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Product(s)

Obsolete

Product(s)

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Product(s)

Power supply and timebase delivery circuit

AN2326

8/20

(110V/60Hz), C2 must be multiplied by two. A 440nF capacitor will limit the current to 4.9

mA.

For the US standard, R2 must be divided by two in order not to limit the current too much on

the LTIC input. A 220k

Ω resistor is enough in this case.

Table 1.

Maximum MCU current

2.2

Hardware protection

To prevent bad measurements due to noisy mains, a filter can be added between the mains

and the input capture of the ST7ULTRALITE. The following figure shows one example of a

filter. This filter is a pass band centered on the mains frequency in order to reject all

frequency which could be understood by the microcontroller as a mains edge.

Please note that this is just a second order filter and that this may not be enough if the mains

is really noisy. Any kind of filter can be added on the LTIC.

Figure 6.

Band pass filter

The pass band filter above must be tuned to mains frequency. The value of the resistors for

this filter is given in the table below.

Table 2.

Resistor values

Capacitor C2

Maximum Current

220nF

4.9mA

330nF

7.3mA

470nF

10.4mA

680nF

15mA

1µF

22.1mA

C1

R1

R2

R3

C2

C2

LTIC

mains

470nF/400V

470nF/400V

470nF/250V

resistors

50Hz/220V

60Hz/110V

R1

6.8K/0.5W

5.6K/0.5W

R2

6.8K/0.5W

5.6K/0.5W

R3

470K/0.5W

220K/0.5W