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Power over Ethernet for MP02 and TP-Link Routers

PoE for a Single Router

Providing Power over Ethernet for MP02 and other routers such as the TP Link desktop routers may be done quite easily with the use of PoE Adapter cables which are available for a cost of typically less that $10.

These are simply passive cables that come in Injector / Extractor pairs, one for the router end and one for the power supply end. These are typically connected by a standard eight wire RJ45 terminated Cat5 cable. At each end there is a RJ45 plug and socket, as well as a power supply plug or socket.

These adapters may be used to place routers in a position where there is not power outlet, or, for example, as part of an arrangement to place a normal indoor router inside a weatherproof enclosure so that it can be used outdoors.

An example of the cables are these ones that are available from DealExtreme (http://dx.com) at this link:

 http://dx.com/p/rj45-network-connector-female-to-rj45-male-plug-dc-5-5mm-female-male-adapter-cable-black-2-pcs-216793#.UwEo7Uu4zWw


PoE for Several Routers

For situations where it is necessary to provide more than one router, it is possible to daisy chain several routers for both network data and power.

This may be useful for providing a router to each of several classrooms in a building where power is not available, or not conveniently located near the router point.

The router must have two LAN ports available in order to support daisy chaining the network.

For an MP02 this means that the WAN port must be re-configured as a LAN port. This is done on the SECN Advanced/WAN configuration page.

Where a site is operated from a solar power system, the routers may be run from a 12V DC supply obtained from the power system.

To provide PoE in a convenient plug-together system, a pair of PoE Injector / Extractor cables, plus a Power Splitter cable is required for for each router.

An example of the cables are these ones that are available from DealExtreme (http://dx.com) at this link:

 http://www.dx.com/p/1-to-2-power-splitter-cable-for-cctv-security-system-camera-dc-12v-112982#.Vpn5H4RTygo

At each router, the Injector / Extractor cables are connected to two LAN Ethernet ports on the router. Data will thus be passed through the switch on the router.

The power connectors are joined using the Power Splitter cable, leaving one power plug to be connected to the router. Thus each router is tapped across the power circuit running through the Injector/Extractor cables.

Design Notes

There are some restrictions on how many routers can be connected this way, from the perspectives of both networking and data.

From a networking perspective, the data from each router will be aggregated in the cable. The routers typically have 10/100Mbps Ethernet ports, so the total data capacity of 100Mbps will be shared between all the routers.

There is also a 100 metre specified maximum limit on the length of the Cat5 cable run, even for a single device on the cable.

From a power perspective, there will be voltage drop along the cable due to the current drawn by the routers and the resistance of the cable.

Cat5 cables have eight wires - four are used for data, and four are available for power.

A single pair of wires in a Cat5 cable has a loop resistance of approximately 2 Ohms per 10 metres of cable length.

PoE adapters generally use two pairs for power, so the effective loop resistance is 1 Ohm per 10 metres.

Note that some specialised adapters such as the MP2-AWD adapter use only a single pair for PoE, thus halving the distance that can be used.

In a 12V solar powered system, the system voltage may fall to as low as 10.5V in a worst case situation, although 11.5V is a more usual design figure.


Example 1: MP02 Routers

The MP02 routers conventionally use a nominal 12 Volt power supply, with a specified range of 9 - 15V. So it is important to make sure that the voltage supplied to the router is at least 9V when used in a PoE arrangement. The peak current drawn by these routers is approximately 100mA.

One router on a 10 metre cable will result in a voltage drop of: 1 x 0.1A x 1 Ohm = 0.1V

Three routers on a 10 metre cable will result in a voltage drop of: 3 x 0.1A x 1 Ohm = 0.3V

Three routers on a 30 metre cable will result in a voltage drop of: 3 x 0.1A x 3 Ohm = 0.9V

Three MP02 routers could thus be theoretically used with a Cat5 cable length of greater than 30 metres from a DC power supply perspective.

Example2: TL-WR841 Routers

The TL-WR841 routers conventionally use a nominal 9 Volt power supply and should not be operated at voltages less than this level. So it is important to make sure that the voltage supplied to the router is at least 9V when used in a PoE arrangement. The peak current drawn by these routers is approximately 250mA.

One router on a 10 metre cable will result in a voltage drop of: 1 x 0.25A x 1 Ohm = 0.25V

Three routers on a 10 metre cable will result in a voltage drop of: 3 x 0.25A x 1 Ohm = 0.75V

Three routers on a 20 metre cable will result in a voltage drop of: 3 x 0.25A x 2 Ohm = 1.5V

The maximum Cat5 cable length that could be theoretically used with three TL-WR841 routers would be 20 metres from DC power supply perspective, assuming a worst case supply voltage of 10.5V..


In addition to the voltage drop due to the resistance of the cable, there may be some small additional voltage drops in the Injector/Extractor and Power Splitter connections.

In a practical situation, the voltage at the routers should be checked while they are operating to ensure that the calculations reflect the reality. This can be done easily with a Power Splitter cable. Compare the voltage at the most distant router to that at the supply end to check that the voltage drop through the distribution cable is as expected.

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