Hard-wired Devices

The simplest solution for powering an IoT device is to connect the device directly to the electrical net. This option is only possible for devices that are intended to be integrated into the infrastructure of a building, such as a thermostat or smoke detector. Of course, the electrical net provides AC power at 110-220V, whereas electronic devices typically run at 3.3V or 5V DC. Consequently, an AC/DC converter needs to be used.

The first option is to use an external power adapter, as shown to the right. These adapters are available off-the-shelf and plug directly into a wall outlet in order to provide DC power to the connected device. Adapters are available in a wide range of voltages, though the voltage is usually fixed. Common output voltages are 12V, 9V, and 5V. Power output is typically somewhat low, with output currents ranging between 0.5A and 2A. A barrel jack is often used as an output connector. These power supplies can be purchased from electronic component suppliers such as Farnell or Mouser.

The second option is to integrate integrate an AC/DC power supply into the design of the device itself. Depending on the power requirements of the device, either a small PCB-mounted (left) or a large enclosed (right) power supply can be used. Frequently, commercial products choose to integrate off-the-shelf power modules: power supply design is an art in and of itself, and commercial modules have the advantage that they are already certified for safety and electromagnetic compliance. The designer needs to choose whether to use an integrated power cable in the product, or to rely on a connector for AC power entry. The advantage of using a power entry connector is that different cables can be used depending on the power socket standards of the region (e.g. EU, UK, US) where the product is used. On the other hand, extra connectors add to the total cost of a product. Common standards for AC power entry cables are IEC C13 (with protective earth) and IEC C7 (no protective earth).

Finally, two special cases deserve attention. The first is Power over Ethernet (PoE). As the name implies, this standard offers a way to transmit both power and data over a single ethernet cable. This standard allows the wiring of wireless access points, IP webcams and VoIP phones to be simplified dramatically. The system comprises two aspects. To begin, power needs to be injected into the system. Some (specialized) network routers and switches do this automatically. Alternatively, a separate PoE injector can be used. The second requirement of the system is that the powered device needs to contain the appropriate circuitry to extract power from the ethernet connection. For instance, the Arduino Ethernet shield can be used in conjunction with a PoE module to prototype such a device.

The second special case is USB power. With the advent of smartphones and tablets, USB has become a de-facto standard for low-voltage power applications, even when no data transfer is involved. USB power adapters have become commonplace, and the USB micro connector is used as a charging port on many types of battery-powered devices. According to the current USB specification, USB devices are limited to 500mA at 5V, though some devices draw up to 2A of current. The newest USB specifications have taken this evolution into account: the USB-C connector is better suited for power delivery, and the USB-PD standard allows for charging at higher voltages, allowing up to 100W of power to be delivered. USB-C and USB-PD are fairly recent standards, so adoption is still low.