Devices unijunction transistor



   Unijunction transistors (double-base diode) are widely used in various devices, automation, pulse and measuring equipment - generators, threshold devices, frequency dividers, timers, etc.
One of the main types of devices on a unijunction transistor relaxation oscillator circuit is shown in Fig. 1a. At power capacitor C1 is charged through resistor R1. Once the voltage on the capacitor voltage becomes equal to the inclusion of unijunction transistor T1, its emitter junction opens and the capacitor is quickly discharged. As the discharge capacitor and the emitter current decreases when a value equal to the current off, the transistor is closed, after which the process is repeated again. As a result, the bases B1 and B2 appear short that multi-polar pulses, which are the output signals of the generator.

Fig. 1. Devices Unijunction transistor. Relaxation oscillator (a), comparator voltage (b)


At power capacitor C1 is charged through resistor R1. Once the voltage on the capacitor voltage becomes equal to the inclusion of unijunction transistor T1, its emitter junction opens and the capacitor is quickly discharged. As the discharge capacitor and the emitter current decreases when a value equal to the current off, the transistor is closed, after which the process is repeated again. As a result, the bases B1 and B2 appear short that multi-polar pulses, which are the output signals of the generator.
Oscillation frequency f of the generator can be calculated by the approximate formula:

where: R - resistor R1, Ohm, C-capacitance C1, F; -transfer coefficient unijunction transistor.
For a given oscillation frequency capacitance should choose the greatest possible in order to obtain the load (R2 or R3) with the desired signal amplitude. An important advantage of unijunction transistor oscillator is that the frequency of its oscillations slightly depends on the magnitude of the supply voltage. Virtually the change in voltage from 10 to 20 V leads to a change in frequency by only 0,5%.
If instead of a resistor R1 in the charging circuit to include a photodiode, photoresistor, thermistor or other element changes its resistance when exposed to external factors (light, temperature, pressure, etc.), then the generator is converted to analog converter of the physical parameter in the repetition rate .
Some change the schema, as shown in Fig. 1b, the same generator can be turned into a comparison device stresses. In this case, the basic transistor circuits are connected to a source of reference voltage, and charging circuit - to the test source. When the voltage exceeds the voltage of the last inclusion, the device will generate pulses of positive polarity.

The device, whose scheme is shown in Fig. 2, the capacitor is charged via the resistor R4 and the resistance area emitter - collector of bipolar transistor T1. The rest of the work of the generator is different from that described previously. Charge current and, consequently, the frequency of the sawtooth voltage, the shooting in this case the emitter unijunction transistor T2, govern the change of bias voltage on the base of transistor T1 through tuning resistor R2. Rejection of the linear form of vibrations produced by such a device does not exceed 1%

 

Fig. 2. Devices Unijunction transistor. Sawtooth generator


The time for inclusion Unijunction transistor can be controlled, giving impetus to the positive polarity in the emitter circuit or negative polarity in the chain of base B2. This principle relies on standby multivibrator circuit is shown in Fig. 3a.
To obtain the desired mode of operation the maximum voltage on the capacitor C1, depending on the ratio of resistors divider R1R2, set a lower voltage transistor integration. The difference between these voltages depending on the potential interference in the chain run, which can lead to false alarms devices. When applying the pulse of negative polarity in the chain of base B2 cross-database UB1B2 voltage is reduced (modulated), resulting in transistor T1 is opened and on the basis of B1 occurs impulse of positive polarity.

 

 

Fig. 3. Devices Unijunction transistor. Single shot leading edge (a), sawtooth generator (b)



Unijunction transistors are used in generators and voltage step form. The input of the device (see Fig. 3b) give a signal of a symmetric (sine, square and etc.) forms. When the positive half-wave signal capacitor C1 is charged via the resistor R2 and the resistance area of the emitter-collector of transistor T1 to a certain voltage, much lower voltage inclusion unijunction transistor T2. During the course of the next positive half-wave voltage on the capacitor increases stepwise by the same amount, and so until then, until it becomes equal to the voltage stepwise inclusion of the transistor T2.Napryazhenie form is removed from its emitter.
On the use of this principle relies on frequency dividers. A cascade of unijunction transistor is able to provide the division factor of 5. By combining into a whole number of such devices, you can get a divider with a much greater rate of division. For example, in Figure 4 shows a circuit frequency divider 100. The first stage device divides the incoming frequency at its input pulses of positive polarity at 4, two others - 5.

 

 

Fig. 4. Devices Unijunction transistor. Frequency divider 100



As seen from the scheme, the frequency divider stages differ from each other only resistances of resistors in circuits charge the capacitors C1-NW. Time constant charge capacitor C1 is determined by the resistors Rl, R2. R4 and R6; C2 - resistors R3. R4 and R6; C3-R5 and R6. When power capacitors C1-NW start charging. Voltage pulses of positive polarity of input devices that are added to the voltage on the capacitor C1, and as soon as their sum reaches a value equal to the voltage integration, unijunction transistor opens and the capacitor discharges through its emitter junction. As a result of the shock increases the voltage drop across resistors R4 and R6, and this leads to a decrease in cross-database voltage transistors T2 and TK. However, the transistor T2 will be opened only when the voltage on the capacitor C2 will be sufficient for its inclusion at low voltage cross-database. Works similarly, and the third cascade divider.

 

Fig. 5. Devices Unijunction transistor. Time Relays



Scheme timers, featuring a very high efficiency, is shown in Fig. 5. In the initial state thyristor D3 is closed, so the device consumes little power (leakage currents are small and can be neglected). When applying to the control electrode of the triggering pulse of positive polarity thyristor opens. As a result, relay P1 and their contacts (in the scheme conditionally not shown) includes actuating device. Simultaneously through resistors R1 and R2 are beginning to charge capacitors C1 and C2. Since the resistance of the first of these resistors is much greater than the second, the first charge on a capacitor C2, and when the voltage on the capacitor C1 reaches the voltage integration, unijunction transistor and a capacitor C1 will discharge through its emitter junction. Emerged at the same time on resistor R2 pulse of positive polarity was formed with the voltage on the capacitor C2, resulting in thyristor D3 closes and disconnect relay P1 before the next triggering pulse.

 

 

Fig. 6. Devices Unijunction transistor. Analog voltage to frequency converter



The device, whose scheme is shown in Fig. 6, is designed to convert the analog voltage to frequency. Here transistor T2 is used in a relaxation oscillator, T1, along with resistors R1 and R2 is included in the charging circuit capacitor C1. When you change the voltage on the base of the transistor T1 changes the resistance of its emitter-collector area, and therefore, depending on the input voltage unijunction transistor T2 opens with a greater or lesser frequency. By repetition rate, removed from the pull-up resistor R3 in the base circuit B1 can judge the voltage at the input device.


V. KONYAEV, Vadim Repin
Source материла: htt: / / radvs.boom.ru. Ed. 12.09 VF Gainutdinov


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