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CIRCUIT OPERATION
THYRISTOR
A thyristor is a controlled silicon diode which is not conductive in
the reversed direction. It will only conduct in the forward direction
when they are triggered by short pulse or steady voltage applied
between the gate and cathode terminals (see Figure 8).
A thyristor family of semiconductors consists of several useful
devices. The most commonly used are silicon-controlled
rectifiers (SCR), triacs, and diacs. They can be thought of as a
solid-state switch with three or more PN junctions.
TRIAC
The block construction of a triac is shown in Figure 9. The triac
is like two SCRs connected in parallel in the opposite direction.
The construction of the triac allows it to conduct in either
polarity. The triac has only one gate that can be triggered by
either polarity. The main function is to control power bilaterally
in an AC circuit.
DIAC
The block construction of a diac or bi-directional diode is shown
in Figure 10. The diac will not conduct in either direction until its
“breakover voltage” (V
BO
) is exceeded. Breakover points range
from 20-36 volt. When this accrues, the device will conduct until
the voltage across its terminals is below the “breakback voltage”
(V
BB
) typical 6V.
CIRCUIT OPERATION
The circuit in Figure 11 is a basic full-wave triac phase control
circuit. The variable resistor VR1 and capacitor C1 are a
single-element phase shift network. When the voltage across
C1 reaches break-over voltage of the diac D3, C1 is then
partially discharged by the diac into the triac gate. The triac is
then triggered (turned on) and conducts for the remainder of
the half-cycle. The problem with this circuit is hysteresis, or
snap back effect. The circuit will not operate until the resistor
VR1 is turned up to an intermediate point. As the resistance of
VR1 is decreased, the voltage across the capacitor C1
increases until the diac first fires at point A, the end of the half
cycle. After the gate is triggered the capacitor voltage drops
suddenly to approximately half the trigger voltage, causing a
different initial condition. The capacitor charges to the diac
trigger voltage at point B in the next half cycle.
The addition of resistor R1 and diodes D1 and D2 in Figure 12
will eliminate the hysteresis problem. The additional parts reset
the timing capacitor to the same level after each positive half
cycle. This provides a uniform initial condition for the timing
capacitor.
Figure 8
Figure 12
Figure 10
Figure 11
Figure 9
P N P N
Anode
Cathode
Gate
Cathode
Anode
Gate
N P N
MT1
MT2
MT2
MT1
MT1
MT2
N
N
N
MT1
MT2
Gate
N
P
P
Current
Flow
Current
Flow
Gate
Current
Flow
SCR
Triac
Diac
Triac
Diac
C1
.082
μ
F
120V
(60Hz)
VR1
250k
Ω
Triac
D3
Diac
C1
.082
μ
F
120V
(60Hz)
VR1
250k
D2
D1
R1
15k
Ω
Load
Black
