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Littelfuse - Thyristors

Numéro de référence AN1003
Description Thyristors
Fabricant Littelfuse 
Logo Littelfuse 





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AN1003 fiche technique
Teccor® brand Thyristors
AN1003
Phase Control Using Thyristors
Introduction
Due to high-volume production techniques, Thyristors
are now priced so that almost any electrical product can
benefit from electronic 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
control. The Thyristor is held in the off condition -- that is, all
current flow in the circuit is blocked by the Thyristor except
a minute leakage 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.
Line
Control
Circuit
Load
Two SCR AC Control
Line
Control
Circuit
Load
Triac AC Control
Line
Line
Control
Circuit
Control
Circuit
Load
Load
One SCR DC Control
Two SCR DC Control
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of Phase Control
Figure AN1003.2 illustrates voltage waveform and shows
common 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.
Full-wave Rectified Operation
Voltage Applied to Load
Delay (Triggering) Angle
Conduction Angle
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 voltage characteristics are plotted against
conduction angle for half- and full-wave phase control
circuits in Figure AN1003.3 and Figure AN1003.4.
HALF WAVE

1.8
1.6
Peak Voltage
1.4
1.2
1.0
RMS
0.8
Power
0.6
0.4
0.2
0
0
AVG
20 40 60
80 100 120 140 160 180
Conduction Angle ()
Figure AN1003.3 Half-Wave Phase Control (Sinusoidal)
©2008 Littelfuse, Inc.
Specifications are subject to change without notice.
Please refer to http://www.littelfuse.com for current information.
Phase Control Using Thyristors
http://www.Datasheet4U.com

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