PM4104APDA
ANALOG INPUTS
Using a Shunt Resistor
The most important external circuitry required for the
SA4104A/B are the voltage sense input and current sense
input networks. These circuits translate the mains voltage
and load current into signals that can be sensed by the
energy metering device. These networks should be
constructed using good quality resistors and capacitors to
ensure adequate immunity to temperature and noise.
Figure 2 shows the circuit diagram of the current sense
input network when using a shunt resistor to sense the line
current as it is implemented on the PM4104APDA. The
shunt is connected externally.
The shunt resistor should be selected so that the voltage
drop generated at maximum rated mains current (IMAX) is
larger than 10mVRMS and smaller than 100mVRMS. At
maximum rated mains current the current input network
should be designed to supply an input current of 16μARMS to
the current sense inputs (IIP and IIN) of the SA4104A/B.
The current sense inputs saturate at an input current of
±17.6μARMS (±25μAPEAK), so this allows about 10%
headroom until saturation occurs. Referring to Figure 2, the
resistors R3 to R6 define the current flowing into the energy
metering device. The optimum input network is achieved by
setting the input resistors equal, i.e. setting R3 = R4 = R5 =
R6. Noting that the energy metering device creates a virtual
short circuit between the differential current sense inputs
(IIP and IIN) the value for the current input resistors can be
calculated as follows:
The functions of the current and voltage input networks are
the following:
n
to sense the load current and mains voltage and
convert them to signals that are appropriate for the
SA4104A/B,
n
n
to allow calibration of the meter,
to compensate for any phase shift present when a
current transformer is used for current sensing and
to filter all high frequency noise and other disturbances
in the current and voltage signals in order to maintain
adequate accuracy when electromagnetic disturbances
are applied to the energy meter.
n
Current Sense Input Network
A typical single phase energy metering system can use
either a shunt resistor or a current transformer (CT) as
current sensing element. The PM4104APDA has therefore
been designed to be used with either a shunt or a current
transformer.
VMAX
R3 = R4 = R5 = R6 =
= RC
4×16µA
where VMAX is the voltage drop across the shunt resistor at
maximum rated mains current. Assuming a 320μΩ shunt
and an IMAX of 40A the VMAX of the shunt is 12.8mVRMS and
therefore the current input resistors need to be RC = 200Ω.
The PM4104APDA evaluation module has been set up for a
meter with an IMAX of 40A and using a shunt resistor with a
resistance of 320μΩ. The current input network can easily
be reconfigured for any other IMAX or any other shunt resistor
by simply changing the current input resistors (R3, R4, R5
and R6) as required. The evaluation module can also be
adapted to use a current transformer by adding the burden
resistors (R1 and R2) and changing the current input
resistors.
To reconfigure the PM4104APDA for a different IMAX or a
different shunt value the current input resistors should be
changed.
JS1 “0”
Current In
GND
JS1 “P”
R5
R3
R6
R4
IIP
16μARMS
C2
C1
SHUNT
RESISTOR
SA4104A
SA4104B
GND
GND
IIN
JS1 “N”
Current Out
Figure 2: Current input network on the PM4104APDA when using a shunt resistor as current sensing element
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