Testthe effect of variation of R, C in UJT triggering technique.

 

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Lab Report: Effect of R and C Variation in UJT Triggering Technique for SCR

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1. Objective:

To study and test the effect of varying resistor (R) and capacitor (C) values on the triggering behavior of an SCR using a UJT (Uni-Junction Transistor) triggering circuit.

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2. Apparatus Required:

• SCR (e.g., TYN612)
• UJT (e.g., 2N2646)
• Resistors (1kΩ to 100kΩ)
• Capacitors (0.01µF to 1µF)
• Oscilloscope
• DC Power Supply (0–30V)
• AC Power Supply (0–230V)
• Load resistor
• Breadboard and connecting wires

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3. Theory:

UJT Triggering Technique:

• The UJT is used to generate sharp triggering pulses for SCR gate control.
• In the circuit, a capacitor charges through a resistor.
• When the capacitor voltage reaches the UJT's intrinsic standoff voltage ($V_p$), the UJT conducts, producing a sharp pulse to trigger the SCR.

Effect of R and C Variation:

• Resistance (R): Controls the charging time of the capacitor, affecting the pulse timing and firing angle ($\alpha$).
• Capacitance (C): Influences the rate of capacitor charging, thereby impacting the triggering frequency and angle.

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4. Circuit Diagram:

Provide a circuit diagram showing:

• UJT connected with a series resistor-capacitor network across the power supply
• Gate of SCR connected to UJT emitter
• AC load connected in series with SCR and AC supply.

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5. Procedure:

Step 1: Circuit Setup

1. Assemble the UJT triggering circuit as per the diagram.
2. Connect the SCR with the load and AC supply.
3. Connect the oscilloscope across the load and the gate-cathode terminals of the SCR.

Step 2: Testing for R Variation

1. Set a fixed value for the capacitor (e.g., 0.1µF).
2. Gradually vary the resistor value from 1kΩ to 100kΩ.
3. Observe the change in firing angle ($\alpha$) on the oscilloscope and record the values.

Step 3: Testing for C Variation

1. Set a fixed resistor value (e.g., 10kΩ).
2. Gradually vary the capacitance from 0.01µF to 1µF.
3. Observe the change in firing angle ($\alpha$) on the oscilloscope and record the values.

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6. Observations:

R (kΩ)	C (µF)	Firing Angle (α, degrees)
	0.01
	0.1
	1
10
50

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7. Results:

• Increasing resistance (R) resulted in a delayed pulse and a higher firing angle ($\alpha$).
• Increasing capacitance (C) increased the charging time, also delaying the triggering pulse and increasing the firing angle.

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8. Conclusion:

The experiment demonstrated that by varying R and C values in the UJT triggering circuit, the firing angle of the SCR can be precisely controlled. This technique is effective for phase control applications in power electronics.

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