
AN1003
Application Notes
http://www.teccor.com
+1 972-580-7777 
AN1003 - 4
2002 Teccor Electronics
Thyristor Product Catalog
Figure AN1003.9
Basic Diac-Triac Phase Control
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
1
 is brought down from its maxi-
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 
θ
i
). After the gate pulse, however, the capacitor voltage 
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 
θ
 for the triac.
Figure AN1003.10
Relationship of AC Line Voltage and Triggering 
Voltage
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
is large, C
 is charged primarily through 
R
3
 from the phase-shifted voltage appearing across C
2
. This 
action provides additional range of phase-shift across C
1
 and 
enables C
 to partially recharge C
 after the diac has triggered, 
thus reducing hysteresis. R
3
should be adjusted so that the circuit 
just drops out of conduction when R
1
is brought to maximum 
resistance.
Figure AN1003.11
Extended Range Full-wave Phase Control
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
Wide-range Hysteresis Free Phase Control
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.
Load
R1
0.C1
Triac
(Q2010L5)
250 k
3.3 k
R2
120 V
(60 Hz)
(For Inductive
Loads)
100
0.1 μF
Diac
HT34B
Diac Triggers at "A"
Diac Does Not
Trigger at "A"
θ
i
B
A
AC Line
Capacitor
Voltage
[+Diac V
BO
]
[–Diac V
BO
]
θ
R4
C1
Diac
HT34B
Triac
(Q2010L5)
68 k
3.3 k
R1
120 V
(60 Hz)
0.1 μF
Load
R2
250 k
R3
100 k
Trim
C2
0.1 μF
R3
C1
Diac
Triac
(Q2010L5)
15 k
1/2 W
3.3 k
R1
120 V
(60 Hz)
0.1 μF
Load
R2
250 k
D1
D2
D1, D2 = 200 V Diodes
(a)
(b)
C1
Diac
Triac
(Q2010L5)
R3
120 V
(60 Hz)
Load
D1
0.1 μF
R1 = 250 k POT
R2, R3 = 15 k, 1/2 W
D3
R4
R1
D4
R2
D2
R4 = 3.3 k
D1, D2, D3, D4 = 200 V Diodes