AN1714 Datasheet, PDF (43/46 Pages) STMicroelectronics – transceiver demonstration kit description

AN1714
Design ideas for auxiliary blocks
4
Design ideas for auxiliary blocks
4.1
Zero-crossing detector
It is possible to synchronize the beginning of the transmission with the mains voltage (phase
0). To implement this function the zero-crossing comparator has to be used and a reduced
reproduction of the mains frequency (with the same phase) has to be present on the ZCin
pin (#16). The maximum voltage of this pin is ±5 V.
In case of a nonisolated application the circuit consists of a simple resistor divider. For an
isolated system a possible solution could be a mains transformer. This solution is more
expensive and is suggested only if such a mains transformer is also used in the application
for another purpose.
It is possible to implement another isolated solution using for example an optocoupler
component also.
In both cases a bidirectional transil has to protect the pin from the burst and surge and a
capacitor have to be added to filter high frequency noise.
Figure 41. Zero-crossing coupling circuit, nonisolated solutions
ST7538Q
Tx Sync
ZCout ₁₅
ZCin
16
25
P
10 MΩ
MAINS
100 KΩ
N
Figure 42. Zero-crossing coupling circuit, isolated solutions
ST7538Q
Tx Sync
ZCout ₁₅
ZCin
16
25
100 KΩ
1
100 KΩ
50
MAINS
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