AN2288 datasheet(11/29 Pages) STMICROELECTRONICS

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AN2288

Meter hardware

11/29

Figure 6.

2.5V reference circuit

4.1.3

Current transformer block

The current transformer used has a turn ratio of 1:5000 and ~350 Ohm resistance, the shunt

resistance used is 36 Ohms (refer fig. 2), so, the effective resistance will be:

After multiplication of secondary current with Rshunt, the voltage Vshunt for max current will

be:

Vshunt is multiplied with gain stage to give actual value to input to microcontroller ADC. The

minimum multiplication factor is x2. The MAX current is chosen such that the operational

amplifier should not saturate. So, with x2 the max value of VAIN is:

The peak to peak voltage is 2.03V. There is safe margin of ~0.47V for a gain factor of x2.

4.1.4

Voltage divider block

The voltage is reduced using a potentiometer and referenced at 3.75V.

Figure 7.

Voltage divider circuit

3.75V - R5*I2

I1+I2

3.75Vref

3.75V

shunt

350

-----------------------

32.64E

shunt

55A

5000

-------------

shunt

359mV

AIN MAX

()

359mV

1.015V

2.5V

Neutral line

Phase

R28

R30

R26

R29

R31

C12

Analog input to ADC

C20

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