SA2005F
ssaammeess
TYPICAL APPLICATION
VOLTAGEDIVIDER
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 voltage divider is calculated for a voltage drop of 14V.
Equationsforthevoltagedividerinfigure5are:
RA=R16+R19+R22
RB=R8||(R13+P1)
Combiningthetwoequationsgives:
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.
(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=1Mischosen.
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.
Substitutingthevaluesresultin:
RB=23.9k
RA=RBx(230V/14V-1)
RA=368.9k.
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).
Resistor values of R16, R19 and R22 is chosen to be 120k,
120kand130k.
CURRENT SENSE RESISTORS
The three voltage channels are identical so R14= R15= R16 =
R17=R18=R19andR20=R21=R22.
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
SH
L
=80A/2500/16µAx2.7W /2
=2.7kW
I =Linecurrent/CTRatio
L
The three current channels are identical so R1 = R2 = R3 = R4
=R5=R6.
6/10
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