Download 6 – UJT Relaxation Oscillator

University of the Immaculate Conception
Engineering Program
ECE 514 – Industrial Electronics Laboratory
Laboratory Activity Report
Unijunction Transistor (UJT) Relaxation Oscillator
Title of the Laboratory Activity
Laboratory Activity No.6
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Date Performed
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Rating
Group No.: _________
Group Members:
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Engr. Aylmer Ronnel L. Sombilla
Instructor
Laboratory Activity No. 6
Unijunction Transistor (UJT) Relaxation Oscillator
Objectives:
1. To observe and graph the output waveforms of a UJT relaxation oscillator
circuit
2. To measure the peak voltage of the UJT in the relaxation oscillator circuit
3. To measure the output frequency of the relaxation oscillator circuit.
4. To observe the effect of increased operating temperature on the UJT’s
peak voltage and output frequency of the oscillator.
Theory:
The relaxation oscillator is the main circuit of most UJT timers and
oscillators. The UJT relaxation oscillator circuit is shown in fig. 1.
Fig. 1 – Relaxation Oscillator Circuit
When power is applied in this circuit, capacitor CE charges through RE.
When the voltage across CE reaches the peak voltage, the UJT fires and a
relatively large amount of current flows through R1. As a result of this current, a
voltage pulse is developed across R1. When the capacitor has fully discharged,
the voltage across it decreases and the UJT turns off again. This cycle of
capacitor charge and discharge is repeated as long as power is supplied to the
circuit. The oscillation frequency of this circuit is approximated by
f = 1 / (RECE).
In the formula above, the frequency of oscillation is dependent on the
values of RE and CE. If we increase their values, the frequency of oscillation
decreases. If we decrease their values, the frequency increases.
In general, the firing behavior of UJTs is more stable compared to SCRs.
This means that UJT is less dependent on temperature as compared to SCRs,
but still the effect of higher operating temperature is to decrease the peak
voltage, Vp and increase the output frequency.
Materials and Equipments:
1 unit DC power supply
1 unit Oscilloscope
1 unit analog multimeter
1 unit breadboard
1 unit soldering iron
1 pc. UJT 2N2646
1 pc. Electrolytic capacitor 100 uF, 16 volts
1 pc. Resistor 10 Kohms, ½ watt
1 pc. Resistor 100 ohms, ½ watt
1 pc. Resistor 20 ohms, ½ watt
Connecting wires per group
Procedure:
1. Connect the circuit of fig. 2.
Fig. 2 – UJT relaxation oscillator circuit
2. Measure the peak voltage Vp of the circuit. Connect a voltmeter across CE
and observe the charging and discharging of the capacitor. The voltmeter
reading will be fluctuating from a high reading to a low reading. Observe
closely the movement of the meter pointer and measure the maximum
voltage reached by the capacitor. Record your measurement below.
Vp = __________
3. Connect the oscilloscope probes across the capacitor CE, draw the output
waveform shown on the oscilloscope below. Measure the maximum voltage
reached by the capacitor Vp as shown on the oscilloscope display. Record
your result below
Vp = __________
4. Measure the output frequency of the signal of procedure 3 by using the
formula f = 1 / T, where T is the period of the signal as measured from the
oscilloscope display. Record your measurement below.
f = __________
5. Compute the frequency of the circuit using the formula f = 1 / RECE. Record
your result below.
f = __________
6. Place the oscilloscope probes across B2 terminal of the UJT and ground.
Draw the output waveform shown on the oscilloscope below.
7. Place the oscilloscope probes across resistor R1 and draw the output
waveform below.
8. Place the oscilloscope probes across the capacitor once again. Heat the
metal casing of the UJT with a soldering iron until you can observe that there
is a change in the frequency and peak voltage of the signal. What happens to
the frequency and Vp of the signal?
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Questions for discussion:
1. Compare your measurements of Vp in procedure 2 and 3. Are they equal
or close? Write your observations.
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2. Compare your measurements of the output frequency in procedures 4 and
5. Are they equal or close? Write your observations.
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3. Are the waveforms shown on the oscilloscope close to what you have
learned from the theory of the relaxation circuit? Write your observations
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4. What do you think will be the effect of increasing the value of the capacitor
to the frequency of the circuit? Explain
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5. Based from the activity, what is the effect of the temperature increase to
the frequency and Vp of the circuit? Is this what you expected based on
what you have learned from theory?
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