AN442
APPLICATION NOTE
TRIAC & MICROCONTROLLERS: THE EASY CONNECTION
INTRODUCTION
The aim of this note is to show how to connect an ST triac and an ST microcontroller.
CONVENTIONAL SOLUTION
For many years the triac has been used to switch load on the AC mains and thanks to the low cost of mi-
crocontrollers (µC) this solution is widely used in the appliance market.
All the system use a buffer transistor between the output port of the microcontroller and the triac as shown
in the Figure 1.
Figure 1. Drive in the 1st and 4th quadrants
+V
DD
µC
I
G
0V
LINE
Because of the low sensivity of the triac in the 4th quadrant this type of drive is often unpractical, and is
replaced by the topology of the Figure 2.
Figure 2. Conventional drive in the 2nd and 3rd quadrants
+V
DD
I
G
µC
LINE
A2
0V
G
A1
To save cost, manufacturers want to use fewer and fewer components and of course want to remove the
buffer transistor, but a problem arises.
REV. 2
April 2004
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AN442 APPLICATION NOTE
Due to the low output current of the microcontroller, the triac had to be very sensitive and consequently
was not able to withstand for example the static dv/dt, and the commutation.
NEW ST SOLUTION
Two parameters have been improved:
– The sensitivity of the triacs.
– The output capability of the microcontrollers in terms of sunk current.
A microcontroller is now able to drive one standard triac or several sensitive triacs, without buffer transis-
tors (see Figure 3).
Figure 3. An easy connection!
+V
DD
I
G
V
OL
R
G
V
G
G
ST
TRIAC
LINE
A2
A1
ST
µC
0V
Table 1 shows the output capability of a range of controllers and the sensitivity of the triacs.
Table 1. Triac and microcontroller characteristics
MICROCONTROLLERS &
OUTPUT CAPABILITIES
TRIAC
T & TW SERIES
Tx05 SERIES
S & SW SERIES
Tx10 SERIES
SENSITIVITY
GATE
PARAMETERS
V
G
=1.5V
at
I
G
= 10mA
V
G
=1.5V
at
I
G
= 20mA
V
G
=1.5V
at
I
G
= 50mA
V
G
= 2V
at
I
G
= 70mA
V
G
= 2V
at
I
G
= 100mA
CONNECTION
I
GT
= 5mA
1 PORT/TRIAC
ST621x SERIES
ST622x SERIES
I
OL
= 20mA
at
V
OL
= 1.3V
I
VSS
=100mA
I
GT
= 10mA
1 PORT/TRIAC
C SERIES
I
GT
= 25mA
2 PORTS
IN PARALLEL/TRIAC
3 PORTS
IN PARALLEL/TRIAC
4 PORTS
IN PARALLEL/TRIAC
CW SERIES
I
GT
= 35mA
B & BW SERIES
I
GT
= 50mA
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AN442 APPLICATION NOTE
Example:
For +5V supply voltage and a LOGIC LEVEL triac with I
GT
= 10 mA, we have:
V
DD
–
V
G
–
V
OL
R
G
= -----------------------------------------
-
I
G
Where:
V
DD
supply voltagage
V
OL
output low voltage of the microcontroller
V
G
gate - anode 1 voltage at I
G
With:
V
DD
= 5 V
V
OL
= 1.3 V
V
G
= 1.5 V
I
G
= 20 mA
Therefore:
R
G
= 110
Ω
To take into account of the dispersion on R
G
, V
DD
and on the temperature variation, we generaly choose
about:
I
G
= 2 .I
GT
t
p
> 20 µs
Where tp is the pulse duration of gate current.
CONCLUSION
Use ST sensitive triacs driven by an ST microcontrollers and remove the buffer transistors.
This can be achieved thanks to the high current capability of our microcontrollers which are compatible
with our new sensitive triacs (T410, T, TW, S, SW series).
Furthemore a non sensitive triac can be driven by several output ports in parallel.
(I
GT
= Specified gate trigger current)
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AN442 APPLICATION NOTE
REVISION HISTORY
Table 2. Revision History
Date
May-1992
5-Apr-2004
Revision
1
2
First Issue
Stylesheet update. No content change.
Description of Changes
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AN442 APPLICATION NOTE
Information furnished is believed to be accurate and reliable. However, STMicroelectronics assumes no responsibility for the consequences
of use of such information nor for any infringement of patents or other rights of third parties which may result from its use. No license is granted
by implication or otherwise under any patent or patent rights of STMicroelectronics. Specifications mentioned in this publication are subject
to change without notice. This publication supersedes and replaces all information previously supplied. STMicroelectronics products are not
authorized for use as critical components in life support devices or systems without express written approval of STMicroelectronics.
The ST logo is a registered trademark of STMicroelectronics.
All other names are the property of their respective owners
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