Abstract :
The
measurement of electric current strength is not always easy, especially when
the measured signal requires further electronic conditioning. Simply connecting
an ammeter to an electrical circuit and reading out the value is no longer enough.
The current signal must be fed into a computer in which sensors convert current
into a proportional voltage with minimal influence on the measured circuit. The
basic sensor requirements are galvanic isolation and a high bandwidth, usually
from DC up to at least 100 kHz. Conventional current measurement systems
therefore tend to be physically large and technically complex.
BY
MEASURING THE VOLTAGE ACROSS THE RESISTORS
The oldest
technique is to measure the voltage drop across a resistor placed in the
current path. To minimize energy losses the resistor is kept very small, so the
measured voltage must be highly amplified. The amplifier’s offset voltage must
be as small as possible and its supply voltage must be at the potential of the
circuit, often 110v mains with high parasitic peaks from which its output must
be isolated. This requirement increases overall system cost
BY USING TRANSFORMER
Another
wide spread principle is the transformer. Its construction is much simpler, but
it doesn’t allow the measurement of DC signals. Isolation between primary and
secondary sides is implicitly given. A problem is the limited frequency range.
HALL CURRENT SENSORS
This is an
improved current sensor. Hall sensors also measure the magnetic field
surrounding the conductor but, unlike current transformers, they also sense DC
currents. A
circular core of soft magnetic material is placed around the conductor to
concentrate the field. The Hall element, which is placed in a small air gap,
delivers a voltage that is proportional to the measured current. This sensor also offers
galvanic isolation.
The very
small output voltage of the hall element must be highly amplified and the
sensitivity is temperature dependent and requires adequate compensation. There
is an inevitable offset, that is; a small DC voltage at zero current; the
offset amplitude and temperature coefficient are subject to significant
fluctuations.
The two
types of Hall Effect current sensors are open loop and closed loop. In the
former, the amplified output signal of the hall element is directly used as the
measurement value. The linearity depends on that of the magnetic core. Offset
and drift are determined by the hall element and the amplifier. The price of
these sensors is low, but so is their sensitivity.
Closed loop
hall sensors are much more précis. The hall voltage is firstly amplified, and
the amplifier’s output current then flows through a compensation coil on the
magnetic core. It generates a magnetization whose direction is opposite to that
of the primary current conductor. The result is that the magnetic flux in the
core is compensated to zero. The nonlinearity and the temperature dependence of
the hall element are thus compensated but the offset remains. Closed loop
current sensors work up of frequencies of 150 kHz. They are not cheap, and for
high currents they become very bulky.
Conventional
sensors are physically large and technically complex; also they have disadvantages
as stated above. Hence they are replaced by magnetoresistive current sensors.
The magnetic field sensors are based on the magnetoresistive effect. These
sensors can be easily fabricated by means of thin film technologies wit widths
and lengths in the micrometer range. To reduce temperature dependence, they are
usually configured as a half bridge or a full bridge. In one arm of the bridge,
the barber poles are placed in opposite directions above the two magnetoresistors,
so that in the presence of a magnetic field the value of the first resistor
increases and the value of the second decreases.
Download :
Download :