SA2005M
ssaammeess
TYPICAL APPLICATION
Voltage Divider
The analog (metering) interface described in this section is
designed for measuring 230V/60A with precision better than
Class1.
The voltage divider for phase one is calculated for a voltage
drop of 14V. Equations for the voltage divider in figure 4 are:
RB = R20 + R23 + R26
RB = R12 || (R17 + P1)
The most important external components for the SA2005M
integrated circuit are the current sense resistors, the voltage
sense resistors and the bias setting resistor. The resistors
used in the metering section should be of the same type so
temperatureeffectsareminimized.
A 5k trimpot will be used in the voltage channel for meter
calibration. The center position on the pot is used in the
calibration P1 = 2.5kW.
Combining the two equations gives:
(RA + RB ) / 230V = RB / 14V
CurrentInputIIN1, IIP1, IIN2, IIP2
Two current transformers are used to measure the current in
the live and neutral phases. The output of the current
transformer is terminated with a low impedance resistor. The
voltage drop across the termination resistor is converted to a
current that is fed to the differential current inputs of the
SA2005M.
Values for resistors R17 = 22kW, P1 = 2.5kW and R12 = 1MW
is chosen.
Substituting the values result in:
RB = 23.914kW
RA = RB x ( 230V / 14V – 1 )
RA = 368.96kW.
CT Termination Resistor
The voltage drop across the CT termination resistor at rated
current should be at least 20mV. The CTs have low phase shift
and a ratio of 1:2500. The CT is terminated with a 3.6W resistor
giving a voltage drop of 86.4mV across the termination resistor
Standard resistor values for R24, R25 and R26 are chosen to
be 130kW.
atratedconditions(I forthemeter).
max
Standard resistor values for R18, R19, R20, R21, R22 and
R23 are chosen to be 120kW.
CurrentSensorInputResistors
The resistors R1 and R2 define the current level into the
current sense inputs of phase one of the SA2005M. The
resistor values are selected for an input current of 16µA on the
current inputs of the SA2005M at rated conditions. For a 60A
meter at 2500:1 CT the resistor values are calculated as
follows:
The capacitor C1 is used to compensate for phase shift
between the voltage sense inputs and the current sense inputs
of the device, in cases where CTs with phase errors are used.
The phase shift caused by the CT may be corrected by
inserting a capacitor in the voltage divider circuit. To
compensate for a phase shift of 0.18 degrees the capacitor
value is calculated as follows:
R1=R2 =(I /16µA)xR /2
SH
L
=60A/2500/16µAx3.6W /2
= 2.7kW
C = 1 / (2 xp x Mains frequency x R12 x tan (Phase shift angle))
C = 1 / ( 2 xp x 50 x 1MW x tan (0.18 degrees ))
C = 1.013µF
I =Linecurrent
L
RSH=CTTerminationresistor
2500=CTratio
Reference Voltage Bias resistor
R7 defines all on chip and reference currents. With R7 = 24kW
optimum conditions are set. Device calibration is done on the
voltage input of the device.
The three current channels are identical so R1 = R2 = R3 =
R4=R5=R6.
VoltageInputIVP
Device setup
The voltage input of the SA2005M (IVP) is driven with a current
of 14µA at nominal mains voltage. This voltage input saturates
at approximately 17µA. At a nominal voltage current of 14µA
allows for 20% overdriving. The mains voltage is divided with a
voltage divider to 14V that is fed to the voltage input pins via a
1MW resistor.
The resistor values calculated above is for a 60A rated meter
so RA must be set to VDD, according to the description under
Input Signals. The RE and IM inputs of the device must be set
to the appropriate states for the meter to function correctly.
8/12
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