
AN1007
Application Notes
http://www.teccor.com
+1 972-580-7777 
AN1007 - 2
2002 Teccor Electronics
Thyristor Product Catalog
Figure AN1007.3
Waveform Across Static Switch
A typical example would be in the application of this type circuit 
for the control of 5 A resistive load with 120 V rms input voltage. 
Choosing a value of 100
 
 for R
1
 and assuming a typical value of 
1 V for the gate to MT1 (V
GT
) voltage, we can solve for V
P
 by the 
following:
V
P
 = I
GT
 (R
L
 + R
1
) + V
GT
Note: R
C
 is not included since it is negligible.
V
P
 = 0.025 (24 + 100) + 1.0 = 4.1 V
Additionally the turn-on angle is
θ
170V
PK
[ 
θ
 = 1.4°]
The power lost by the turn-on angle is essentially zero. The 
power dissipation in the gate resistor is very minute. A 100
 
, 
0.25 W rated resistor may safely be used. The small turn-on 
angle also ensures that no appreciable RFI is generated.
The relay circuit shown in Figure AN1007.1 and Figure AN1007.2 
has several advantages in that it eliminates contact bounce, 
noise, and additional power consumption by an energizing coil 
and can carry an in-rush current of many times its steady state 
rating.
The control device S
1
 indicated can be either electrical or 
mechanical in nature. Light-dependent resistors and light- acti-
vated semiconductors, optocoupler, magnetic cores, and mag-
netic reed switches are all suitable control elements. Regardless 
of the switch type chosen, it must have a voltage rating equal to 
or greater than the peak line voltage applied. In particular, the 
use of hermetically sealed reed switches as control elements in 
combination with triacs offers many advantages. The reed switch 
can be actuated by passing DC current through a small coiled 
wire or by the proximity of a small magnet. In either case, com-
plete electrical isolation exists between the control signal input, 
which may be derived from many sources, and the switched 
power output. Long life of the triac/reed switch combination is 
ensured by the minimal volt-ampere switching load placed on the 
reed switch by the triac triggering requirements. The thyristor rat-
ings determine the amount of load power that can be switched.
Normally Closed Circuit
With a few additional components, the thyristor can provide a 
normally closed static switch function. The critical design portion 
of this static switch is a clamping device to turn off/eliminate gate 
drive and maintain very low power dissipation through the clamp-
ing component plus have low by-pass leakage around the power 
thyristor device. In selecting the power thyristor for load require-
ments, gate sensitivity becomes critical to maintain low power 
requirements. Either sensitive SCRs or sensitive logic triacs must 
be considered, which limits the load in current capacity and type. 
However, this can be broader if an extra stage of circuitry for gat-
ing is permitted.
Figure AN1007.4 illustrates an application using a normally 
closed circuit driving a sensitive SCR for a simple but precise 
temperature controller. The same basic principle could be applied 
to a water level controller for a motor or solenoid. Of course, SCR 
and diode selection would be changed depending on load current 
requirements.
Figure AN1007.4
Normally Closed Temperature Controller
A mercury-in-glass thermostat is an extremely sensitive measur-
ing instrument, capable of sensing changes in temperature as 
small as 0.1 °C. Its major limitation lies in its very low current-
handling capability for reliability and long life, and contact current 
should be held below 1 mA. In the circuit of Figure AN1007.4, the 
S2010LS2 SCR serves as both current amplifier for the Hg ther-
mostat and as the main load switching element.
With the thermostat open, the SCR will trigger each half cycle 
and deliver power to the heater load. When the thermostat 
closes, the SCR can no longer trigger and the heater shuts off. 
Maximum current through the thermostat in the closed position is 
less than 250 μA rms.
Figure AN1007.5 shows an all solid state, optocoupled, normally 
closed switch circuit. By using a low voltage SBS triggering 
device, this circuit can turn on with only a small delay in each half 
cycle and also keep gating power low. When the optocoupled 
transistor is turned on, the gate drive is removed with only a few 
milliamps of bypass current around the triac power device. Also, 
by use of the BS08D and 0.1 μF, less sensitive triacs and alter-
nistors can be used to control various types of high current loads.
120 V rms (170 V peak)
V
P
-
1 V rms or 1.6 V peak MAX
V
P
+
V
+
V
T
-
θ
Sin1
–
-----4.1
=
1000 W Heater Load
120 V ac
60 CPS
D2015L
CR
1
—CR
4
CR
4
CR
3
CR
1
CR
2
S2010LS2
0.1 μF
R
1
510 k
SCR
1
Twist Leads to Minimize
Pickup
Hg in Glass Thermostat