
Application Notes
AN1007
2002 Teccor Electronics
Thyristor Product Catalog
AN1007 - 3
http://www.teccor.com
+1 972-580-7777
Figure AN1007.5
Normally Closed Switch Circuit
Optocoupled Driver Circuits
Random Turn-on, Normally Open
Many applications use optocouplers to drive thyristors. The com-
bination of a good optocoupler and a triac or alternistor makes an 
excellent, inexpensive solid state relay. Application information 
provided by the optocoupler manufacturers is not always best for 
application of the power thyristor. Figure AN1007.6 shows a stan-
dard circuit for a resistive load.
Figure AN1007.6
Optocoupled Circuit for Resistive Loads (Triac or 
Alternistor)
A common mistake in this circuit is to make the series gate resis-
tor too large in value. A value of 180
 
 is shown in a typical appli-
cation circuit by optocoupler manufacturers. The 180
 
 is based 
on limiting the current to 1 A peak at the peak of a 120 V line 
input. This is good for protection of the optocoupler output triac, 
as well as the gate of the power triac on a 120 V line; however, it 
must be lowered if a 24 V line is being controlled, or if the R
(resistive load) is 200 W or less. This resistor limits current for 
worst case turn-on at the peak line voltage, but it also sets turn-
on point (conduction angle) in the sine wave, since triac gate cur-
rent is determined by this resistor and produced from the sine 
wave voltage as illustrated in Figure AN1007.2. The load resis-
tance is also important, since it can also limit the amount of avail-
able triac gate current. A 100
 
 gate resistor would be a better 
choice in most 120 V applications with loads greater than 200 W 
and optocouplers from Quality Technologies or Vishay with opto-
coupler output triacs that can handle 1.7 A
PK
 (I
TSM
 rating) for a 
few microseconds at the peak of the line. For loads less than 
200 W, the resistor can be dropped to 22
 
. Remember that if the 
gate resistor is too large in value, the triac will not turn on at all or 
not turn on fully, which can cause excessive power dissipation in 
the gate resistor, causing it to burn out. Also, the voltage and dv/
dt rating of the optocoupler's output device must be equal to or 
greater than the voltage and dv/dt rating of the triac or alternistor 
it is driving.
Figure AN1007.7 illustrates a circuit with a dv/dt snubber network 
included. This is a typical circuit presented by optocoupler manu-
facturers.
Figure AN1007.7
Optocoupler Circuit for Inductive Loads (Triac or 
Alternistor)
This “T” circuit hinges around one capacitor to increase dv/dt 
capability to either the optocoupler output triac or the power triac. 
The sum of the two resistors then forms the triac gate resistor.
Both resistors should then be standardized and lowered to 
100
 
. Again, this sum resistance needs to be low, allowing as 
much gate current as possible without exceeding the instanta-
neous current rating of the opto output triac or triac gate junction. 
By having 100
 
 for current limit in either direction from the 
capacitor, the optocoupler output triac and power triac can be 
protected against di/dt produced by the capacitor. Of course, it is 
most important that the capacitor be connected between proper 
terminals of triac. For example, if the capacitor and series resis-
tor are accidentally connected between the gate and MT2, the 
triac will turn on from current produced by the capacitor, resulting 
in loss of control.
For low current (mA) and/or highly inductive loads, it may be nec-
essary to have a latching network (3.3 k
 + 0.047 μF) connected 
directly across the power triac. The circuit shown in Figure 
AN1007.8 illustrates the additional latching network.
Load
Triac
51 k
0.02 μF
(4) IN4004
PS2502
 +
120 V ac
Q2008L4
BS08D
R
in
V
CC
1
6
4
180
G
R
L
120 V 
60 Hz
MT2
MT1
Hot
Neutral
Load Could Be
in Either Leg
2
R
in
V
CC
1
6
4
100
G
Neutral
2
100
Z
L
120 V
60 Hz
MT2
MT1
Hot
0.1 μF
C
1