sames

sames

PM2002DP

3/10

http://www.sames.co.za

ANALOG SECTION

The analog (metering) interface described in this section is

designed for measuring 220V/80A with precision better than

Class 1.

The most important external components for the SA2002D

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.

Voltage Input IVP

The voltage input of the SA2002D (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

Voltage Divider

The voltage divider is (Figure 3) calculated for a voltage drop of

14V. Treat C7 as a short circuit and ignore J16. Equations for

the voltage divider in figure 3 are:

RB = R1 + R2 + R3 + R15 + R10

RB = R12 || ( R11 + P1 / 2)

Combining the two equations gives:

(RA + RB) / 220V = RB / 14V

Values for resistors R10 = 47W, R11 = 22kW, P1=10kW and

R12 = 1MW is chosen.

Substituting the values result in:

RB = 26.29kW

RA = RB x (220V / 14V - 1)

RA = 386.84kW

Figure 3: Mains voltage divider

R1

R2

R3

R10

R11

R12

NEUTRAL

C7

R15

J16

P1

LIVE IN

Standard resistor values of R1, R2, R3 and R15 are chosen to

be 47kW, 150kW, 47kW and 150kW. The resistor RA is divided

so that the voltage drop across one resistor is within the

specified limits of the resistor type that is used, usually

200VDC. The resistor value ratios are chosen so that shorting

J16 the module could be used on a 110VAC mains system.

CURRENT SENSOR INPUT RESISTORS FIGURE 4 (USING

A SHUNT RESISTOR)

The resistors R6, R7 define the current level into the current

sense inputs the device. The voltage drop across the shunt

should be at least 20mV at rated conditions. A shunt resistor

with a value of 50mV @80A was chosen for the application

module for its ease of use. The resistor values are calculated

for an input current of 16µA on the current inputs at rated

conditions. For a 80A meter the resistor values are calculated

as follows:

R6 = R7 = (I / 16µA) x R / 2

L

SH

= 80A / 16µA x 625uW / 2

= 1.5625kW

I

Line current

L =

A standard value of 1.6kW is chosen.

Figure 4: Current input configuration

R6

R7

R14

LIVE IN

LIVE OUT

L