What is a UJT Relaxation Oscillator – Circuit Diagram and Applications

Oscillators are the electronic circuits used to generate waveforms without using any input signal. Waveforms such as sine waves, cosine waves, triangular waves, pulse waves, etc.. are generated using an Oscillator circuit. Basically there are two types of electronic oscillators- the Linear oscillators and the Relaxation Oscillators. The Linear oscillators are used to generate the sinusoidal waveforms whereas the relaxation oscillators are used for generating the non-sinusoidal waveforms. Relaxation oscillator consists of a feedback loop with a switching device such as a transistor, Op-Amp, relay,etc.. that repetitively charges and discharges the capacitor through a resistor. In UJT Relaxation Oscillator, UJT is used as the switching device.

What is UJT Relaxation Oscillator?

To generate waveforms without using any input signal we use Oscillators. Relaxation Oscillators are the circuits that produce the non-sinusoidal waveforms. These Oscillators consists of a feedback loop with a switching device, which charges and discharges a capacitor through a resistor until it reaches the threshold value. Here, the period of the oscillator depends on the time constant of the capacitor. In UJT Relaxation Oscillator, UJT is used as the switching device which charges and discharges the capacitor.

UJT Characteristics and Relaxation Oscillator

To understand the functioning of UJT in the relaxation Oscillator it is important to know the characteristics of the UJT.UJT is the short form for UniJunction Transistor. It is a three-terminal device used as an ON-OFF switching transistor. These are constructed using P and N-type semiconductor material, forming a single PN junction in the N-type channel of the device. It has unidirectional conductivity and negative resistance characteristics. It acts as a variable voltage divider during breakdown conditions. Here, the P-type material is fused into the N-type silicon channel. The N-Type channel of the UJT acts as the main current-carrying channel with two outer connections Base1 and Base2. The P-type material forms the emitter connection.

UJT Relaxation Oscillator
UJT Relaxation Oscillator

In UJT the emitter terminal E is forward biased. Here, the Intrinsic stand-off ratio indicates the resistive ratio of RB1 to RB2, denoted by η. η values range from 0.5 to 0.8.

η = RB1/(RB1+RB2)

UJT gets switched OFF when a small input voltage, less the voltage across RB1, is applied to the emitter terminal. When the Emitter terminal is applied with voltage greater than the voltage across the RB1, the device gets forward biased and starts conducting.

UJT Relaxation Oscillator Circuit Diagram

UJT Relaxation Oscillator consists of a UJT circuit with its emitter connected to a resistor and a capacitor. The timing of the output waveform is determined using the RC time constant. Supply voltage VBB is applied to the circuit. The capacitor starts charging through the resistor R1.

UJT Relaxation Oscillator Theory

When the capacitor charges to the threshold peak value of the UJT, UJT gets switched ON and the capacitor starts discharging. The capacitor discharges through the resistor R2. The capacitor discharges till the voltage reduces to the valley point of the UJT, where the UJT gets switched OFF and the capacitor starts charging again. The output voltage collected across R2 forms the non-sinusoidal waveform. The voltage waveform is generated when the UJT is in ON state.

Initially the voltage across the capacitor Vc =0. The capacitor starts charging through the resistor R1, V = V0(1- e1/R1 C). The capacitor continues to charge till the UJTis switched ON, where it starts discharging through the resistor R2.

This process of charging and discharging continues. The voltage across the capacitor when plotted on the graph shows a sweep waveform. The continuous charging and discharging of the capacitor have generated a sweep waveform across the capacitor. Thus, the output of the relaxation oscillator generates continuous non-sinusoidal waveforms.

ujt relaxation oscillator waveform obtained across the discharge resistor also generates a continuous with relaxation and AC signal. The relaxation is caused when the UJT is switched OFF and the AC signal is generated when UJT is switched ON.

There are some design parameters to be considered while designing this relaxation oscillator. The time period of the output waveform depending on the time constant RC is given as T=R2C log(1/1-η) whereas frequency is represented as 1/T. As the speed of charging of the capacitor depends on the resistance value of R1, the efficient resistance value of R1 can be chosen as R1 = 104 /η VBB, VBB is the supply voltage. The discharging values of the capacitor depending on the resistance value of R2. Thus RMax = (VBB -Vp)/Ip and RMin = (VBB – Vv)/ Iv . where Vp and Ip are the peak voltage and peak current of UJT respectively. Vv and Iv are the valley voltage and valley current of UJT respectively.


The UJT relaxation oscillator applications are

Relaxation oscillator stays in rest position for some time and produces AC signals. These oscillators produce low-frequency signals. UJT Relaxation Oscillator is used in function generator to produce sweep signals, Electronic Beepers, SMPS, blinking lights, Voltage-controlled oscillators, inverters, etc..

 Advantages and Disadvantages

The UJT Relaxation Oscillator Advantages and Disadvantages are

The negative resistance characteristic of the UJT adds an advantage to the UJT relaxation Oscillator. UJT requires a low value of triggering current. It is of low cost and is a low power absorbing device. UJT has a stable triggering voltage.

The disadvantages of the UJT Relaxation Oscillator are that they are unstable and for the good control features require complex circuitry.

UJT Relaxation Oscillator can be used as a pulse generator when the voltage across the discharge resistor is used. By connecting a potentiometer at the place of the charging resistor R1, sawtooth waveforms with different frequency ranges can be obtained across the capacitor. Pulses with different frequency range can be obtained across the discharge resistor when ujt relaxation oscillator experiment with different values of capacitor and resistors R1 and R2.

The mathematical model of the relaxation oscillator is used in many fields of science to analyze the dynamical systems that produce non-linear oscillations. In the output of a relaxation oscillator, there is only a single ramp that takes up the entire time period. Here voltage across the capacitor is a sawtooth wave whereas the current through UJT is a sequence of short pulses. What is the peak voltage of UJT?

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