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AN1003 ©2004 Littelfuse, Inc.
AN1003 - 1
http://www.littelfuse.com
Thyristor Product Catalog
+1 972-580-7777
AN10039
Phase Control Using Thyristors
Introduction
Due to high-volume production techniques, thyristors are now
priced so that almost any electrical product can benefit from elec-
tronic control. A look at the fundamentals of SCR and triac phase
controls shows how this is possible.
Output Power Characteristics
Phase control is the most common form of thyristor power con-
trol. The thyristor is held in the off condition that is, all current
flow in the circuit is blocked by the thyristor except a minute leak-
age current. Then the thyristor is triggered into an on condition
by the control circuitry.
For full-wave AC control, a single triac or two SCRs connected in
inverse parallel may be used. One of two methods may be used
for full-wave DC control a bridge rectifier formed by two SCRs
or an SCR placed in series with a diode bridge as shown in
Figure AN1003.1.
Figure AN1003.1
SCR/Triac Connections for Various Methods of
Phase Control
Figure AN1003.2 illustrates voltage waveform and shows com-
mon terms used to describe thyristor operation. Delay angle is
the time during which the thyristor blocks the line voltage. The
conduction angle is the time during which the thyristor is on.
It is important to note that the circuit current is determined by the
load and power source. For simplification, assume the load is
resistive; that is, both the voltage and current waveforms are
identical.
Figure AN1003.2
Sine Wave Showing Principles of Phase Control
Different loads respond to different characteristics of the AC
waveform. For example, some are sensitive to average voltage,
some to RMS voltage, and others to peak voltage. Various volt-
age characteristics are plotted against conduction angle for
half- and full-wave phase control circuits in Figure AN1003.3
and Figure AN1003.4.
Control
Circuit
Line
Load
Two SCR AC Control
Control
Circuit
Triac AC Control
Line
Load
Control
Circuit
One SCR DC Control
Control
Circuit
Line
Line
Load
Two SCR DC Control
Load
Full-wave Rectified Operation
Voltage Applied to Load
Delay (Triggering) Angle
Conduction Angle
AN1003 AN1003
Application Notes
http://www.littelfuse.com
AN1003 - 2
©2004 Littelfuse, Inc.
+1 972-580-7777
Thyristor Product Catalog
Figure AN1003.3
Half-Wave Phase Control (Sinusoidal)
Figure AN1003.4
Symmetrical Full-Wave Phase Control (Sinusoidal)
Figure AN1003.3 and Figure AN1003.4 also show the relative
power curve for constant impedance loads such as heaters.
Because the relative impedance of incandescent lamps and
motors change with applied voltage, they do not follow this curve
precisely. To use the curves, find the full-wave rated power of the
load, and then multiply by the ratio associated with the specific
phase angle. Thus, a 180° conduction angle in a half-wave circuit
provides 0.5 x full-wave conduction power.
In a full-wave circuit, a conduction angle of 150° provides 97%
full power while a conduction angle of 30° provides only 3% of full
power control. Therefore, it is usually pointless to obtain conduc-
tion angles less than 30° or greater than 150°
.
Figure AN1003.5 and Figure AN1003.6 give convenient direct
output voltage readings for 115 V/230 V input voltage. These
curves also apply to current in a resistive circuit.
Figure AN1003.5
Output Voltage of Half-wave Phase
Figure AN1003.6
Output Voltage of Full-wave Phase Control
Peak Voltage
RMS
AVG
Power
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
0 20 40 60 80 100
120
140
160
180
Conduction Angle ( )
Normalized Sine Wave RMS Voltage Power
as Fraction of Full Conduction
HALF WAVE Peak Voltage
RMS
1.8
1.6
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0
0 20 40 60 80 100
120
140
160
180
Conduction Angle (
)
Normal Sine Wave RMS Voltage Power
as Fraction of Full Conduction
FULL WAVE
Power
AVG
Peak Voltage
180
160
140
120
100
80
60
40
20
0 0 20 40 60 80 100 120 140 160 180
Conduction Angle (
)
RMS
AVG
Output Voltage
360
320
280
240
200
160
120
80
40
0
Input
Voltage
230 V 115 V
HALF WAVE Peak Voltage
RMS
0 20 40 60 80 100
120
140
160
180
Conduction Angle (
)
AVG
Output Voltage
360
320
280
240
200
160
120
80
40
0
Input
Voltage
230 V 115 V
180
160
140
120
100
80
60
40
20
0
FULL WAVE Application Notes
AN1003
©2004 Littelfuse, Inc.
