Introduction - Control of Power Inverters in Renewable Energy and Smart Grid Integration - page 36

Table 1.3 Recommended PCB connections for LAH 25-NP (LEM n.d.a.)

Primary
maximum
current (A)

Primary
nominal
current (A)

Nominal
output
current (mA)

Recommended
PCB connections Turns ratio

55 25 1:1000 25

27 12 2:1000 24

18 8 3:1000 24

is connected on the primary side so that the current caused by the measured voltage is around
10 mA. The output is also a current source, which can be converted into a voltage signal with
the measuring resistor R M . In order to obtain the best accuracy, the current flowing through
R 1 should be designed to be around 10 mA according to the voltage measured. Note that the
turns ratio is 2500:1000 and the primary coil resistance is 250 .

Assume that the circuit with LV 25-P shown in Figure 1.34(b) is adopted to measure the DC
bus voltage of an inverter, which is up to 800 V, and the maximum voltage for analog inputs
is 3 V. If the measuring resistor is chosen as 100

, then the maximum secondary current

3 V 30 mA

2

is

resistors are connected in series as the primary resistor R 1 , then the maximum DC bus voltage
allowed is 12 mA

R M =

30 mA, which corresponds to a primary current of = 12 mA. If two 33 k

. 5

× 2 × 33 . 25 k = 798 V. The maximum power dissipated by each primary

resistor is (12 mA) 2

4.752 W so the power rating can be chosen as 5W. Note that
the primary current is around the rated value 10 mA.

× 33 k =

1.3.5 Signal Conditioning
In general, signals obtained from sensors are often not compatible with the requirements of the
inputs to the controller and need further conditioning, which includes impedance matching,
scaling, level-shifting, filtering, converting, linearisation, isolation, etc. For example, the output
voltage provided by a voltage sensor may be bipolar but the voltage level required by the analog
inputs of a DSP is often 0 ∼ 3V.

1.3.5.1 Impedance Matching

Since the input impedance of an analog input of micro-controllers and DSP is often not very
high, it is a good practice to use an op-amp driver circuit for signal conditioning of analog
input signals and also as a buffer. This reduces the loading effect to the sensor circuits and