sames

sames

PM2002DP

4/10

http://www.sames.co.za

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 / 16µA) x R / 2

L

SH

= 80A / 16µA x 2.7W / 2

= 2.7kW

I Line current

L =

RSH = CT Termination resistor

2500 = CT ratio

Figure 5: Current input configuration

LIVE IN

LIVE OUT

R16

IIN

IIP

J5

L

CT1

R6

R7

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

C = 1 / (2 x

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.