K0900E70 Datasheet, PDF (173/224 Pages) Teccor Electronics – Thyristor Product Catalog

AN1007
7
Thyristors Used as AC Static Switches and Relays
Introduction
Since the SCR and the triac are bistable devices, one of their
broad areas of application is in the realm of signal and power
switching. This application note describes circuits in which these
thyristors are used to perform simple switching functions of a
general type that might also be performed non-statically by vari-
ous mechanical and electromechanical switches. In these appli-
cations, the thyristors are used to open or close a circuit
completely, as opposed to applications in which they are used to
control the magnitude of average voltage or energy being deliv-
ered to a load. These latter types of applications are described in
detail in “Phase Control Using Thyristors” (AN1003).
Static AC Switches
Normally Open Circuit
The circuit shown in Figure AN1007.1 provides random (any-
where in half-cycle), fast turn-on (<10 µs) of AC power loads and
is ideal for applications with a high-duty cycle. It eliminates com-
pletely the contact sticking, bounce, and wear associated with
conventional electromechanical relays, contactors, and so on. As
a substitute for control relays, thyristors can overcome the differ-
ential problem; that is, the spread in current or voltage between
pickup and dropout because thyristors effectively drop out every
half cycle. Also, providing resistor R₁ is chosen correctly, the cir-
cuits are operable over a much wider voltage range than is a
comparable relay. Resistor R₁ is provided to limit gate current
(I_GTM) peaks. Its resistance plus any contact resistance (R_C) of the
control device and load resistance (R_L) should be just greater
than the peak supply voltage divided by the peak gate current
rating of the triac. If R₁ is set too high, the triacs may not trigger
at the beginning of each cycle, and phase control of the load will
result with consequent loss of load voltage and waveform distor-
tion. For inductive loads, an RC snubber circuit, as shown in Fig-
ure AN1007.1, is required. However, a snubber circuit is not
required when an alternistor is used.
Figure AN1007.2 illustrates an analysis to better understand a
typical static switch circuit. The circuit operation occurs when
switch S₁ is closed, since the triac Q₁ will initially be in the block-
ing condition. Current flow will be through load R_L, S₁, R₁, and
gate to MT1 junction of the thyristor. When this current reaches
the required value of I_GT, the MT2 to MT1 junctions will switch to
the conduction state and the voltage from MT2 to MT1 will be V_T.
As the current approaches the zero crossing, the load current will
fall below holding current turning the triac Q₁ device off until it is
refired in the next half cycle. Figure AN1007.3 illustrates the volt-
age waveform appearing across the MT2 to MT1 terminals of Q₁.
Note that the maximum peak value of current which S₁ will carry
would be 25 mA since Q₁ has a 25 mA maximum I_GT rating. Addi-
tionally, no arcing of a current value greater than 25 mA when
opening S₁ will occur when controlling an inductive load. It is
important also to note that the triac Q₁ is operating in Quadrants I
and III, the more sensitive and most suitable gating modes for tri-
acs. The voltage rating of S₁ (mechanical switch or reed switch)
must be equivalent to or greater than line voltage applied.
Load
R_L
V_RMS
S₁
Control
Device
Reed
Switch
R₁
100 Ω
R₂
100 Ω
Triac
For
Inductive
Loads
C₁
0.1 µF
R₁ ≥
√2•V
I_GTM
(R_L + R_C) Where I_GTM is Peak Gate Current
Rating of Triac
Figure AN1007.1 Basic Triac Static Switch
AC Voltage Input
120 V rms, 60 Hz
V_IN
Load
R_L
Q₁
S₁
Q2008L4
+ I _GT
- I GT
G
R₁
V _GT
MT2
MT1
Figure AN1007.2 Analysis of Static Switch
©2002 Teccor Electronics
Thyristor Product Catalog
AN1007 - 1
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