sames

sames

SA2005F

6/10

http://www.sames.co.za

TYPICAL APPLICATION

In figure 5, the components required for the three phase

power/energy metering section of a meter, is shown. The

application uses current transformers for current sensing. The

4-wire meter section is capable of measuring 3x230V/80A with

precision better than Class 1.

The most important external components for the SA2005F

integrated circuit are the current sense resistors, the voltage

sense resistors as well as the bias setting resistor.

BIAS RESISTOR

R7 defines all on-chip and reference currents. With R7=24kW,

optimum conditions are set. Device calibration is done on the

voltage inputs of the device.

CT TERMINATION RESISTOR

The voltage drop across the CT termination resistor at rated

current should be at least 20mV. The CT’s used have low

phase shift and a ratio of 1:2500.The CT is terminated with a

2.7W resistor giving a voltage drop across the termination

resistor 864mV at rated conditions (Imax for the meter).

CURRENT SENSE RESISTORS

The resistors R1 and R2 define the current level into the

current sense inputs of phase one 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:

R1 = R2

= (I / 16µA ) x R / 2

LSH

= 80A /2500 / 16µA x 2.7W / 2

= 2.7kW

I = Line current / CT Ratio

L

The three current channels are identical so R1 = R2 = R3 = R4

= R5 = R6.

VOLTAGE DIVIDER

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

Equations for the voltage divider in figure 5 are:

RA = R16 + R19 + R22

RB = R8 || (R13+P1)

Combining the two equations gives:

( RA + RB ) / 230V = RB / 14V

A 5k trimpot is used in each of voltage channel for meter

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

calculations. P1 = 2.5k and values for resistors R13 = 22k and

R8 =1M is chosen.

Substituting the values result in:

RB = 23.9k

RA = RB x (230V / 14V - 1)

RA = 368.9k.

Resistor values of R16, R19 and R22 is chosen to be 120k,

120k and 130k.

The three voltage channels are identical so R14= R15= R16 =

R17 = R18 = R19 and R20 = R21= R22.