AN392

Light dimmer

Doc ID 1863 Rev 2

5/11

2

Light dimmer

For a light dimmer application, the board can be plugged in series with the line wire like a

mechanical switch. The line synchronization and the auxiliary supply are obtained from the

voltage across the TRIAC (Figure 4).

Figure 4 shows a schematic which can operate either on 110 V or 240 V mains. It uses an

MCU ST6210 and a logic level TRIAC. This circuit drives halogen or incandescent lamps

supplied directly from the mains or through a low voltage transformer. It includes soft start

and protection against transformer saturation and against open load. The user interfaces

are the same as previously presented.

Figure 4.

Light dimmer circuit diagram

2.1

Power control

referred to the zero crossing of the line voltage. To avoid a connection to the mains neutral

as the circuit is in series with the load, the trigger delay is referred to the zero crossing of the

current (see Figure 1). When the TRIAC anode current reaches zero, the mains voltage is

reapplied across the TRIAC. Synchronization is achieved by measuring this voltage. This

voltage is monitored over each half cycle with a network of resistances connected to two I/O

lines of the ST6210. This allows detection of spurious open load and the retriggering of the

TRIAC with multipulse operation if it is not latched after the first gate current pulse.

5.6 V

1N4148

100 uF/10 V

22 pF

22 pF

BTA08 -600SW

820-1/2W

4.7 M

4.7 M

4.7 M

100

220 nF/400 V

Fuse

Potentiometer

Version

Push button

1

2

8 MHz

VPP

6

PB1

14

PB0

15

VSS

20

NMI

5

PA1

18

PA2

17

PA3

16

PB3

12

RESET

7

PB4

11

PB2

13

PA0

19

ST6210

5.6 V

1N4148

BTA08 -600SW

820-1/2W

100

Fuse

1

2

8 MHz

VPP

6

PB1

14

PB0

15

VSS

20

NMI

5

PA1

18

PA2

17

PA3

16

PB3

12

RESET

7

PB4

11

PB2

13

PA0

19

ST6210

GND

GND

GND

GND

GND

GND

GND

Optional

user interface

Neutral

Line

+5 V

+5 V

+5 V

Touch sensor

PB5

100k

+5 V

200k

200k

GND

22k

T?

Trans

100k

12 V

10 nF

GND