
Application Notes
AN1001
2002 Teccor Electronics
Thyristor Product Catalog
AN1001 - 3
http://www.teccor.com
+1 972-580-7777
Sidac
Basic Operation
The sidac is a multi-layer silicon semiconductor switch. Figure 
AN1001.7 illustrates its equivalent block construction using two 
Shockley diodes connected inverse parallel. Figure AN1001.7 
also shows the schematic symbol for the sidac.
Figure AN1001.7
Sidac Block Construction
The sidac operates as a bidirectional switch activated by voltage. 
In the off state, the sidac exhibits leakage currents (I
DRM
) less 
than 5 μA. As applied voltage exceeds the sidac V
BO
, the device 
begins to enter a negative resistance switching mode with char-
acteristics similar to an avalanche diode. When supplied with 
enough current (I
S
), the sidac switches to an on state, allowing 
high current to flow. When it switches to on state, the voltage 
across the device drops to less than 5 V, depending on magni-
tude of the current flow. When the sidac switches on and drops 
into regeneration, it remains on as long as holding current is less 
than maximum value (150 mA, typical value of 30 mA to 65 mA). 
The switching current (I
S
) is very near the holding current (I
H
) 
value. When the sidac switches, currents of 10 A to 100 A are 
easily developed by discharging small capacitor into primary or 
small, very high-voltage transformers for 10 μs to 20 μs.
The main application for sidacs is ignition circuits or inexpensive 
high voltage power supplies.
Geometric Construction
Figure AN1001.8
Cross-sectional View of a Bidirectional Sidac Chip 
with Multi-layer Construction
Diac
Basic Operation
The construction of a diac is similar to an open base NPN tran-
sistor. Figure AN1001.9 shows a simple block construction of a 
diac and its schematic symbol.
Figure AN1001.9
Diac Block Construction
The bidirectional transistor-like structure exhibits a high-imped-
ance blocking state up to a voltage breakover point (V
BO
) above 
which the device enters a negative-resistance region. These 
basic diac characteristics produce a bidirectional pulsing oscilla-
tor in a resistor-capacitor AC circuit. Since the diac is a bidirec-
tional device, it makes a good economical trigger for firing triacs 
in phase control circuits such as light dimmers and motor speed 
controls. Figure AN1001.10 shows a simplified AC circuit using a 
diac and a triac in a phase control application.
Figure AN1001.10
AC Phase Control Circuit
Geometric Construction
Figure AN1001.11
Cross-sectional View of Diac Chip
P
N
P
N
N
P
N
P
2
3
4
5
2
3
4
1
Equivalent Diode Relationship
Schematic Symbol
MT2
MT2
MT1
MT1
P
3
P
1
N
2
N
4
P
5
MT1
MT2
MT1
MT2
N
N
P
MT1
MT2
Block Construction
Schematic Symbol
Load
N
N
P
MT1
MT2
Cross-section of Chip
Equivalent Diode
Relationship
MT1
MT2