Circuit description

UM1559

8/13

Doc ID 023542 Rev 1

3

Circuit description

Power supply

The power for the STM8 and the associated control circuit is derived from the mains input

using a capacitive drop power supply followed by a 5 V linear regulator, as shown in

Figure 3.

Figure 3.

Power supply

C2, R15, Z1, D1 and C4 form the capacitive drop supply that provides an unregulated 10 V

to the L7805 linear regulator. Zener diode Z1 sets the peak value of the unregulated supply.

A value of 10 V was chosen to maintain the minimum voltage of 8 V at the input of the

L7805. The power supply section was designed to use with a 120 V/60 Hz mains. The

power supply can be modified to be used on 230 V/50 Hz mains by changing C2.

Component changes needed to modify the board to work on a 230 V/50 Hz mains are

shown in Table 2.

Line sync interface

To synchronize the gate drive for the Triac to the AC mains, the STM8 senses the zero

crossings of the AC mains voltage at the timer1 capture input. A pair of series current

limiting resistors, R1 and R2, connect the AC line input to the timer1 capture input pin. Two

1206 case resistors are used in series to provide sufficient voltage rating. The voltage at the

pin is clamped to 5 V and ground by the internal diodes in the STM8. The values of the

resistors are selected to limit the current to within the allowable range for the STM8.

Triac power stage

Triac Q1 is connected in between the mains and the load and functions as a phase

controlled switch to provide power to the motor. The gating signal is generated by the STM8

at GPIO pins PC6 and PC7. The pins are paralleled in order to increase the available gate

current drive and the control must operate these two outputs in unison.

For best operation, the Triac should be operated in the second and third quadrant, always

using a negative pulse on the gate to turn the device on. Since the STM8 operates from a

positive 5 V supply, it is not possible to directly drive the gate with a negative pulse.

Capacitor C1 AC couples the gate signal from the STM8 to the Triac so that a falling edge on

the output of PC6 and PC7 generates a negative voltage on the Triac gate for turn-on. The

AM12564v1

115VAC

out

1

gnd

2

gnd

3

NC

4

NC

5

gnd

6

gnd

7

in

8

U2

L7805AC

1

2

P2

R15

47

C4

220uF 10V

GND

Z1

10V

5V

C3

470uF 10V

D1

1N4148

C2

2.2uF 400V