UJT Pulse Generation in Motor Control

In motor speed control circuits utilizing a TRIAC and a Unijunction Transistor (UJT), the UJT typically functions as a relaxation oscillator to generate triggering pulses for the TRIAC.

Potentiometer Role in UJT Oscillator

A UJT relaxation oscillator circuit involves a capacitor charging through a resistor until it reaches the UJT's peak voltage ($V_P$). Once $V_P$ is reached, the UJT fires, allowing the capacitor to discharge rapidly and produce a pulse.

The charging process is governed by an RC time constant. A potentiometer, being a variable resistor, is often used as part of this charging resistance ($R$).

By adjusting the potentiometer's resistance, the rate at which the capacitor charges is changed:

• Increasing resistance ($R$) slows down capacitor charging, increasing the time it takes to reach $V_P$. This leads to a lower frequency of generated pulses.
• Decreasing resistance ($R$) speeds up capacitor charging, reducing the time to reach $V_P$. This results in a higher frequency of generated pulses.

Therefore, the potentiometer directly controls the frequency of the pulses generated by the UJT.

Pulse Control of TRIAC

These UJT-generated pulses are used to trigger the TRIAC gate. The timing (frequency) of these pulses determines the phase angle at which the TRIAC conducts the AC power to the motor, thus controlling the motor's speed.

Conclusion: The potentiometer's primary function in this setup is to vary the charging rate of the capacitor in the UJT oscillator, thereby controlling the pulse frequency fed to the TRIAC.