SA9903B
sames
VOLTAGE DIVIDER
The voltage divider is calculated for a voltage drop of
14V+2.3% (14.33V). Equations for the voltage divider in figure
8 are:
RA = R1 + R2 + R3
RB = R4 || R5
Combining the two equations gives:
( RA + RB ) / 230V = RB / 14.33V
Values are chosen for R4 = 24kW and R5 =1MW. Substituting
the values result in:
RB = 23.4375kW
RA = RB ( 230V / 14.33V - 1 )
RA = 352.7kW.
Resistor values of R1, R2 and R3 are chosen to be 110kW,
110kW and 130kW.
TYPICAL APPLICATION
In figure 8, the components required for a two wire single phase
power/energy metering section of a meter, is shown The
application uses a shunt for the mains current sensing. The
metering section described in this section will be designed for
measuring 230V/80A with precision better than Class 1
The most important external components for the SA9903B
integrated circuit are the current sense resistors, the voltage
sense resistors as well as the bias setting resistor. The
resistors used in the metering section should be of the same
type so temperature effects are minimized.
BAIS RESISTOR
R8 defines all on-chip and reference currents. With R8=24kW,
optimum conditions are set. The meter calibration is
implemented in software.
SHUNT RESISTOR
The voltage drop across the shunt resistor (RSH) at rated
current should be at least 20mV. A shunt resistor of 625µW is
chosen. The voltage drop across the shunt resistor is 50mV at
rated conditions (Imax for the meter).
CRYSTAL OSCILLATOR
A color burst TV crystal with f = 3.5795MHz is used for the
oscillator. The oscillator frequency is divided down to
1.7897MHz on-chip, to supply the A/D converters as well as
the digital circuitry.
CURRENT SENSE RESISTORS
The resistors R6 and R7 define the current level into the current
sense inputs of the device. The resistor values are selected for
an input current of 16µA on the current inputs at rated
conditions.
According to equation described in the Current Sense inputs
section:
R6 = R7 = (I
L
/ 16µA) x R
SH
/ 2
= 80A / 16µA x 625µW / 2
=1.5625kW
A resistor value of 1.6k is chosen, the -2.3% deviation from the
calculated value will be compensated for when calculating the
resistor values for the voltage path.
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