CLOUD ACCESS
How to deliver reinforced current and power measurement for IoTrelated
devices. By Hans-Günter Kremser
With the exponential growth of
connected devices such as
refrigerators, and washing
machines in private households, the
demand for IoT-connected devices
has also increased in industrial
applications, for example in the form
of remote monitoring of machines to
predict potential failures.
In order to monitor any mainspowered
devices, there is a simple
way to design a system that
measures the current and voltage of
a 230-VAC-powered device and the
placement of the monitored current-
and power-data into the cloud.
It is important to note that the
described design can be used to
measure current, voltage, active
power, apparent power and power
factor but it is not a VDE-certified
solution for an electricity meter
because such systems must sample
voltage and current simultaneously.
The only difference is the multiplexed
sampling but the rest of the system
could also be applied to an e-meter/
smart meter.
It is possible to prototype an
isolated current sensing system
with an isolated amplifier and a
SimpleLink Wi-Fi CC3200 LaunchPad
development kit. The system has
a peak galvanic isolation of up to
7 kV, compliant with the Verband
der Elektrotechnik, Elektronik und
Informationstechnik V 0884-11 and
Underwriters Laboratories 1577
standards.
Using evaluation modules
(EVMs) shortens development time
drastically because you don’t have to
develop the hardware, and can test
the main functions before starting
development.
In the example described here,
we have used the AMC1311 EVM to
match the input voltage range of the
Launchpad development kit, which is
intended to measure ground-based
voltages.
This EVM includes two operational
amplifiers (TLV6001) which are used
to convert the differential output of
the isolated amplifier to a single
ended output and adjust the output
common mode voltage to match the
ADC’s input common mode voltage.
A multiplexer before the ADC
alternately samples the mains voltage
and current. A single-chip wireless
microcontroller with an Arm Cortex-M4
core calculates the root-mean-square
(RMS) voltage, RMS current, active
power, apparent power and power
factor, as shown in Figure 1.
Because the European Union set
the standard for the mains voltage
in Europe as 230 V, ±10%, the
measured current in the prototype
system will also vary by ±10% (plus
Figure 1: Power
meter block diagram
Figure 2: Voltage
measurement
schematics
component tolerances). For voltage
and current measurements, the
isolation amplifier evaluation module
provides not only an isolation barrier
for the signal but also an isolated
power supply.
Due to relatively small signal
amplitudes and to avoid additional
spikes on the signals, the secondary
side of the isolated amplifier is
powered by a non-switching power
supply based on a transformer,
rectifier and the LM7805 fixed voltage
regulator.
The average current drawn from
the power supply has been measured
with 200 mA by an ampere-meter.
The drop voltage of the lowdropout
regulator (LDO) could be an
area of concern; therefore this has
to be analysed: The voltage drop
is 5 V, which results in a power
dissipation of 1 W. With a thermal
resistance of 23.9°C/W, the LDO
will heat up to ambient temperature
plus 23.9°C. That is about 45°C,
which is well below the operating
free-air temperature range of 125°C,
specified in the LM7805 datasheet.
The primary side is powered by an
isolated supply based on the SN6501
22 28 April 2020 www.newelectronics.co.uk
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