sames

PM2005M/P

5/12

http://www.sames.co.za

http://www.sames.co.za

CIRCUIT DESCRIPTION

ANALOG SECTION

BIAS RESISTOR

The analog (metering) interface described in this section is

designed for measuring 3x

with precision better

than Class 1.

The most important external components for the SA2005M

and SA2005P 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 that temperature effects are minimized.

Pin VREF (SA2005M or P pin 19) is connected to VSS via R7

which determines the on chip bias current. With R7 = 24k

optimum conditions are set. VREF does not require any

additional circuitry.

The voltage drop across the CT termination resistor at rated

current should be at least 16mV. The CT’s used have low

phase shift and a ratio of 1:2500. The CT is terminated with a

3.6

resistor giving a voltage drop across the termination

resistor 86.4mV at rated conditions (Imax for the meter).

Referring to figure 4 the resistors R1 and R2 define the current

levels into the SA2005’s current sense inputs (phase on IIP1

and IIN1). The resistor values are selected for an input current

of 16µA into the current inputs at rated conditions. According

the equation described in the Current Sense inputs section of

the datasheet:

R1=R2=(I / 16µA)xR

/2

=

/ 2500 / 16µA x

/ 2

= 2.7k

I Line current / CT Ratio

The three current channels are identical so R1=R2=R3=R4=

R5=R6.

W

W

W

W

CT TERMINATION RESISTOR

CURRENT SENSOR INPUT RESISTORS

=

SH

230V/60A

60A

3.6

VOLTAGE DIVIDER

Referring to figure 5 the connections for the voltage sense

input for one phase is shown. The current into the A/D

converter (IVP) is set 14µA

at nominal mains voltage. This

voltage sense input saturates at approximately 17µA

. A

nominal voltage current of 14µA allows for 20% over driving.

Each mains voltage is divided down by a voltage divider to 14V.

The current into the voltage sense input is set at 14µA via a

1M

resistor.

The following equation is used to calculate the 14V voltage

drop:

RA = R22 + R23 + R24 + R25

RB = R8 || (R13 + P1)

Combining the two equations gives:

( RA + RB ) / 230V = RB / 14V

A 24k

resistor is chosen for R13 and P1 combined. A 1M

resistor is used for R8.

Substituting the values result in:

RB = 23.44k

RA = RB x ( 230V / 14V - 1 )

RA = 361.6k

Resistor values of R22, R24 are chosen to be 82k

and

resistors R23 and R25 is chosen to be100k

each.

The three voltage channels are identical so

R14 = R16 = R18 = R20 = R22 = R24 = 82k and R15 = R17 =

R19 = R21 = R23 = R25 = 100k

RMS

RMS

W

WW

W

W

W

W

W

Figure 4: Current Input Configuration

R1

2.7k

R2

2.7k

R26

3.6R

CT1

TZ76

GND

I1 In

Neutral

Pin 23

Pin 22

The capacitors C1, C2 and C3 is used to compensate for

phase shifts between the SA2005 voltage sense inputs and

current sense inputs. The on-board Ct’s were characterized

and found to have a constant phase shift of 0.18 degrees. The

value of the phase shift compensation capacitors were

calculated as follows:

C = 1 / (2 x

x Mains frequency x R5 x tan (Phase shift angle))

C = 1 / (2 x

x 50 x 1M

x tan (0.18 degrees))

C = 1.013µF

p

pW

Figure 5: Mains Voltage Divider

R8

1M

R13

22k

V1In

C1

1u

GND

P1

10k

R22

82k

R23

100k

R24

82k

R25

100k

J3

Pin 21

Neutral