Live voltages are connected to two positions in the same connector (J2). These lines are connected to the inputs of the A/D converter through a resistor divider with a ratio of 0.6057V/1000V. The TCR of the resistors is 25 ppm/ºC.
The two inputs (L1 and L2) have the same resistor divider schematic and layout. The
metering ground reference, GND, is connected to the Line Voltage input VN.
Important: The resistors
installed by default in the divider (ratio of 0.6057V/1000V) allow a maximum input
voltage of 291 Vrms. It is recommended that the divider ratio be reduced to be able
to measure higher input voltage values. Also, take into account the AC power supply
input range (from 90 to 264 Vac and from 47 to 63 Hz). If this range is not
accomplished, removing the AC power supply (U1) is required to not damage
it.
Figure 1. Lines Voltage Sense
Schematic
An anti-aliasing filter is required for band-limiting the input signals. Typically, a
single-pole RC filter is sufficient for metrology applications. The filter is tuned with
the following component values:
- R34 and R46: 4.42 kΩ, 1%, 25 ppm/ºC
- C55 and C58: 1600 pF NP0/COG
Some test points close to the resistor dividers allow monitoring the input voltage value
at the A/D converter inputs:
- TP23: VP1 before RC filter input measuring referring to GND
- TP24: VP1 input measuring referring to GND
- TP26: VP2 before RC filter input measuring referring to GND
- TP27: VP2 input measuring referring to GND
Figure 2. VP1 and VP2 Test
Points
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