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| 型号： | 71M6543F-DB-CT |
| 厂商： | Maxim Integrated Products |
| 文件页数： | 51/91页 |
| 文件大小： | 0K |
| 描述： | DEMO BOARD 71M6543F-DB-CT |
| 标准包装： | 1 |
| 系列： | * |
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71M6543 Demo Board User’s Manual
2.5.1 SELF-HEATING
The effect of self-heating will be most pronounced at maximum current and depends on the following parame-
ters:
?
?
?
?
?
Nominal shunt resistance
Current through the shunt resistor
Thermal mass
Heat conduction away from the shunt (thermal resistance towards the environment)
Temperature coefficient of copper and resistive material.
It is quite obvious that the nominal resistance of the shunt resistor should be kept as low as possible. Table
1-10 shows a few combinations of shunt resistance and 71M6x0x part number. The parts with part numbers
corresponding to higher current capacity are designed to work with low shunt resistance. Lowering the shunt re-
sistance below the recommended limits decreases accuracy and repeatability.
Good heat conduction can help to maintain the shunt temperature. Attaching the shunt to solid metallic struc-
tures such as meter terminal blocks helps decreasing the thermal resistance. This, of course, applies to meters
where the terminals and other mechanical parts can be considered heat sinks, i.e. they do not heat up due to
other effects.
The thermal mass will control how long it takes the sensor to reach its maximum temperature. Meters, for which
only short-time maximum currents are applied, can benefit from a large thermal mass, since it will increase the
time constant of the temperature rise.
The temperature coefficient (TC) of the shunt is a very important factor for the self-heating effect. Shunts with a
TC of just a few PPM/°C can maintain good shunt accuracy even in the presence of significant self-heating.
There are several methods that can be applied in the meter design to minimize the effects of self-heating:
?
?
Software algorithms emulating the thermal behavior of the shunt(s).
Direct temperature measurement, ideally with the 71M6xx3 mounted directly on the shunt (collocation)
or employing some other method of temperature sensing (PTC resistor, NTC resistor, discrete temper-
ature sensor).
The effect of shunt self-heating can be described by the following formulae. First, the relative output of a shunt
resistor is:
Δ V/V = Δ R/R
Δ R is a function of the change in temperature, the temperature coefficient (TC R ), the thermal resistance (R TH ),
and, of course, the applied power, which is proportional to the square of the current:
? V
V
=
? R
R
=
R ? ? T ? TC R
R
= I 2 R ? R TH ? TC R
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.
2.5.2 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 an
ANSI-type 16S meter, the distance between the phase A sensor and the phase B sensor is roughly 25 mm,
which makes these two phases most critical for cross-talk. For the ANSI form 2S meter, which is a frequently
used single-phase configuration, the distance between the sensors is in the range of 70 mm, 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.
2.5.3 PLACEMENT OF SENSORS (IEC)
The arrangement of the current terminals in a typical IEC meter enclosure predetermines the spacing of the
shunts, and usually allows for only for 20 to 22 mm center-to-center spacing between the shunts. This means
that the clearance between adjacent shunts is typically only 10 mm or less. A typical arrangement is shown in
Page: 51 of 91
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