Introduction to UJT:
A Uni Junction Transistor (UJT) is an electronic semiconductor device. It is made of a lightly doped (high resistance) silicon bar, which can either be an n-type or p- type. It is a three terminal device with characteristics very different from the conventional 2 junction, bipolar transistor. It is a pulse generator with the trigger or control signal applied at the emitter.
There are two types of Uni Junction Transistors:-
1. The original Uni Junction Transistor (UJT), is a simple device that is essentially a bar of N type semiconductor material into which P type material has been diffused somewhere along its length.
2. The Programmable Uni Junction Transistor (PUT), is a close cousin to the Thyristor. Like the Thyristor, it consists of four P-N layers and has an Anode and a Cathode connected to the first and the last layer, and a Gate connected to one of the inner layers. They are not directly interchangeable with conventional UJTs but perform a similar function.
For both types, their main use is as a trigger device for thyristors and as the active device in relaxation oscillators. The graph of emitter voltage against emitter current of a Uni Junction Transistor shows an area of negative resistance; this is what makes it very useful.
Tests:-
Test #1:-
1. Set a Digital Multimeter in the Ohms position.
2. Read the resistance between Base 1 and Base 2.
3. Read the resistance between Base 2 and Base 1 (reversing the leads).
Both of the readings should approximately be equal (high resistance).
Test #2:-
1. Connect the negative lead to the Emitter.
2. Measure the resistance from Emitter to Base 1 using the positive lead.
3. Measure the resistance from Emitter to Base 2 using the positive lead.
Both of the readings should approximately be equal (high resistance).
Test #3:-
1. Connect the positive lead to the Emitter.
2. Measure the resistance from Emitter to Base 1 using the negative lead.
3. Measure the resistance from Emitter to Base 2 using the negative lead.
Both of the readings should approximately be equal (low resistance).
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