AN1003 - 3
http://www.littelfuse.com
Thyristor Product Catalog
+1 972-580-7777
Control Characteristics
A relaxation oscillator is the simplest and most common control
circuit for phase control. Figure AN1003.7 illustrates this circuit
as it would be used with a thyristor. Turn-on of the thyristor
occurs when the capacitor is charged through the resistor from a
voltage or current source until the breakover voltage of the
switching device is reached. Then, the switching device changes
to its on state, and the capacitor is discharged through the thyris-
tor gate. Trigger devices used are neon bulbs, unijunction tran-
sistors, and three-, four-, or five-layer semiconductor trigger
devices. Phase control of the output waveform is obtained by
varying the RC time constant of the charging circuit so the trigger
device breakdown occurs at different phase angles within the
controlled half or full cycle.
Figure AN1003.7
Relaxation Oscillator Thyristor Trigger Circuit
Figure AN1003.8 shows the capacitor voltage-time characteristic
if the relaxation oscillator is to be operated from a pure DC
source.
Figure AN1003.8
Capacitor Charging from DC Source
Usually, the design starting point is the selection of a capacitance
value which will reliably trigger the thyristor when the capaci-
tance is discharged. Trigger devices and thyristor gate triggering
characteristics play a part in the selection. All the device charac-
teristics are not always completely specified in applications, so
experimental determination is sometimes needed.
Upon final selection of the capacitor, the curve shown in Figure
AN1003.8 can be used in determining the charging resistance
needed to obtain the desired control characteristics.
Many circuits begin each half-cycle with the capacitor voltage at
or near zero. However, most circuits leave a relatively large
residual voltage on the capacitor after discharge. Therefore, the
charging resistor must be determined on the basis of additional
charge necessary to raise the capacitor to trigger potential.
For example, assume that we want to trigger an S2010L SCR
with a 32 V trigger diac. A 0.1 µF capacitor will supply the neces-
sary SCR gate current with the trigger diac. Assume a 50 V dc
power supply, 30° minimum conduction angle, and 150
°
maxi-
mum conduction angle with a 60 Hz input power source. At
approximately 32 V, the diac triggers leaving 0.66 V
BO
of diac
voltage on the capacitor. In order for diac to trigger, 22 V must be
added to the capacitor potential, and 40 V additional (50-10) are
available. The capacitor must be charged to 22/40 or 0.55 of the
available charging voltage in the desired time. Looking at Figure
AN1003.8, 0.55 of charging voltage represents 0.8 time constant.
The 30° conduction angle required that the firing pulse be
delayed 150° or 6.92 ms. (The period of 1/2 cycle at 60 Hz is
8.33 ms.) To obtain this time delay:
6.92 ms = 0.8 RC
RC = 8.68 ms
if C = 0.10 µF
then,
To obtain the minimum R (150° conduction angle), the delay is
30° or
(30/180) x 8.33 = 1.39 ms
1.39 ms = 0.8 RC
RC = 1.74 ms
Using practical values, a 100 k potentiometer with up to 17 k min-
imum (residual) resistance should be used. Similar calculations
using conduction angles between the maximum and minimum
values will give control resistance versus power characteristic of
this circuit.
Triac Phase Control
The basic full-wave triac phase control circuit shown in
Figure AN1003.9 requires only four components. Adjustable
resistor R
1
and C
1
are a single-element phase-shift network.
When the voltage across C
1
reaches breakover voltage (V
BO
) of
the diac, C
1
is partially discharged by the diac into the triac gate.
The triac is then triggered into the conduction mode for the
remainder of that half-cycle. In this circuit, triggering is in Quad-
rants I and III. The unique simplicity of this circuit makes it suit-
able for applications with small control range.
Switching
Device
Voltage
or
Current
Source
Triac
R
C
SCR
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0
1
2
3
4
5
6
Time Constants
Ratio of
(
Capacitor Voltage
Supply Source Voltage
)
R
8.68
3 ×10
0.1
6 ×10
--------------------------
86,000 =
=
R
1.74
3 ×10
0.1
6 ×10
---------------------------
17,400 =
= AN1003
Application Notes
http://www.littelfuse.com
AN1003 - 4
©2004 Littelfuse, Inc.
+1 972-580-7777
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