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PM9604

THREE PHASE POWER/ENERGY METERING MODULE FOR SPI BUS AND PULSE OUTPUT DEVICES

South African Micro Electronic Systems 

PM9604/5AP

7. Circuit Description

The module is supplied from an on-board DC supply derived from the incoming

three phases.

The Application Circuit (Figure 2) details the components required for a three phase

power metering module. Terminated current transformers are used as current

sensors.

The most important external components are:

Terminated current sensors (current transformers) are connected to the current

sensor inputs of the devices through current setting resistors (R8...R13).

The resistor values should be selected for an input current of 16µARMS into the

integrated circuit current sensor inputs at the rated line current.

The values of these resistors are calculated as follows:

Phase 1:

R8 = R9 = (IL1/16µARMS) x R17/2

Phase 2:

R10 = R11= (IL2/16µARMS) x R18/2

Phase 3:

R = R = (I /16µA ) x R /2

12

13

L3

RMS

19

Where ILX

=

R17, R18 and R19 =

Secondary CT current at rated conditions.

Termination resistors of the three current

transformers.

R1A, R1B, R4 and R14 set the current for the phase 1 voltage sense input. R2A + R2B,R5

and R15 set the current Phase 2 and R3A + R3B, R6 and R16 set the current for Phase

3. The values should be selected so that the input current into the voltage sense

input (virtual ground) is set to 14µARMS at nominal line voltage. Capacitors C1, C2

and C3 are for decoupling and phase compensation.

Resistors R7A + R7B and trimpot T7 defines all on-chip bias currents. R7 is made up

of two resistors which can be connected in parallel using JP7 to allow convertion of

the module between the SA9604A and SA9605A devices.

The SA9604A uses a reference current of 25µA ± 10% and the SA9605A reference

current is 50µA ± 10%. Inserting the jumper will connect R7A and R7B in parallel,

thereby doubling the reference current to an appropriate level. T may be varied for

7

calibration purposes.

XTAL is a colour burst TV crystal (f = 3.5795 MHz) for the oscillator. The oscillator

frequency is divided down to 1.7897 MHz on-chip to supply the digital circuitry and

the A/D converters.

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