Download Figure 6–1

TRIAC
• Triac was developed by the need to control the current in both
directions which a single SCR was not able to do. Before triacs,
two SCRs in paralle to accomplish this bidirectional task.
• The triac is three-terminal device used to control the average
current flow to a load.
• The triac, like SCR, is often used in an AC circuit to control the
power on load.
• Triac can carry current in either direction (positive and negative
directions),so theoratically it is possible for a triac to conduct for
a 360 per cycle with appropriate triggering. SCRs carry current
in positive direction only.
• A triac can operate in full-wave phase control circuits while an
SCR can operate in half-wave phase control circuits.
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Figure 6–1
(a) Schematic symbol and terminal names of a triac. (b) Triac circuit showing how the supply
voltage, the load, and the triac are connected.
Equivalent Circuit of the triac
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Operation Of the Triac
• The symbol of the triac is shown in Figure 6-1(a)
with the names of its terminals.
• When the triac is turned OFF, no current can flow
between the main terminals whatever the polarity of
the applied voltage. It acts as open switch.
• When the triac is turned ON, there is a very lowresistance current flow path between the main
terminals.
• The direction of the current flow will depend on the
polarity of applied voltage.
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• When the voltage is more positive on MT1 with
respect to MT2 the current will flow from MT1 to
MT2. If the voltage is more positive on MT2 with
respect to MT1 the current will flow from MT2 to
MT1.
• In Figure 6-1(b), the average current delivered to the
load can be varied by varying the amount of time per
cycle that the triac spends in its ON state.
• With proper triggering arrangement, triac can conduct
for a full 360o per cycle.
• Triacs (SCRs, Transistors) have no contact bounce,
no arcing across partially opened contacts and they
operate much faster (more precise control) than
mechanical switches.
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Triac Waveforms
(b)
(a)
Figure 6–2
Waveforms of triac main-terminal voltage and load
voltage for three different conditions. (a) Firing delay
equals 30 for both the positive half cycle and the
negative half cycle. (b) Firing delay equals 120 for
both half cycles. (c) Unequal firing delay angles for
the positive and negative half cycles. This is usually
undesirable.
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(c)
• Triac waveforms are very much like SCR waveforms except
that they can fire on the negative half cycle.
• In figure 6-2 (a), the triac is OFF during the first 30o(firing
delay angle) of each half cycle. During this time the entire
line voltage is dropped across the main terminals of the
triac, with no voltage applied to the load.
• After 30o has elapsed, the triac fires (turn ON) and it
becomes like a closed switch and start conducting current to
the load thru its main terminals for the remainder of the half
cycle (for the conduction angle).
• for the negative half cycle the triac will be turned ON after
210o=180o+30o and it will conduct current in the opposite
direction (from load to supply) for the remainder of the
negative half cycle.
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Electrical Characteristics of Triac
• Refer to Figure 6-2 (c), there is mismatch in the delay angles for the positive and
negative half cycles. This is a common problem for the triacs; some have tendency
to easily fire on the positive half cycle and some have the reverse tendency.
• This because IGT for a triac for the forward main terminal polarity (See figure -3)
may be quite a bit different from IGT for the reverse main terminal bias.
• For most medium sized triacs, VGT=0.6V—2V and IGT=0.1mA—20mA.
• Like SCR, Triac does not require continuous gate current once it has been fired. It
will remain ON until the bias is changed or its terminal current drops below IHO
(medium sized triac IHO=100mA).
Definition of some terms of a triac:See page# 243 of your text book
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Figure 6–3
(a) The situation when a triac has forward main terminal bias. Normally the gate current
and gate voltage would have the polarities indicated. (b) The situation at a different
point in time when the triac is reverse biased. Normally the gate current and voltage are
also reversed.
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Triggering Methods For Triacs
Figure 6–4
(a) Simple gate control circuit (triggering circuit) for a triac. Firing
delay is adjusted by potentiometer R2. (b) Improved gate control
circuit, which allows a wider range of adjustment of firing delay.
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Triggering Methods For Triacs
• RC Gate Control Circuits
– Fig.6-4(a) shows the simplest triggering circuit; C
charges thru R1, R2 during the delay angle of each
half cycle.
– When C is charged to a value enough to deliver
sufficient gate current (IGT) thru R3 the Triac will
fire.
– The charging rate of C is set by R2; big R2 will give
charging rate slow causing long firing delay (big
delay angle) and vice versa.
– To establish wider range of delay angle, the double
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RC network is used (Fig.6-4(b)).