What will you learn? show

At a glance of PUT

In today’s blog, we will study PUT, characteristics of PUT, and the PUT relaxation oscillator.

Introduction

PUT stands for programmable unijunction transistor.

You all must be thinking that we have learned the UJT, then what is the need to learn the PUT? The reason is hidden in the name itself. PUT is basically a programmable UJT device in which intrinsic standoff ratio (η ) and peak voltage (Vp) can be programmed with the help of external resistors, which will be explained in the latter part of this blog.

And yes, you’re thinking right that we cannot program the above parameter in UJT that is intrinsic standoff ratio ( η ) and peak voltage (Vp) is fixed for UJT.

PUT has a four-layer structure, same as thyristors, and has three terminals Anode, Cathode, and Gate. You might get confused that in the previous line, I mentioned the word thyristors. Still, in the long form, it is a programmable unijunction transistor. The reason behind this is that PUT has the same characteristics and parameters as UJT.

The below figure shows the symbol and structure of PUT.

As the PUT is a four-layer device, its topmost layer that is P- layer, is connected to the Anode terminal. N- Layer, which is next to the anode, is connected to by the ohmic contact to the gate terminal layer next to the gate. The P- layer is left alone, and the N- layer situated at the bottom is connected to the cathode terminal. Ohmic contacts are made on anode, cathode, and gate terminals, also known as external contact.

Characteristics of PUT

Characteristics of PUT are taken with respective anode voltage (Va) and anode current (Ia)

Anode plays an important role in PUT; hence anode is provided with the positive voltage concerning the cathode, which is always grounded. The Gate terminal is connected to the external resistors R1 and R2 together, forming a voltage divider.

In the PUT characteristics, intrinsic standoff ratio ( η ) and peak voltage (Vp) is determined by the value of R1 and R2

As the anode to cathode voltage increases, anode current starts increasing; thus, PUT behaves as a PN junction. However, we cannot increase Va beyond certain limits for that particular point.

At the particular point when the number of charges gets injected in the device junction saturates. Once we reach that point, the anode current increases, and the anode voltage is reduced beyond that point.

A question might have popped in your brain that why are we using the particular term point? What is that point, and what is its significance in PUT?

This particular point and beyond this point is equal to the negative resistance. This region in the PUT characteristics is specially used for the PUT relaxation oscillator. Which I will explain to you in the latter part of this blog.

As the anode voltage is reduced to a particular point level, this point is known as the valley point. After this device becomes fully saturated and no decrease in anode voltage (Va) value is possible. However, the UJT behaves like a fully saturated PN junction diode after saturation.

Now you all might think about what is peak voltage and intrinsic standoff ratio and why the hell she is talking about it repeatedly, so let’s look at these terms.

Important terms

Peak voltage (Vp): peak voltage is known as the anode to cathode voltage. After that, PUT jumps into the negative resistance region.

Intrinsic standoff ratio (η ): as the name suggests it is the ratio of two entities. The intrinsic standoff ratio of PUT is the ratio of external resistor R1 to the sum of both external resistors R1 and R2.

PUT relaxation oscillator

Now let’s come to the most important topic of today’s blog. Let me present you one and only PUT relaxation oscillator.

In the wide applications of PUT, the PUT relaxation oscillator is the most essential application.

And yes, you all are thinking right PUT relaxation oscillator is used for generating a wide range of sawtooth waveforms.

Ok, so first, we need to understand what is an oscillator and what is sawtooth waveform is?

The oscillator is the circuit that converts direct current (DC) from a power supply to an AC signal without using any power inverters. And sawtooth waveform is a non-sinusoidal waveform. So in this circuit, PUT is used to generate this sawtooth waveform from the input DC signal.

Why the name relaxation oscillator? Because the time interval of this oscillator starts by charging the capacitor, and the time interval is terminated by discharging the capacitor, that’s where the relaxation oscillator name came from.

I especially drew a circuit of PUT relaxation oscillator for you to observe various elements such as resistor, capacitor, and UJT so let explain their functions in detail.

Back-to-back connected resistors R1 and R2 are the circuit’s external resistors, which determine the value of intrinsic standoff ratio (η) and peak voltage (Vp).

Rk is the limiting resistor that limits cathode current. R and C are useful to control the frequency of the oscillator.

working of a PUT relaxation oscillator

Let’s move toward the working part of the circuit when we apply the voltage E_dc as the positive plate of the capacitor is connected to supply capacitor starts charging through the resistor (R)as the capacitor is charging voltage across capacitor starts increasing and reaches up to peak voltage (Vp),

as soon as the voltage of the capacitor reached the peak voltage, the PUT goes into the negative resistance mode.

Due to this, a low resistance path is created for through anode to cathode is created. By using this path, the capacitor starts discharging.

And while discharging when the voltage across the capacitor decreases below the valley point, PUT reverses its initial condition, and there will be no more discharging path for the capacitor.

Thus, the capacitor starts charging again, and this cycle is repeated numerous times. The result of which sawtooth waveform is created. If you observe the waveform carefully, you see the charging and discharging pattern of the capacitor in the whole waveform.

In this way, by using PUT, we can generate a sawtooth waveform and can be used as a relaxation oscillator.