sames

sames

SA2002E

7/10

http://www.sames.co.za

TYPICAL APPLICATION

In figure 10, the components required for stand alone power

metering application, is 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

MAX

components for the SA2002E integrated circuit are the current

sense resistors, the voltage sense resistors as well as the bias

setting resistor.

BIAS RESISTOR

R13 defines all on-chip and reference currents. With

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

done on the voltage input of the device.

SHUNT RESISTOR

The voltage drop across the shunt resistor at rated current

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

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 625µW = 1W.

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 of

the SA2002E at rated conditions. According to equation

described in the Current Sense inputs section:

R6 = R7 = ( I / 16µA ) x RSH / 2

L

= 40A / 16µA x 625µW / 2

= 781.2W

A resistor with value of 820W is chosen, the 5% deviation from

the calculated value will be compensated for when calculating

resistor values for the voltage path.

VOLTAGE DIVIDER

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

5%(14.7V). Equations for the voltage divider in figure 9 are:

RA = R1 + R2 + R3

RB = R12 || (R11+P1)

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 = 2.5kW and values for resistors R11 = 22kW

and R12 =1MW is chosen.

Substituting the values will result in:

RB = 23.91kW

RA = RB x (230V/14.7V - 1) = 333kW so the resistor values of

R1, R2 and R3 are chosen to be 110kW.