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Powering the Internet of Things

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article image A moderately small solar panel is perfectly capable of supplying sufficient power to run a small, basic wireless network node

The Internet of Things holds great potential; however it is important to establish how the devices will be powered, especially where mains power or cabled-in power supplies are not available. 

Some of the options that can provide a portable power supply for IoT nodes include solar power, energy harvesting and batteries.

Solar photovoltaic systems are often chosen for sensing, control or measurement devices that are located outdoors where sunlight is available, and consume a relatively small amount of power. For a small, low-power embedded device that receives a reasonable amount of sun each day, a moderately small solar panel is perfectly capable of supplying sufficient power to run a small, basic wireless network node consisting of a microcontroller, some sensors and an embedded low-power Wi-Fi, Bluetooth or 802.15.4/ZigBee radio transceiver. 

However, since solar power is only available for a fraction of the day, solar-powered wireless devices almost always need to incorporate a small amount of energy storage in the form of a battery or supercapacitor in conjunction with the solar cell. Also, solar cells or solar panels typically have a relatively low output voltage if a small number of cells are used, and their non-linear V-I curve makes it desirable to employ Maximum Power Point Tracking (MPPT) where practical.

This is necessary to keep the system operating near the maximum power point so that the limited energy available is harvested most efficiently. A solar power supply for a remote wireless system ideally tracks the maximum power point of the cell along the V-I curve and is able to charge a small battery or supercapacitor to fill in the demand when sufficient sunlight is not available.

The Linear Technology LTC3105 is a high efficiency step-up DC/DC converter that can operate from input voltages as low as 225 millivolts, with a built-in maximum power point controller (MPPC). In addition to solar cells, this device is well suited to other low voltage, high impedance energy harvesting transducers such as thermoelectric generators and fuel cells.

While it is not a true maximum power point tracker, the user-programmable maximum power point setting helps to optimise the efficiency of energy extraction from any energy source, such as a thermoelectric pile or a solar cell, where the voltage across the transducer may vary with changing environmental conditions as well as with the load current. The LTC3105 is capable of supplying 70 mA of output current at 3.3V from an input voltage of 1 volt - sufficient power to run a small, well designed basic sensor node consisting of a microcontroller, RF transceiver and a sensor or two.

Energy harvesting is a type of power supply, made possible by parts such as the Linear LTC3108, which is designed to accommodate energy harvesting from transducers with extremely low output voltages, as low as 20 millivolts. This makes it particularly well suited for use with thermopiles and thermoelectric generators, which can generate a very low potential difference from a realistic temperature difference – a potentially convenient energy source for remote sensing in industrial automation or process monitoring in high-temperature systems where wired communications and power are not convenient.

Energy can also be derived from vibration, and using a part such as the Linear LTC3588 – a piezoelectric energy harvesting power supply controller, which connects to a piezoelectric crystal to harvest mechanical energy in the form of vibrations from the ambient environment. 

Output voltage selections between 1.8V and 3.6V are available with a continuous output current capability of up to 100 milliamps, compatible with a range of modern power-efficient microcontrollers and RF mesh systems-on-chip.

An electromechanical energy harvester of this sort can be employed to provide a continuous source of a small amount of ‘free’ energy for a small, efficient wireless network mote, particularly in applications such as vehicles and industrial machinery where plenty of vibrational energy is available to be harvested in the environment.

For some systems, it is also practical to use batteries such as lithium-ion, lithium-polymer or nickel-metal hydride batteries, and rely on user intervention to simply recharge and replace the batteries where needed. The batteries may be left internally, inside the device, with the system being plugged into a power supply via a charging port when the device requires a recharge, as opposed to the traditional method of removing and swapping the batteries.

In this sort of application, battery management and charging ICs such as the Microchip MCP73833 Li-polymer/ Li-ion charge management controller can be of use to control the recharge of a Li-ion cell, as can buck/boost converters such as the Texas Instruments TPS63031. Buck/boost converters allow a regulated output voltage to be generated from input voltages both higher and lower than the desired output voltage. This enables a battery such as a two-cell NiMH, three-cell NiMH, or single-cell Li-ion/ Li-polymer to be used efficiently and charged and discharged across the entire usable part of its discharge curve without hitting the minimum input voltage of an LDO or buck regulator.

Finding an appropriate source of power for an IoT device is possible; it just takes a little research and a team of dedicated engineers with the experience and knowledge to understand the requirements. 

Product manufacturers can partner with LX Group and benefit from their experience with connected devices, embedded and wireless hardware/software design, and ability to transfer ideas from the whiteboard to the white box while covering the entire gamut of testing, standards, compliance or market dynamics abroad. 

Clients seeking a reliable implementation can partner with LX Group, which is equipped to create or tailor just about anything from a wireless temperature sensor to a complete Internet-enabled system within the required timeframe and budget. 

An award-winning electronics design company based in Sydney, Australia, LX Group specialises in embedded systems design and wireless technologies.

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