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 16µA
RMS
into the
integrated circuit current sensor inputs at the rated line current.
The values of these resistors are calculated as follows:
Phase 1:
R
8
= R
9
= (I
L1
/16µA
RMS
) x R
17
/2
Phase 2:
R
10
= R
11
= (I
L2
/16µA
RMS
) x R
18
/2
Phase 3:
R
12
= R
13
= (I
L3
/16µA
RMS
) x R
19
/2
Where
I
LX
R
17
, R
18
and R
19
=
=
Secondary CT current at rated conditions.
Termination resistors of the three current
transformers.
R
1A
, R
1B
, R
4
and R
14
set the current for the phase 1 voltage sense input. R
2A
+ R
2B
,R
5
and R
15
set the current Phase 2 and R
3A
+ R
3B
, R
6
and R
16
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 14µA
RMS
at nominal line voltage. Capacitors C1, C2
and C3 are for decoupling and phase compensation.
Resistors R
7A
+ R7
B
and trimpot T
7
defines all on-chip bias currents. R
7
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 R
7A
and R
7B
in parallel,
thereby doubling the reference current to an appropriate level. T
7
may be varied for
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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sames
SAMES [ SAMES ]
