AN1007
Application Notes
Load could be here
instead of lower location
Rin
1
6
180
180
G
180 Ω for 120 V ac
360 Ω for 240 V ac
Vcc
R
in
6
5
1
2
3.3 k
MT2
MT1
240 V ac
5
4
Hot
2
3
MT2
MT1
Input
0.1 µF
100 Ω
G
120/240 V ac
4
0.047 µF
Load
3
Triac or
Alternistor
0.1µf
Neutral
Load
Figure AN1007.8
Optocoupler Circuit for Lower Current Inductive
Loads (Triac or Alternistor)
Figure AN1007.10 Random Turn-on Triac Driver
Select the triac for the voltage of the line being used, the current
through the load, and the type of load. Since the peak voltage of
a 120 V ac line is 170 V, you would choose a 200 V (MIN) device.
If the application is used in an electrically noisy industrial envi-
ronment, a 400 V device should be used. If the line voltage to be
controlled is 240 V ac with a peak voltage of 340 V, then use at
least a 400 V rated part or 600 V for more design margin. Selec-
tion of the voltage rating of the opto-driver must be the same or
higher than the rating of the power triac. In electrically noisy
industrial locations, the dv/dt rating of the opto-driver and the
triac must be considered.
In this circuit, the series gate resistors are increased to 180 Ω
each, since a 240 V line is applied. Note that the load is placed
on the MT1 side of the power triac to illustrate that load place-
ment is not important for the circuit to function properly.
Also note that with standard U.S. residential 240 V home wiring,
both sides of the line are hot with respect to ground (no neutral).
Therefore, for some 240 V line applications, it will be necessary
to have a triac switch circuit in both sides of the 240 V line input.
If an application requires back-to-back SCRs instead of a triac or
alternistor, the circuit shown in Figure AN1007.9 may be used.
The RMS current through the load and main terminals of the triac
should be approximately 70% of the maximum rating of the
device. However, a 40 A triac should not be chosen to control a
1 A load due to low latching and holding current requirements.
Remember that the case temperature of the triac must be main-
tained at or below the current versus temperature curve specified
on its data sheet. As with all semiconductors the lower the case
temperature the better the reliability. Opto-driven gates normally
do not use a sensitive gate triac. The opto-driver can supply up to
1 A gate pulses and less sensitive gate triacs have better dv/dt
capability. If the load is resistive, it is acceptable to use a stan-
dard triac. However, if the load is a heavy inductive type, then an
alternistor triac, or back-to-back SCRs as shown in Figure
AN1007.9, is recommended. A series RC snubber network may
or may not be necessary when using an alternistor triac. Nor-
mally a snubber network is not needed when using an alternistor
because of its high dv/dt and dv/dt(c) capabilities. However,
latching network as described in Figure AN1007.8 may be
needed for low current load variations.
100
120 V ac
1
2
6
G
V
cc
A
K
A
5
4
NS-
SCR
NS-SCR
R
in
G
K
100
0.1µF
3
Load
Figure AN1007.9
Optocoupled Circuit for Heavy-duty Inductive Loads
All application comments and recommendations for optocoupled
switches apply to this circuit. However, the snubber network can
be applied only across the SCRs as shown in the illustration. The
optocoupler should be chosen for best noise immunity. Also, the
voltage rating of the optocoupler output triac must be equal to or
greater than the voltage rating of SCRs.
Zero Crossing Turn-on, Normally Open
Relay Circuits
Summary of Random Turn-on Relays
When a power circuit is mechanically switched on and off
mechanically, generated high-frequency components are gener-
ated that can cause interference problems such as RFI. When
power is initially applied, a step function of voltage is applied to
the circuit which causes a shock excitation. Random switch
opening stops current off, again generating high frequencies. In
addition, abrupt current interruption in an inductive circuit can
lead to high induced-voltage transients.
The latching characteristics of thyristors are ideal for eliminating
interference problems due to current interruption since these
devices can only turn off when the on-state current approaches
zero, regardless of load power factor.
As shown in Figure AN1007.10, if the voltage across the load is
to be phase controlled, the input control circuitry must be syn-
chronized to the line frequency and the trigger pulses delayed
from zero crossing every half cycle. If the series gate resistor is
chosen to limit the peak current through the opto-driver to less
than 1 A, then on a 120 V ac line the peak voltage is 170 V;
therefore, the resistor is 180 Ω. On a 240 V ac line the peak volt-
age is 340 V; therefore, the resistor should be 360 Ω. These gate
pulses are only as long as the device takes to turn on (typically,
5 µs to 6 µs); therefore, 0.25 W resistor is adequate.
On the other hand, interference-free turn-on with thyristors
requires special trigger circuits. It has been proven experimen-
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AN1007 - 4
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Thyristor Product Catalog
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