1: ZigBee Essential Facts and Features > Low Power Consumption

Low Power Consumption

An important aim of ZigBee and the IEEE 802.15.4 standard is the provision for building autonomous, low-powered devices. These devices are powered from an internal source, such as a battery pack or solar cells, and therefore need no external power supply or power cabling. There are many wireless applications that require this type of device, from light-switches, active tags and security detectors to remote industrial control and monitoring. This page describes the advantages of and the methods of achieving such devices.

Advantages of Autonomy

From a user perspective, autonomously powered devices have the following advantages:

• Easy and low-cost installation of devices: No need to connect to a separate power supply
• Flexible location of devices: Can be installed in difficult places where there is no power supply, and can even be used as mobile devices
• Easily modified network: Devices can easily be added or removed, on a temporary or permanent basis

Achieving Autonomy

Autonomous network devices are generally battery-powered or solar powered:

• Battery-powered devices: Since these devices are generally small, they use low-capacity batteries. Infrequent device. maintenance is often another requirement, meaning long periods between battery replacement and the need for long-life batteries
• Solar-powered devices: Solar power is an example of “energy harvesting” in which the device absorbs and stores energy from its surroundings. In this case, the energy is collected in the form of light and the device must be located in a well-lit environment.

The power consumption of these devices must be carefully managed to make optimum use of very limited power resources over an extended period of time. These devices therefore present significant technical challenges to keep power consumption low. The methods employed to minimise power consumption include the following:

• Low duty cycle: Most of the power consumption of a wireless network device corresponds to the times when the device is active - that is, transmitting or receiving. The active time as a proportion of the time interval between activity is called the duty cycle. Power use is optimised by using extremely low duty cycles, so that the device is active for a very small fraction of the time. This is helped by making the transmission/reception time short and the time interval between transmission/reception long.
• Sleep mode: When not transmitting or receiving, the device should revert to a sleep mode during which the power consumption is minimal.
• Modulation: The modulation schemes used to transmit data (BPSK for 868/915 MHz, O-QPSK for 2400 MHz) minimise power consumption by using a peak-to-average power ratio of one.

	In practice, not all devices in a network can be autonomously powered, particularly those that need to be switched on all the time (and cannot sleep), such as Routers and Co-ordinators. Such devices can often be installed in a mains-powered appliance that is permanently connected to the mains supply (even if not switched on) - for example, a ceiling lamp or an electric radiator.

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