MODULE OVERVIEW

ANALOG SECTION

The analog (metering) interface described in this section is

designed for measuring 230V/60A with precision better than

Class 1.

The most important external components for the SA2007

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

temperature effects are minimized.

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

SA2007.

Current Input IIN1, IIP1, IIN2, IIP2

The termination resistor on the second current channel is

adjustable by means of P1 to facilitate channel equalization

calibration. The termination resistor is chosen so that a 10

trimpot in parallel will provide a sufficient channel range. If the

SA2007P is being used, P1 can be removed and R16 replaced

with a 3.6

resistor, as channel equalization data can be read

from the external EEPROM, requiring no trim-pots.

The resistors R8, R9 and R10, R11 define the current level into

the current sense inputs of the SA2007. The resistor values

are selected for an input current of 16µA on the current inputs.

For a 60A meter at 2500:1 CT the resistor values are

calculated as follows:

R8 = R9 = ( I / 16µA ) x R

/ 2

= 60A / 2500 / 16µA x 3.6

/ 2

= 2.7k

I

Line current

RSH = CT Termination resistor

2500 = CT ratio

The two current channels are identical so R8 = R9 = R10 =

R11.

The voltage input of the SA2007 (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

1M

resistor.

The voltage divider is calculated for a voltage drop of 14V.

Equations for the voltage divider in figure 4 are:

RA = R1 + R2 + R3 + R4

RB = R6 || (R5 + P1)

W

W

W

W

W

Current Sensor Input Resistors

Voltage Input Ivp

Voltage Divider

LSH

L

=

sames

PM2007M/PPE

R7

3.6R

R8

2.7k

R9

2.7k

Live Line

Neutral Line

Live Load

Neutral Load

P1

10R

CT2

Phase 2

CT1

Phase 1

R10

2.7k

R11

2.7k

R16

6.2R

IIN1

GND

GND

DR-01576

IIP1

IIN2

IIP2

4/16

Figure 3: Current input configuration

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 3.6

resistor

giving a voltage drop of 86.4mV across the termination resistor

at rated conditions (I

for the meter).

W

max

Figure 4: Mains voltage divider

http://www.sames.co.za

NEUTRAL

LIVE IN

R1

R2

R3

R5

R6

C4

X1

RB

RA

R4