S2006FS221V Teccor Electronics, Inc., S2006FS221V Datasheet - Page 136

• Manufacturer Part Number: S2006FS221V
• Description: Thyristor Product Catalog
• Manufacturer: Teccor Electronics, Inc.

AN1003
Figure AN1003.9
The hysteresis (snap back) effect is somewhat similar to the
action of a kerosene lantern. That is, when the control knob is
first rotated from the off condition, the lamp can be lit only at
some intermediate level of brightness, similar to turning up the
wick to light the lantern. Brightness can then be turned down until
it finally reaches the extinguishing point. If this occurs, the lamp
can only be relit by turning up the control knob again to the inter-
mediate level. Figure AN1003.10 illustrates the hysteresis effect
in capacitor-diac triggering. As R
mum resistance, the voltage across the capacitor increases until
the diac first fires at point A, at the end of a half-cycle (conduction
angle
drops suddenly to about half the triggering voltage, giving the
capacitor a different initial condition. The capacitor charges to the
diac, triggering voltage at point B in the next half-cycle and giving
a steady-state conduction angle shown as
Figure AN1003.10
In the Figure AN1003.11 illustration, the addition of a second RC
phase-shift network extends the range on control and reduces
the hysteresis effect to a negligible region. This circuit will control
from 5% to 95% of full load power, but is subject to supply volt-
age variations. When R
R
action provides additional range of phase-shift across C
enables C
thus reducing hysteresis. R
just drops out of conduction when R
resistance.
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3
AC Line
from the phase-shifted voltage appearing across C
(60 Hz)
120 V
i
). After the gate pulse, however, the capacitor voltage
Capacitor
Voltage
2
to partially recharge C
Load
Diac Triggers at "A"
0.1 µF
Basic Diac-Triac Phase Control
Relationship of AC Line Voltage and Triggering
Voltage
C 1
R 1
R 2
1
A
is large, C
250 k
3.3 k
3
should be adjusted so that the circuit
HT34B
Diac Does Not
Trigger at "A"
i
Diac
(Q2010L5)
1
B
Triac
1
is brought down from its maxi-
1
after the diac has triggered,
1
is charged primarily through
is brought to maximum
(For Inductive
for the triac.
Loads)
0.1 µF
100
2
. This
[+Diac V
[–Diac V
1
and
BO
BO
AN1003 - 4
]
]
Figure AN1003.11
By using one of the circuits shown in Figure AN1003.12, the hys-
teresis effect can be eliminated entirely. The circuit (a) resets the
timing capacitor to the same level after each positive half-cycle,
providing a uniform initial condition for the timing capacitor. This
circuit is useful only for resistive loads since the firing angle is not
symmetrical throughout the range. If symmetrical firing is
required, use the circuit (b) shown in Figure AN1003.12.
Figure AN1003.12
For more complex control functions, particularly closed loop con-
trols, the unijunction transistor may be used for the triggering
device in a ramp and pedestal type of firing circuit as shown in
Figure AN1003.13.
(60 Hz)
120 V
(a)
(b)
(60 Hz)
120 V
120 V
(60 Hz)
Load
R 1 = 250 k POT
R 2 , R 3 = 15 k, 1/2 W
D 1 , D 2 = 200 V Diodes
0.1 µF
Load
Load
Extended Range Full-wave Phase Control
Wide-range Hysteresis Free Phase Control
C 2
R 2
R 2
D 2
R 2
D 2
68 k
R 3
0.1 µF
15 k
1/2 W
100 k
Trim
D 1
C 1
R 4
R 1
R 4 = 3.3 k
D 1 , D 2 , D 3 , D 4 = 200 V Diodes
0.1 µF
D 1
0.1 µF
C 1
R 4
R 1
D 3
R 3
D 4
R 1
C 1
R 3
3.3 k
©2002 Teccor Electronics
250 k
Thyristor Product Catalog
3.3 k
250 k
HT34B
(Q2010L5)
Diac
Diac
Diac
Triac
(Q2010L5)
Triac
(Q2010L5)
Triac
Application Notes