PM2002DP
CT TERMINATION RESISTOR
Provision is made for the use of a current transformer on the
module. The existing shunt needs to be removed and the
current sense resistors will have to be recalculated for the
specific current transformer. The voltage drop across the CT
termination resistor at rated current should be at least 20mV.
The CT's used should have a low phase shift and a ratio of
1:2500.The CT is terminated with a 2.7W resistor giving a
voltage drop of 864mV across the termination resistor at rated
conditions
CURRENT SENSOR INPUT RESISTORS FIGURE 5 (USING
A CURRENT TRANSFORMER)
The resistors R6, R7 define the current level into the current
sense inputs the device. The resistor values are selected for
an input current of 16µA on the current inputs at rated
conditions. For an 80A-rated meter the resistor values are
calculated as follows:
R6 = R7 = (I
L
/ 16µA) x R
SH
/ 2
= 80A / 16µA x 2.7W / 2
= 2.7kW
I
L =
Line current
RSH = CT Termination resistor
2500 = CT ratio
L
CT1
IIN
R16
IIP
R7
J5
R6
sames
Phase Compensation (When using a current transformer)
Phase shift caused by the current transformer may be
corrected by inserting a capacitor in the voltage divider circuit.
See Figure 6, Capacitor C7. To compensate for a phase shift of
0.18 degrees the capacitor value is calculated as follows:
C = 1 / (2 x
p
x Mains frequency x R5 x tan (Phase shift angle))
C = 1 / (2 x
p
x 50 x 1M x tan( 0.18 degrees))
C = 1.013µF
Reference Voltage VREF
The VREF pin of the SA2002D is connected to a resistor that
determines the on chip bias current.
Ground GND
The GND pin of the SA2002D is to the live phase, which is
halfway between VDD and VSS. Note that supply bypass
capacitors C1 and C2 are positioned as close as possible to the
supply pins of the device, and connected to a solid ground
plane.
LIVE IN
LIVE OUT
Figure 5: Current input configuration
http://www.sames.co.za
4/10
SAMES [ SAMES ]
