UJT Triggering Circuit

Spread the love

This blog post summarize UJT Triggering Circuit

UJT Triggering

It includes UJT Triggering SCR (Silicon Controlled Rectifier), Load resistor.

unijunction-transistor.jpg (116×87)

unijunction transistor (UJT) is an electronic semiconductor device that has only one junction. The UJT has three terminals: an emitter (E) and two bases (B1 and B2). The base is formed by lightly doped n-type bar of silicon. Two ohmic contacts B1 and B2 are attached at its ends. The emitter is of p-type and it is heavily doped. The resistance between B1 and B2, when the emitter is open-circuit is called interbase resistance.

interbase-resistance.jpg (159×224)

Working of Capacitor

Initially the capacitor charges through R whose voltage is applied to the emitter of UJT. When the capacitor voltage reaches peak point voltage of UJT. the UJT will switch to on condition. Now the capacitor discharges through the output resistance. Thus the pulse is generated in the circuit.

Lab Experiment


To Study and verify UJT Triggering circuit of SCR.

Apparatus required

UJT Triggering circuit kit

☞Patch cards

☞Power card


☞60W Bulb

Circuit Diagram for UJT Triggering Circuit

UJT Triggering Circuit

Procedure of UJT Triggering Circuit

CRO Settings

☞Time (X) Axis : 5ms/div

☞Voltage (Y) Axis : 20V/div

☞Probe : 1:10 CRO Probe is suitable one.

Initial Settings

☞Ensure the 230V Power Supply is in proper with Tester.

☞Ensure the CRO is working properly with probe checking and proper ground line axis.

Mimic Diagram for UJT Triggering circuit

mimic-diagram-for-r-triggering.jpg (569×473)

Experiment Steps

☞The connections are made as per the circuit diagram given above.

☞The R Load must be 60W bulb.

☞When the load is Rheostat, then it must be in maximum position.

☞Turn on the SW2 and observe the waveform across between G1&K1

☞Turn ON SW1 and observe the waveform across the R load.

☞Measure the X-axis time interval of ON time of Load Voltage and OFF Time of Load Voltage.

☞Adjust the value of Firing angle and note down the load voltage.

☞Repeat the step 6.

☞Repeat the experiment for different values of α and note down Vo.


Model Graph

model-graph-for-ujt-triggering-circuit.jpg (322×307)

Total Time Period


The Average Output Voltage


Leave a Reply

Your email address will not be published. Required fields are marked *

eleven + 6 =

This site uses Akismet to reduce spam. Learn how your comment data is processed.