Meter hardware

AN2288

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4.1

Main blocks

These are the main blocks in the Meter:

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5V Power supply block

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3.75V reference block

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Current Transformer block

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Voltage divider block

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Tamper detection block

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Gain switching block

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EEPROM block

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RTC block

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LCD Module

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In-Circuit Programming block

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Calibration through PC GUI

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Microcontroller block

4.1.1

5V power supply block

The 5V capacitive power supply is based on full-wave rectification. The positive half wave is

used to charge the capacitor and as well as negative half wave of sinewave is also used to

charge the capacitor in the same direction. The zener diode (ZD1) tells to which voltage the

C3 is charged. Voltage regulator U1 utilizes the energy stored in C3 to produce a stable

output voltage. Resistor R1 controls the charge and discharge of C1 and also limits the

current flow through zener diode ZD1. Two inductors (L1 and L2) are used to reduce the

noise in power supply.

Figure 5.

Capacitive power supply

4.1.2

2.5V reference block

A potential divider is used to get a 3.75V reference. As some current (I2) flows towards the

voltage follower (refer to Figure 6), the current through R5 is higher than that through R6. If

R5=R6, there is a voltage drop of R5*I2 in the reference. This voltage drop is variable, which

depends on the current flow through reference I2. If I2 is negligible and constant, this

3.75Vref will be ~3.75 and constant. So, a voltage follower is used to get ~3.75V constant

reference.

L7805

R1

C1

D1

D4

C2

R3

ZD1

C5

C3

C6

Phase line

Neutral line

5V

L1

L2

C4

R2

D2

D3