sames

SA2002D

7/10

http://www.sames.co.za

TYPICAL APPLICATION

In figure 7, the components required for stand alone power

metering application, are shown. The application uses a shunt

resistor for the mains current sensing. The meter is designed

for 220V/40A I

operation.

The most important external components for the SA2002D

integrated circuit are the current sense resistors, the voltage

sense resistors as well as the bias setting resistor.

R13 defines all on-chip and reference currents. With

R13=24k , optimum conditions are set. Device calibration is

done on the voltage input of the device.

The voltage drop across the shunt resistor at rated current

should be at least 20mV. A shunt resistor with a value of 625µ

is chosen. The voltage drop across the shunt resistor is 25mV

at rated conditions (Imax). The power dissipation in the current

sensor is:

P=(40A)² x 625µ = 1W.

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 of

the SA2002D at rated conditions.

According to equation described in the Current Sense inputs

section:

R6 = R7 = ( I / 16µA ) x R / 2

= 40A / 16µA x 625µ

/ 2

= 781.25

A resistor with value of 820

is chosen, the 5% deviation from

the calculated value will be compensated for when calculating

resistor values for the voltage path.

MAX

LSH

BIAS RESISTOR

SHUNT RESISTOR

CURRENT SENSE RESISTORS

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VOLTAGE DIVIDER

The voltage divider is calculated for a voltage drop of 14V +

5%(14.7V). Equations for the voltage divider in figure 7 are:

R1+R2+R3=RA and R12 || (R11+P1)=RB. Combining the two

equations gives:

(RA + RB) / 220V = RB / 14.7V

A 5k trimpot will be used in the voltage channel for meter

calibration. The center position on the pot is used in the

calculations. P1 = 2.5k

and values for resistors R11 = 22k

and R12 =1M is chosen.

Substituting the values will result in:

RB = 23.91k and RA = RB x (230V/14.7V - 1) resulting in RA =

333k

so the resistor values of R1, R2 and R3 are chosen to

be 110k

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PROGRAMMING

The resistor values are calculated for a 40A rated meter. The

LED pulse rate must be set accordingly by programming pins

R0, R1 and R2. Using the table Rated Conditions Select (On

page 5) pins R0 and R2 is set to V

and R1 set to V . These

settings will configure the SA2002D for 220V/40A operation

with a LED pulse rate of 1600 pulses/kWh. The FAST pin is set

to V for normal operation.

SS

DD

SS