Why is ujt used in relaxation oscillator

If the UJT is used in conjunction with an RC timing circuit it will produce an output that consists of short duration pulses with a very fast rise time, ideal for the triggering of SCRs, GTO thyristors and triacs. From the emitter characteristic V—I curve shown in Figure 4 it can be seen that the resistance of the UJT emitter to base B 1 decreases significantly once the peak point voltage is reached. The decrease occurs quite rapidly. Figure 4 UJT emitter characteristics curve.

UJT Relaxation Oscillator. The circuit shown in Figure 5 is applicable to low-level power-control applications with SCRs and triacs.

The only modification to the circuit may be to use a pulse transformer. Figure 5 UJT relaxation oscillator circuit diagram. When the supply to the timing circuit is first turned on, the voltage across the capacitor will increase at a rate determined by the values of the resistor and the capacitor. The time may be modified by varying either the value of the resistor or the value of the capacitor.

In practice it is found that it is easier to alter the value of the resistor. The operation of a UJT relaxation oscillator is straightforward and may be summarized as follows:. Example 2. The frequency of the pulses from this circuit will depend on the time constant.

The actual calculation of frequency can be complex but if the standoff ratio is equal to, or near, 0. Example 3. In the circuit in Figure 5 , determine the output frequency if the standoff ratio is 0. Determine the frequency if RV 1 is set to 7. It is worth noting that in most cases when a relaxation oscillator is used as a trigger circuit for thyristors, the calculation of frequency is not important.

What is more important is the time delay from the start of a cycle to the point where a trigger pulse is delivered to the thyristor. This time delay will determine both the average value and the RMS value of load voltage , and hence the load power. The waveforms produced by the relaxation oscillator are shown in Figure 6.

Figure 6 Relaxation oscillator waveforms. The resistor R provides a path for the capacitor C to charge through the voltage applied. The capacitor usually starts charging and continues to charge until the maximum voltage V BB. But in this circuit, when the voltage across capacitor reaches a value, which enables the UJT to turn ON the peak voltage then the capacitor stops to charge and starts discharging through UJT.

This process continues and the voltage across the capacitor, when indicated on a graph, the following waveform is observed. So, the charge and discharge of capacitor produces the sweep waveform as shown above. The charging time produces increasing sweep and the discharging time produces decreasing sweep.

The repetition of this cycle, forms a continuous sweep output waveform. The positive feedback provided to the op-amp fed some part of the output to the non-inverting input terminal. Let for a particular time the input at the terminal of the op-amp is negative or less than 0. Hence the capacitor starts charging exponentially with V Z2. Now, the output of the comparator gets reversed to -V Z1.

This causes the charging of the capacitor with -V Z1. So, here also, the charging and discharging phenomenon of the capacitor determines the time period of the output waveform. The figure below shows the output and capacitor voltage waveform for op-amp relaxation oscillator:.

The circuit of op-amp relaxation oscillator is also known as an astable multivibrator, as the circuit holds 2 quasi-stable states. Hence, the signal performs the transition between 2 states. Thereby generating the square wave signal at the output.

These are used to produce internal clock signalling in any digital circuits. These also find applications in thyristor triggering circuits, in oscilloscopes as well as television receivers etc.

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