As charging continues, V
r
is eventually high enough
to turn the Zener on,
which then fires SCR
2
.
Once SCR
2
is on, the voltage divider network of R
2
and
R
3
will result in a level at V
2
which is too small to turn
SCR
1
on. Thus the regulator recharges the battery
whenever its voltage drops and prevents overcharging
when it is fully charged.

Thyristors
31
•
SCRs are unidirectional devices since current can flow
only in one direction.
•
Diacs and triacs are bidirectional thyristors, where
current can flow in either direction.
•
A diac has two terminals (anode
1
and anode
2
), whilst a
triac has a third terminal (gate).
•
The triac can be viewed as a bidirectional SCR.
•
Most common application for the diac is as a triggering
device for a triac in AC control circuits.
Bidirectional Thyristors

A
1
A
1
Thyristors
32
A
2
Schematic symbol
Equivalent circuit
A
2
The Diac

A
1
A
2
A
1
A
2
Q
1
and Q
2
forward-
biased, Q
3
and Q
4
reverse-biased.
Q
1
and Q
2
reverse-
biased, Q
3
and Q
4
forward-biased.
I
I
Thyristors
33
The Diac
Two possible bias conditions

Thyristors
34
The Diac
V-I Characteristic

Schematic symbol
Equivalent circuit
Thyristors
35
The triac is basically a diac with a gate terminal for
controlling the turn-on conditions in either direction.
Equivalent to a bidirectional SCR.
The Triac

A
1
I
A
1
A
1
A
1
A
2
A
2
A
2
A
2
G
G
G
G
I
36
Thyristors
The Triac

A
2
Thyristors
37
A
2
A
2
A
2
A
2
Triac operating curve
The Triac
V-I Characteristic

Thyristors
38
Operation in quadrant I is identical to the operation of
the SCR.
Operation in quadrant III is simply a reflection of
quadrant I operation.
The triac can be triggered into conduction by either:
1.
Applying a negative gate trigger while A
2
is positive
(quadrant II) or
2.
Applying a positive gate trigger while A
2
is negative
(quadrant IV).
The Triac
Opration regions

V
in
A
2
Basic triac phase
control
A
1
R
L
V
in
V
G
I
L
triac on
I
L
Thyristors
39
The Triac
Phase control

Triac phase control circuit
Thyristors
40
V
in
D
1
D
2
R
1
R
L
A
1
A
2
D
1
conducts during the
positive half-cycle of V
in
.
A
1
and G are positive with
respect to A
2
.
D
2
conducts during the
negative half-cycle of V
in
.
A
2
and G are positive with
respect to A
1
.
R
1
sets the trigger point for
both cycles.
G
The Triac
Phase control

3.36
A
.
Thyristors
41
R
L
V
in
V
triac
I
L
Let V
in
= 75 V, R = 82
k
Ω
, C = 1
μ
F, and R
L
=
22
Ω
.
Example 1:
If the triac has fired,
what is the current
through R
L
, assuming the
voltage across the triac is
1 V?
The Triac
Phase control

Thyristors
42
Let V
in
= 75 V, R = 82 k
Ω
,
C = 1
μ
F, and R
L
= 22
Ω
.
Example 2:
The diac has a breakover
voltage of 32 V. If the triac
has a trigger voltage of 1 V
and a trigger current of 10
mA, what is the capacitor
voltage that triggers the
triac?
V
BO
(
diac
)
V
GT
(
triac
)
33
V
.
The Triac
Phase control

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