Download 10 – Testing a Triac with a Multimeter

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University of the Immaculate Conception
Engineering Program
ECE 514 – Industrial Electronics Laboratory
Laboratory Activity Report
Testing a Triac with a Multimeter
Title of the Laboratory Activity
Laboratory Activity No. 10
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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. 10
Testing a Triac with a Multimeter
Objectives:
1. To demonstrate the normal resistance readings of a triac
2. To demonstrate the forward main terminal bias and reverse main terminal
bias triggering of a triac using a multimeter
Theory:
Triacs are bidirectional thyristor devices that permit the flow of electric
current in the forward and reverse directions. A single triac can be represented
by two SCRs connected in antiparallel with the gates connected together. Just
like the SCR, the triac has three terminals, but instead of a cathode, anode, and
gate, the triac terminals are designated as Main Terminal 1 (MT1), Main Terminal
2 (MT2), and gate (G). The schematic symbol of a triac is shown with its SCR
equivalent circuit in fig.1.
Fig. 1 – Schematic symbol for a triac and its SCR equivalent circuit
Triac bias voltages are specified with respect to terminal MT1. A triac can
be turned on by either a positive or negative pulse in the gate terminal depending
on the polarity of the voltage across MT1 and MT2. If MT2 is more positive than
MT1, the triac can be triggered by a positive pulse in the gate terminal (i.e. gate
is more positive than MT1). This type of bias is called the Forward Main Terminal
Bias. If MT2 is more negative than MT1, the triac can be triggered by a negative
pulse in the gate terminal (i.e. gate is more negative than MT1. This type of bias
is called the Reverse Main Terminal Bias.
The ohmmeter function of a multimeter can be used to show the normal
resistance readings of a triac as well as the latching behavior of the triac in the
forward and reverse main terminal biases.
Materials and Equipments:
1 unit
1 pc.
Analog multimeter
Triac Q4004L4
Procedure:
1. Set the multimeter to x 1 ohm range. Measure the resistance between MT1
and MT2 as shown in fig. 2. Record your result below.
RMT1-MT2 = __________ (reading should be close to ∞)
Fig. 2 – Measuring RMT1-MT2
2. Reverse the leads and measure the resistance once again between the same
terminals. Record your result below.
RMT1-MT2 (reversed) = __________ (reading should be close to ∞)
3. Measure the resistance between MT2 and G as shown in fig. 3. Record your
result below.
RMT2-G = __________ (reading should be closed to ∞)
Fig. 3 – Measuring RMT2-G
4. Reverse the leads and measure the resistance once again between the same
terminals. Record your result below.
RMT2-G (reversed) = __________ (reading should be closed to ∞)
5. Measure the resistance between MT1 and G as shown in fig. 4. Record your
result below.
RMT1-G = __________ (reading should be low)
Fig. 4 – Measuring RMT1-G
6. Reverse the leads and measure the resistance once again between the same
terminals. Record your result below.
RMT1-G (reversed) = __________ (reading should be low)
7. Check the triac if it latches or triggers under forward main terminal bias. Place
the black lead on MT2 and red lead on MT1. The resistance reading should
be the same as the reading of procedure 1. Now, without removing the black
lead on MT2, short the MT2 and G terminals. The resistance reading should
change to a low value. The resistance should stay low even if the black lead
is removed from G but still in contact with MT2.
8. Check the triac if it latches or triggers under reverse main terminal bias. Place
the red lead on MT2 and black lead on MT1. The resistance reading should
be the same as the reading of procedure 2. Now, without removing the red
lead on MT2, short the MT2 and G terminals. The resistance reading should
change to a low value. The resistance should stay low even if the red lead is
removed from G but still in contact with MT2.
Questions for discussion:
1. Looking at the SCR equivalent circuit of the triac, why is the resistance
across the MT1 and MT2 terminals always high even if the test leads are
reversed?
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2. Looking at the SCR equivalent circuit of the triac, why is the resistance
across the MT2 and G terminals always high even if the test leads are
reversed?
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3. Looking at the SCR equivalent circuit of the triac, why is the resistance
across the MT1 and G terminals always low even if the test leads are
reversed?
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4. If the triac did not latch in procedure 7 and 8, can we conclude right away
that the device is defective? Explain.
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