UJT Application, Uses, Advantages, Diagram, Symbol, Circuit

UJT stands for Unijunction Transistor. UJT is a two-layered, three-terminal, solid-state switching device. Unlike a BJT, it has only one junction that's why it is called Unijunction Transistor.
UJT can act as an electrically operated solid-state switch. UJT is a current-controlled device.
UJT is also known as Double base diode because like a diode it also has two-layer and one junction. Just difference is diode has two terminals whereas UJT has three terminals.

UJT Symbol and Diagram

Here you can see the symbol and construction diagram of UJT. You can see the UJT has three terminals, Base 2, Emitter, and Base 1.

You can see in the constructional diagram, the emitter terminal is connected to the P-type layer and the Base terminals are connected to the N-layer.

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UJT Equivalent Circuit

Here you can see the equivalent circuit of Unijunction Transistor.

Here, you can see in the above equivalent circuit, that UJT has two resistors connected in series with an equivalent diode. The resistance value of RB2 is always less than RB2.

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Application and Uses of UJT

1. The main application of UJT is, that it is used for triggering of SCR and TRIAC.

2. Unijunction Transistors are used in synchronized oscillators and pulse generator circuits that work at low to moderate frequencies.

3. Unijunction transistors are used in both voltage-regulated and current-regulated switch-mode power supply circuits.

4. Unijunction Transistor(UJT) is used for a large amount of AC power control with static devices which increases safety, integrating control, accurate control, and reliability.

5. Unijunction Transistor(UJT) is used in digital voltage, and frequency measurement circuits.

6. UJTs are used as an electronic solid-state switch.

7. UJT is used in Hall Effect sensors for magnetic flux measurement purposes.

8. UJTs are used to make relaxation oscillator circuits.

9. UJTs are used in phase control and timing circuits.

10. UJTs are used for different types of waveform generation such as Sawtooth Wave generation.

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Advantages of UJT

1. UJT is a very low-cost device but has a wide variety of applications.

2. UJT has good electrical and temperature characteristics.

3. UJT provides a very good switching response.

4. UJT provides unidirectional conductivity that's why it is good for reverse current blocking circuits.

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Disadvantages of UJT

1. UJT cannot be used in linear amplifiers.

2. UJT can work with only low to moderate frequency, it is not suitable for very high frequency.

3. UJT has negative impedance property that's why it is not suitable to use in some circuits.

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Motor Speed Control Circuit using UJT

Here you can see the motor speed control circuit using UJT.

Here, UJT is used to trigger the SCR which is connected in series with the motor.

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