SA2005P
ssaammeess
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
VoltageDivider
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.
Equationsforthevoltagedividerinfigure5are:
RA = R16 + R19 + R22
RB = R8 || R13
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.
Combiningthetwoequationsgives:
Bias Resistor
(RA + RB ) / 230V = RB / 14V
Resistor values R11 = R12 = R13= 24kW and R8 =1MW is
chosen.
R7 defines all on-chip and reference currents. With R7=24kW,
optimum conditions are set.
CT Termination Resistor
Substitutingthevaluesresultin:
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).
RB = 23.4375kW
RA = RB x (230V / 14V - 1)
RA = 361.607kW.
Resistor values of R16, R19 and R22 is chosen to be 130k,
130kand100k.
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.
The three voltage channels are identical so R14= R15= R16 ,
R17=R18=R19andR20=R21=R22.
According to equation described in the Current Sense inputs
section:
R1 = R2 = ( I / 16µA ) x R / 2
SH
L
= 80A /2500 / 16µA x 2.7W / 2
= 2.7kW
I =Linecurrent/CTRatio
L
The three current channels are identical so R1 = R2 = R3 = R4
=R5=R6.
12/16
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