current becomes zero, it will turn-off. The output circuit is designed to have discharge current of less than a milli-second duration. The capacitor will again get charged in the following positive half-cycle and the SCR will be triggered again in the negative half-cycle. Thus the frequency of the output pulse will be equal to the frequency of the input supply. For limiting the charging current resistor R is used. High voltage/current pulses can be used in spot welding, electronic ignition in automo biles, generation of large magnetic fields of short duration, and in testing of insulation.

6. Battery Charging Regulator.

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The basic components of the circuits are shown in figure. Diodes D1 and D2 are to establish a full-wave rectified sig nal across SCR1 and the 12 V battery to be charged. When the battery is in dis charged condition, SCR2 is in the off-state as will be clear   after discussion. When the full-wave rectified input is large enough to give the required turn-on gate current (controlled by resistor R 1), SCR1 will turn on and the charging of the battery will commence. At the commencement of charging of battery, voltage VR determined by the simple volt age-divider circuit is too small to cause 11.0 V zener conduction. In the off-state Zener diode is effectively an open-circuit maintaining SCR2 in the off-state because of zero gate current. The capacitor C is included in the circuit to prevent any voltage transients in the circuit from accidentally turning on of the SCR2. As charging continues, the battery voltage increases to a point when VR is large enough to both turn on the 11.0 V Zener diode and fire SCR2. Once SCR2 has fired, the short circuit representation for SCR2 will result in a voltage-divider circuit determined by R1 and R2 that will maintain V2 at a level too small to turn SCR1 on. When this occurs, the battery is fully charged and the open-circuit state of SCR1 will cut off the charging current. Thus the regulator charges the battery whenever the voltage drops and prevents overcharging when fully charged. There are many more applications of SCRs such as in soft start circuits, logic and digital circuits, but it is not possible to discuss all these here.

Research Source

http://www.circuitstoday.com

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Colegio de San Juan de Letran- Calamba

School of Engineering

Industrial Electronics

Final Research in Industrial Electronics

Tolentino, Sherwin Anthony B.

5EE

October 07, 2010