Maxim Integrated 71M6541 Demo Board - Manual (Page 54)

71M6541 Demo Board REV 3.0 User’s Manual
54 Rev 4.0
The effect of shunt self-heating can be described by the following formulae. First, the relative output of a shunt
resistor is:
ΔV/V = ΔR/R
ΔR is a function of the change in temperature, the temperature coefficient, the thermal resistance, and, of
course, the applied power, which is proportional to the square of the current:
R TH
R TC R R I
R
TC T R
R
R
V
V
⋅ ⋅ =
⋅ ∆ ⋅
=
∆
=
∆
2
Ultimately, it is up to the meter designer to select the best combination of shunt resistance, TC, shunt geometry
and potential software algorithms for the given application.
2.6.2 PLACEMENT OF SENSORS (ANSI)
The arrangement of the current terminals in an ANSI meter enclosure predetermines shunt orientation, but it al-
so allows for ample space in between the sensors, which helps to minimize cross-talk between phases.
A good practice is to shape the shunts like blades and to place them upright so their surfaces are parallel. In a
16S meter, the distance between the phase A sensor and the phase B sensor is roughly 1”, which makes these
two phases most critical for cross-talk. For the form 2S meter, which is a very frequently used single-phase con-
figuration, the distance between the sensors is in the range of 2.75”, which makes this configuration much less
critical. However, even for this case, good sensor placement is essential to avoid cross-talk.
Sensor wires should be tightly twisted to avoid loops that can be penetrated by the magnetic fields of the sen-
sors or conductors.