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Guide PoE: Power supply via LAN cable


Power over Ethernet (PoE) has long been the standard in the business environment for powering cameras, telephones etc. Thanks to falling prices, PoE is now also interesting for private users.

The Internet of Things needs electricity and internet access. Power over Ethernet has established itself so that you don’t need additional power supplies, power cables and sockets in addition to a LAN cable. IP phones, surveillance cameras and network equipment almost all run via PoE as standard.

In the meantime, a wide accessories market has established itself. In addition to switches and injectors that feed the power into the network cable and telephones or IP cameras with integrated PoE, there are adapters that connect USB devices such as the Raspberry Pi with power and LAN – ideal for smart home applications, for example. Network switches or WLAN access points that do not require any additional power supply are of course also a problem against cable clutter – and those who have connected a garage or garden shed with LAN can use the power distribution switches to not only use the network equipment, but also to forward the electricity also supply other devices with energy.

Passive PoE offers almost endless possibilities: You do not do without your own power supply for components such as LED strips or network receivers, but at least you do not need a socket locally – the network cable is used for the purpose of power supply.

There is “standard PoE”. This usually means the standards IEEE802.3af and IEEE802.3at. An injector or network switch feeds a voltage of around 48 volts into the LAN cable – and that is “safely”, that is, it is irrelevant whether the device connected to the switch needs or supports PoE. End devices may use a maximum of just under 13 watts (802.3af) or 25.5 watts (803.at) of energy, potential losses due to the cable length are already included.

For comparison: A single-board computer like the Raspberry Pi has the current version 4 an energy consumption of 7.6 watts under load. Even with the “small” PoE standard, there is still enough reserve to operate a 7-inch LC screen with a touchscreen, for example. Even the iPad charger from Apple only produces 12 watts, so even operating a tablet via PoE is not a problem – provided the appropriate adapter is used.

The alternative to the standard is Passive PoE. One adapter is connected to both ends of the network cabling; on one side you feed the electricity, on the other you take it out again – without any big rules. This is extremely inexpensive and flexible, since you can ultimately do everything you want via the network cable. However, you have to know exactly what you are doing here – and you should never plug any devices into the cable without having previously connected the counterpart of the PoE injector. In addition, there is no control over line losses – the voltage at the other end of the cable will be lower than at the entry point – and over the maximum power, which can cause cables, plugs, etc. to heat up.

A PoE-compatible switch.

Network switches with integrated PoE have a stronger power supply unit than normal switches and feed the energy themselves. They also take over the “handshake” with the end device, so make sure that the devices fit together. Since corresponding products with four outlets are available from around 25 euros, there is hardly any reason to look for other solutions. A switch is connected to the free network port of the DSL router like the Fritzbox with a network cable and provides several ports for other end devices – in this case with a power supply.

When buying such a switch, you should still keep your eyes open: in addition to products that feed the energy supply for other devices, there are also switches that operate themselves via PoE so that they do not need their own socket. This is also called PoE Powered Switch or PD. Pass-through switches such as the Netgear GS105PE take on a special form, which are both powered by PoE themselves and can supply two additional devices with power without the need for their own power supply.

A PoE injector

In the past, PoE injectors were common, which have taken on the same task for only one end device, that is, so to speak, are “plugged in”. However, due to the drop in the price of PoE switches, there is hardly any left. Only the passive PoE injectors, which are usually supplied in a double pack, have a certain reason to exist: on the one hand, the energy is fed in from any source, such as the supplied power supply unit of an IP camera, on the other hand, the electricity is drawn again and connects LAN and power to the end device. Such sets are available from around 5 euros, but as I said: be careful when using them, you should know what you are doing.

The various PoE adapters are particularly interesting for hobbyists, which supply devices such as single-board computers (Raspberry Pi & Co.) with energy, but can also operate tablets or smartphones thanks to the USB or Lightning connection – or ultimately any other product with USB Power supply.

There is a small trap with USB-C: While many devices like the Raspberry Pi like “old” USB devices want to be supplied with 5 volts, there are other voltages with USB-C. The author of these lines, for example, stumbled upon the fact that a Nest camera with USB-C does not work on the PoE adapter. The reason: The camera needs 19 volts. Corresponding adapters are currently in short supply, but can be obtained with a little research in China, for example from Aliexpress. However, the adapter has not yet been delivered, which is why further test reports are still pending.

Admittedly, the power supply via PoE is a bit nerdy – but it is super practical, saves cables and power supplies, and with the right products also cheap and efficient. A changeover without a reason is certainly not necessary, but you should pay attention to the support as a precaution when purchasing network equipment in the future. Anyone who is looking for a solution for the power supply in the smart home environment for Raspberry, camera, phone or tablet including wall bracket will find inexpensive solutions.

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