Download 7 – UJT Relay Time-delay Circuit

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University of the Immaculate Conception
Engineering Program
ECE 514 – Industrial Electronics Laboratory
Laboratory Activity Report
UJT Relay Time – delay Circuit
Title of the Laboratory Activity
Laboratory Activity No.7
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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. 7
UJT Relay Time – delay Circuit
Objectives:
1. To observe the operation of a UJT relay time-delay circuit
2. To measure the maximum and minimum time delay of the circuit and
compare them with the calculated values.
3. To observe what happens to the circuit if the relay is not allowed to latch.
Theory:
A UJT relay time-delay circuit is an example of the application of a UJT
relaxation oscillator. The circuit shown in fig. 1 provides an adjustable time delay
in energizing the relay. In this circuit, power is applied to the load when the 12 V
relay is energized. This occurs after a certain no. of seconds has lapsed after S1
is closed. The length of the time delay can be varied by adjusting the 1Megaohm
potentiometer.
When S1 is closed and 20 V is applied to the top of the 820 - ohm resistor,
a small amount of current flows in the relay coil. The value of the 820 – ohm
resistor is chosen so that the current flowing through it is not large enough to pick
the relay, but large enough to keep the relay coil energized once it has already
been picked up. This is possible because the holding current for a relay coil is
generally less than the pick-up current. The holding current is the minimum
current required to flow through the coil so that the relay contact will stay closed
once it is picked up. The pick-up current is the current required to flow through
the coil so that the relay armature will move and switch the contact. The 22 uF
capacitor charges through the 22 K resistor and the 1 Megaohm potentiometer at
a rate determined by the setting of the potentiometer. When the capacitor
reaches the peak voltage of the UJT, the UJT fires and allows an additional
amount of current to flow through the relay coil. This current is enough to
energize the coil, allowing the relay contacts to close. The current pulse stops
almost immediately, but now the current through the 820 ohm resistor is sufficient
to hold the contacts in the closed position applying power to the load.
If we look at the circuit of fig. 1 closely, this circuit is just an adjustable UJT
relaxation oscillator with additional circuits for the 220 V lamp. If we get rid of the
820 - ohm resistor in the circuit, this will not allow the relay to latch (i.e. stay
closed once it has been picked-up). If this is the case, then our circuit will be
purely just a relaxation oscillator delivering pulses and controlling a 220 V lamp
as load a through the 12 V relay.
Materials and Equipments:
1 unit DC power supply
1 unit analog multimeter
1 unit breadboard
1 pc. UJT 2N2646
1 pc. Electrolytic capacitor 22 uF, 25 volts
1 pc. Resistor 820 ohms, ½ watt
1 pc. Resistor 680 ohms, ½ watt
1 pc. Resistor 1 Megaohm, ½ watt
1 pc. Resistor 22 Kohms, ½ watt
1 pc. Potentiometer, 1 Megaohm
1 pc. Relay 12 V, 10 Amps
1 pc. lamp or light bulb, 220V
1 pc. Toggle switch
1 pc. AC power cord
Connecting wires
Procedure:
1. Connect the circuit of fig. 1 below.
Fig. 1 – UJT Relay Time - delay circuit
2. Adjust the 1 Megaohm potentiometer so that it is at maximum value. Turn on
the power in the circuit by closing the switch S1. Measure the minimum time
delay of the circuit by using a stop watch. The minimum time delay can be
measured by starting the stop watch at the same time that the switch S1 is
closed and stopping the stop watch at the same time the lamp turns on.
Record your result below.
tdelay (min) = __________
(measured)
3. Calculate the minimum time delay by using the formula below, and record
your result. ( RE = 22K, CE = 22 uF)
tdelay (min) = RECE
tdelay (min) = __________
(calculated)
4. Turn off the power in the circuit by opening S1. Make sure that the 22 uF
capacitor has completely discharged by shorting its terminals with a wire.
Adjust the 1 Megaohm potentiometer so that it is at maximum value.
5. Connect an analog voltmeter across the capacitor and turn on the power in
the circuit. Now, adjust the potentiometer in the opposite direction (toward its
minimum) very slowly and stop turning its shaft the moment the lamp turns
on. Do not change the setting of the potentiometer once the lamp has turned
on. (You can see that the voltage across the capacitor is slowly rising by
observing the analog voltmeter and it also decreases drastically as the UJT
fires.)
6. Turn off the power in the circuit by opening S1. Disconnect the 1Megaohm
potentiometer from the circuit and measure its resistance as set in procedure
5. (Be careful to measure the correct resistance since the potentiometer has
three terminals.) Record your measurement below.
Rmax = __________
7. Without changing the potentiometer setting of procedure 5 and 6, connect it
back to the circuit. Turn on the power once again and measure the maximum
time delay as in procedure 2 by using a stop watch. Record your
measurement below.
tdelay (max) = __________
(measured)
8. Calculate the maximum time delay by using the formula below and record
your result. (RT = 22 K + Rmax, Rmax is the value measured in procedure 6)
tdelay (max) = RTCE
tdelay (max) = __________
(calculated)
9. Turn off the power and remove the 820 ohm resistor from the circuit. Set the
potentiometer to its minimum value. Turn on the power once again and
observe what happens to circuit. Write your observations below. (We have
not allowed the relay to latch this time since we removed the 820 ohm
resistor.)
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Questions for discussion:
1. Explain the purpose / function of the 820 ohm resistor in the circuit.
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2. In procedure 8, explain why the circuit behaved as observed when the 820
ohm resistor was removed.
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3. Compare the values of the minimum and maximum time delays (computed
and measured values). Are they close to each other? Write your
observations.
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4. If the 820 ohm resistor in the circuit is replaced by a much smaller value,
390 ohms for example, what do you think will be the effect on the circuit?
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5. If the 22 uF capacitor is replaced by a capacitor of larger value, what
would be the effect on the minimum and maximum time delay values?
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6. If the 22 uF capacitor is replaced by a capacitor of lesser value, what
would be the effect on the minimum and maximum time delay values?
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