AN2451
Application ideas
Doc ID 12791 Rev 3
43/55
and
show two possible zero crossing detector circuit implementations.
N and P mean neutral and phase lines respectively at the meter/concentrator side, while
ZC_OUT is a digital output to the application microcontroller.
Particular attention should be paid to current rating (see solution in
) The
maximum allowable current for 1W dissipation, sustainable by two ½ W resistors in a series,
is 4.5 mA rms = 6.4 mA peak.
Such a current flowing into the LED of the optocoupler can minimize the delay between the
actual zero crossing of the mains voltage and the edge of the ZC_OUT signal, if the
optocoupler has been chosen to have an activation current I
F
about 10 times smaller than
the peak current.
shows the behavior of the ZC_OUT digital signal versus the AC Mains Input for
both circuits.
Warning:
The circuit in
is only applicable to a non-isolated
board topology. It is not possible to implement it directly on
the ST7540 reference design.
Figure 41.
Schematic of a zero crossing detection circuit for non-isolated coupling
VDC
1
3
D1
BZX79C12A
R1
470k
R2
220k
Q1
BS170F/SOT
R3
450
ZC_OUT
P
N
