32
effectively set the minimum resolvable current)
of +/- 20 mV out of a full scale value of +/- 4 V.
Consequently, a 500 A full scale model will be
unable to resolve differences of less than 2.5 A
accurately. Another downside is the result of the
magnetic core, which puts a hard upper limit
on the overload rating because once the nulling
circuit runs out of current to counteract the field
from the conductor the core will quickly satu-
rate. The aforementioned 500 A full scale model
has an overload rating of 900 A, which might
seem impressive until you consider that most
shunts can easily tolerate brief overloads of 10x
or more (though, again, whether a useful signal is
still obtainable depends on the signal condition-
ing circuit).
A similar type of sensor - especially in physical
form factor and typical output scaling - uses the Giant
MagnetoResistance effect, or GMR, in which the bulk
resistance of alternating layers of magnetic and non-
magnetic materials changes in proportion to magnetic
field intensity. The main advantage of GMR over Hall
effect is higher sensitivity - so it is capable of measur-
ing lower currents accurately - but one major downside
for OEMs is that relatively few manufacturers offer
current sensors based on GMR at this time (it first
found wide application in hard disk drive read heads).
The last commonly used type of current sensor is
the current transformer, which comes in two very
different flavors: conventional and Rogowski. TheRogowski Coil
IpBurdenIsNp : NsIpIdeal Current Transformer
The last commonly used
type of current sensor in
EV systems is the current
transformer, which comes
in two very different
flavors: conventional and
Rogowski.
conventional current transformer usually consists of a
single-turn primary and a 10- to 1,000-turn secondary
wound on a magnetic core material appropriate for the
frequency range. Since transformers conserve amp *
turns, the current in the secondary will be reduced by
the turns ratio, and only a load, or burden, resistor is
then needed to scale the output voltage. For example,
to generate a 1 V signal at 10 A with a shunt requires a
resistance of 0.1 Ω which will have a peak loss of 10 W
(I^2 R again), but with a 1:100 current transformer the
secondary current will be 100 mA, requiring a 10 Ω
burden resistor to develop the same 1 V while reduc-
ing the loss to 100 mW. Current transformers are also
highly linear and can achieve nearly as wide a dynamic
range as shunts, with isolation thrown in for free. TheTHE TECH