sames

sames

SA2005P

12/16

http://www.sames.co.za

TYPICAL APPLICATION

CALCULATION OF EXTERNAL RESISTOR VALUES

In figure 8, all the components required for a three-phase

power/energy metering section, 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 SA2005P

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.

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 three voltage divider for voltage measurement are

identical so resistor values for one phase will be calculated.

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

Combining the two equations gives:

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

Resistor values R11 = R12 = R13= 24kW and R8 =1MW is

chosen.

Substituting the values result in:

RB = 23.4375kW

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

RA = 361.607kW.

Resistor values of R16, R19 and R22 is chosen to be 130k,

130k and 100k.

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

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