Question

Explain the operation of PUT relaxation oscillator with diagram.

Answer 1

A PUT (Programmable Unijunction Transistor) relaxation oscillator is a type of oscillator that utilizes the unique properties of the PUT to produce periodic oscillations. The PUT is a semiconductor device that has a single transistor junction and can be programmed to trigger at a specific voltage level.

Operation of PUT Relaxation Oscillator

The basic operation of the PUT relaxation oscillator involves charging and discharging a capacitor through a resistor, and using the PUT's characteristics to trigger the oscillation when a threshold voltage is reached.

Components Used in PUT Relaxation Oscillator:

1. PUT (Programmable Unijunction Transistor): This is the key component that controls the triggering and switching behavior.
   

1. Capacitor (C): This stores the electrical charge and helps in determining the timing of the oscillations.
   

1. Resistor (R): The resistor limits the charging rate of the capacitor.
   

1. DC Power Supply: Provides the required voltage to the circuit.
   

1. Triggering Circuit: Helps in triggering the PUT at a certain voltage level.
   

Working Principle:

1. Initial Condition: When the circuit is powered on, the capacitor is initially uncharged (or partially charged).
   

   

1. Charging the Capacitor: The capacitor starts charging through the resistor R. The voltage across the capacitor increases gradually.
   

1. Triggering: When the voltage across the capacitor reaches a certain threshold value (which is determined by the PUT’s intrinsic properties and the external components), the PUT turns on and starts conducting.
   

1. Discharging the Capacitor: Once the PUT turns on, the capacitor discharges rapidly through the PUT. The discharging rate is very fast compared to the charging rate.
   

1. Resetting the Oscillator: After the capacitor discharges to a low voltage, the PUT turns off and the cycle repeats. The capacitor starts charging again, and the oscillation continues.
   

The frequency of the oscillation is determined by the values of the resistor (R) and capacitor (C), as well as the specific characteristics of the PUT, like its triggering voltage.

PUT Relaxation Oscillator Circuit Diagram:

Below is the basic circuit diagram for a PUT relaxation oscillator.

`
         +Vcc
           |
           R
           |
          ---  
         |   |
         |   |----+
         |   |    |
          ---     |
           |      |
          (C)     |
           |      |
           |    PUT
           |      |
           |      |
           +------+
           |
          GND
`

Explanation of the Diagram:

1. Resistor (R): This limits the current charging the capacitor.
   

1. Capacitor (C): It charges and discharges, controlling the timing of the oscillator.
   

1. PUT: The programmable unijunction transistor is the heart of the oscillator. It controls the discharge of the capacitor once the voltage reaches a triggering point.
   

1. Power Supply (+Vcc): Provides the necessary voltage to the circuit.
   

Frequency of Oscillation:

The frequency \( f \) of the PUT relaxation oscillator is determined by the following formula:

\[
f = \frac{1}{R \cdot C \cdot \ln \left(\frac{V_{cc} - V_{th}}{V_{th}} \right)}
\]

Where:

1. \( R \) = Resistance (ohms)
   

1. \( C \) = Capacitance (farads)
   

1. \( V_{cc} \) = Power supply voltage
   

1. \( V_{th} \) = Triggering voltage of the PUT
   

Applications:

1. Timing circuits: Used in timing applications where a periodic signal is required.
   

1. Pulse generation: Can be used to generate pulses for clock circuits.
   

1. Signal generators: Used for generating audio or radio frequency signals in simple circuits.
   

Key Points:

1. The PUT in this circuit acts as a switch that triggers when the capacitor reaches a certain voltage.
   

1. The frequency of the oscillator depends on the resistor-capacitor combination and the properties of the PUT.
   

1. PUT relaxation oscillators are commonly used in simple timing and pulse-generation circuits.
   

If you have any more questions or need further clarification, feel free to ask!