isolated current voltage transducers

Hall Effect Technologies
V A
Î S
V CE(sat)
= ?
R S
= 80 Ω
V S
V Cmin
= 14,25 V
R M
= 90 Ω
Figure 12: Diagram for calculation of the available voltage, V A
, at
the output stage
V A
V CE(sat)
= 2,35 V
Example 3: Measuring current higher than the maximum
value given in the datasheet
LEM datasheets indicate the transducer operating conditions,
with a measuring range that is generally limited to 150 to
200 % of the nominal current (I PN
). However, it is possible to
measure currents exceeding this range if the following two
parameters are considered:
• the measurement resistance must not be smaller than the
R Mmin
value given in the datasheet; this minimum value is
required to limit the power dissipated in the transducer
output stage; depending on the selected transducer R Mmin
can be very small, in some cases equal to zero
• the primary conductor temperature must not exceed the
value specified in the datasheet (e.g. 90 °C), to keep the
transducer within acceptable operating temperature levels
The following examples demonstrate the calculation of the
maximum measurable current. Various scenarios are
examined to address different types of LEM transducers.
a) The datasheet requires an R Mmin
value greater than zero.
For example, the following parameters are defined for the
LA 55-P transducer: V C
= 15 V ± 5 %, T A
= 70 °C, R S
=
80 Ω, R Mmin
= 50 Ω, VA = 11.9 V (as calculated in example
2a). The maximum value of the measurable secondary
current is:
I S
= V A
/ (R S
+ R Mmin
) = 11.9 V / (80 + 50)Ω = 91.5 mA
The corresponding maximum measurable primary current
is 91.5 A.
V M
R S
= 80 Ω
R M
= ?
Î S
V S
V Cmin
= 14,25 V
Figure 13: Equivalent diagram to calculate the measuring
resistance, R M
V M
b) The datasheet allows an R Mmin
value equal to zero.
For example, the following parameters are defined for an
LA 305-S transducer: V C
= 15 V ± 5 %, T A
= 70 °C, I P
= 300
A, R S
= 35 Ω (at 70 °C), R Mmin
= 0 Ω, N P
:N S
= 1:2500. The
minimum measurement resistance R M
can be zero and it is
up to the user to select the most suitable value. The
smaller the value the larger the maximum measurable
current, but the smaller the available output voltage signal.
The available voltage at the amplifier output (V A
) is
determined as follows:
R Mmax
= 75 Ω at 300 A for V C
= 15 V, I S
= 300 A/2500 = 120 mA
So:
V A
= (R S
+ R Mmax
) • I S
= (35 + 75)Ω • 0.12 A = 13.2 V
The maximum value of the measurable secondary current
depends on the selected measurement resistance.
For example, with R M
= 5 Ω, we have:
I S
= (V A
-∆V C
)/(R S
+ R M
) = (13.2 V) / (35 + 5)Ω = 330 mA
The maximum measurable primary current is consequently
equal to 330 mA • 2500 = 825 A, corresponding to
2.75 • I PN
. The voltage measured on the measurement
resistance is V M
= 330 mA • 5 Ω = 1.65 V.
With R M
= 2 Ω, we have I S
= 357 mA, a maximum
measurable primary current of 3 • I PN
(892 A) with a smaller
output voltage of 0.71 V.
If a zero ohm measurement resistor is considered, the
transducer can provide I S
= 377 mA, which represents
3.14 • I PN
(943 A). This arrangement provides no output
voltage, so a virtual ground referenced current to voltage
converter will be required to obtain a usable output signal.
It is up to the user to decide exactly what is needed.
Under these higher current conditions it is especially
important to keep the temperature of the primary conductor
below the maximum allowable temperature to avoid
permanent damage.
Example 4: Measuring transient current higher than the
maximum value given in the datasheet
In some cases higher currents can be measured using the
current transformer effect, provided the frequency and/or the
di/dt are high enough. This can be useful for short circuit
detection, as this is typically a high di/dt event, but care must
be exercised when attempting to use this capability for
standard or continuous measurement. See § 3.2.3 for more
details.
It is important to note that some transducers (e.g. LTS) have
an internal measurement resistor value that cannot be
adjusted and, in these circumstances, the measuring range
cannot be adjusted.
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