P. 1
AN001: Introduction to ABC’s CT3613xxxxSA CT Products (Jan. 2021)
(1). Introduction to Current measurement:
The basic issues regarding the measurement of
electric current:
(i). No matter in traditional or modern
electronic/electrical engineering, for the safety,
reliability and functionality of the whole system,
voltage and current at critical points are always to
be measured and/or controlled all the time.
(ii). Voltage measurement, excluding the extra
high voltage, is not difficult. It can be achieved by
a few basic electronic components in the circuitry,
with very low cost, little loss and little troubles.
However, current measurement is usually not very
convenient to carry out. It could easily cause
interference with the circuit it measures, cause
significant power loss plus safety concerns in
some circumstances.
(iii). If the accuracy of current measurement with
a wide range of magnitude is also demanded, it
will significantly increase the complexity and
difficulty of this task.
(2). How to carry out the proper current
measurement in electronic/electrical systems:
(i). The most simple and popular way: To place a
current sensing resistor (R
SENSE
in Fig. 1) on the
path of that current flow.
Acquire the voltage across R
SENSE
, and get the
value of the current flown by using the equation:
I
LOAD
= V
OUT
/ (R
SENSE
*Gain). (as Fig. 1)
(ii). The basic precautions of R
SENSE
in Fig.1 are:
* The resistance needs to be as small as possible
when compare to load impedance, so that it
won’t affect the original circuit’s performance.
* To reduce the unwanted power loss, the
resistance also needs to be as small as possible.
* The resistance needs to be reliable and stable
enough, so that there is always a reliable reading
of the voltage drop on it.
(iii). Other precautions in current measurement:
* Voltage rating: If the VSUPPLY is too high (below
Fig. 2), one may not find appropriate amplifier
circuit to suit this application.
* Safety Isolation: If the VSUPPLY is hazardous
voltage to human (e.g. mains electricity, grid
power), then the next-stage circuit needs to be
isolated from the grid power if the next-stage is
accessible by men.
* High frequency noise & surge spike on the
current sensed need to be expelled from
next-stage circuitry for a precise and stable
measurement: certain type of isolation devices
needs to be placed in between them.
(3). CT (abbreviated as CT) is the best measure
to all above issues:
(i). Theory of operation of CT: All principles (e.g.
Faraday’s & Ampere’s Law) related to a voltage
transformer are to be obeyed in CT as well.
Fig. 1
Fig. 2
P. 2
(ii). Allow the “large” current flow thru the
primary side and acquire a scaled clone of that
current on the secondary side. (scaling ratio =
reciprocal of turn ratio)
Equivalent circuit of a typical transformer:
(iii). Bring everything from secondary side to
primary side: For a CT with a turn ratio of 1:1000.
If we have a load resistor (Z2 in Fig. 4) of 100 Ω,
then it appears only as 0.1mΩ on the primary side,
which is very good for a current measurement
device and the whole system operation.
(4). Basic requirements of a CT:
* Magnetic cores with high permeability & good
saturation capability to achieve very high
magnetizing inductance and allow large flux
density (volt-second product).
* B-H curve of magnetic cores with good linearity.
* Low dissipation loss: Low copper loss (low
DCR/ACR), low magnetic core loss.
* Acceptable accuracy under wide operating
frequency range: to achieve this, it must have low
leakage inductance, low parasitic capacitance and
low dissipation loss.
* All characteristics with low temperature drift.
*Appropriate turn ratio to best suit the desired
applications.
Basic problems of a CT:
* Simple CT can only work on AC current but not
DC current.
* It is not practical, if not impossible, to demand
high accuracy of measurement of AC current with
a very wide operating frequency range. Accuracy
is a big problem due to the reactive loss at high
frequency.
* Waveform of primary current (current to be
sensed) and accuracy of measurement may get a
bit distortion at secondary side due to the nature
of nonlinearity of magnetic cores, especially when
it is at two extreme sides of flux density level
(very large and very small ones). (Fig. 5)
* Higher cost of CTs compared to using electronic
circuit for AC current measurement (in the case of
non-isolated application).
(5). Applications: In regard to the magnitude
of the current measured:
(i). For very small magnitude of current (roughly
1uA ~ 100mA), there are special techniques
(circuit) to do a precise measurement, which is
out of the scope of this application note.
(ii). For extra-large magnitude of current (200A ~
up to tens of kiloamperes), there are large current
measurement device (such as the one shown in
Fig. 3
Fig. 4
Fig. 5