sames

sames

SA9903B

8/12

http://www.sames.co.za

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).

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 / 16µA) x R / 2

LSH

= 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.

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.

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.