sames

SA2002P

10/14

http://www.sames.co.za

TYPICAL APPLICATION

The analog (metering) interface described in this section is

designed for measuring 230V/40A with precision better than

Class 1.

The most important external components for the SA2002P

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.

The voltage drop across the shunt resistor at rated current

should be at least 20mV. A shunt resistor with a value of 625µ

is chosen. The voltage drop across the shunt resistor is 25mV

at rated conditions (I

). The power dissipation in the current

sensor is:

P = (40A) x 625µ

= 1W.

Reference Voltage Bias resistor

Current Sense Resistors

R6 defines all on chip and reference currents. With R13 = 24k

optimum conditions are set.

The resistors R6 and R7 define the current level into the

current sense inputs of the device. The resistor values are

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

the SA2002P at rated conditions.

According to equation described in the current sense inputs

section:

R6 = R7 = ( I / 16µA ) x R

/ 2

= 40A / 16µA x625

/ 2

= 781.25

A resistor with a value of 820

is chosen, the 5% deviation

from the calculated value will be compensated for when

calculating the values in the voltage path.

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Shunt Resistor

max

2

L

SH

Voltage Input IVP

Voltage Divider

Programming

The voltage input of the SA2002P (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 + 5%

(14.7V). Equations for the voltage divider in figure 4 are:

RB = R1 + R2 + R3 + R8

RB = R11 || R12

Combining the two equations gives:

(RA + RB) / 230V = RB / 14.7V

Values for resistors R8 = 10 , R11 = 24k

and R12 = 1M

is

chosen.

Substituting the values result in:

RB = 23.437k

RA = RB x (230V / 14.7V – 1 )

RA = 343.27k .

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

150k , 150k and 43k .

A combination for R1, R2 and R3 with 110k, 110k and 110k

would also be sufficient.

In order for the meter to function correctly the EEPROM must

be programmed with the relevant parameters eg. LED pulse

rate, Motor pulse rate, Creep threshold, AC mode

measurement direction and rated current.

The calibration is preformed as described in the previous

section.

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