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I'm wondering if it's possible to power a microcontroller (e.g. ATmega328) from a Ethernet RJ45 motherboard port. Unlike USB, there is no power line, all pins/lines seem to be data only.

I am guessing I don't have much hope?

Thanks

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    \$\begingroup\$ Maybe you can find a motherboard with power over ethernet? \$\endgroup\$ – Wesley Lee Mar 15 at 4:32
  • \$\begingroup\$ if the port is only 10/100 then you have 4 "unused" wires you can use for low-current power. \$\endgroup\$ – dandavis Mar 15 at 20:20
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Power over Ethernet (PoE) is a standard way of powering things up to a few watts or more from an ethernet cable. You need to inject the power if your devices don't already support it. It's commonly used on things like IP telephones.

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  • \$\begingroup\$ Can you please comment on my 100Base solution? \$\endgroup\$ – Sunnyskyguy EE75 Mar 15 at 21:28
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It depends what you are trying to achieve. The standard for power devices through a ethernet cable is PoE (Power Over Ethernet) which allow you to drain up to 15.4W, but if what you are after is to power your microcontroller without having ethernet communications, and just harvesting the power from the connector, then it is a different thing.

The voltage levels on the twisted pairs are +/- 2.5V, and potentially you can use them to drain power, but don't expect to be able to drain much current (10-15mA).

The question has already been asked before, and I cannot phrase the answer better than this guy has done: How could I power an IC over the Ethernet?

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  • \$\begingroup\$ 2.5V into 100 ohms is 2.5mA, potentially enough to run a small microcontroller. Commanding the ethernet port to transmit continuously may be tricky. \$\endgroup\$ – Jasen Mar 15 at 7:52
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You can use middle Pins 4,5 for DC

4 blue solid DC+
5 white/blue stripe DC-

DC is used on 1Gbps NIC so not permitted but no connection(NC) for 100Mbps NIC may be used .

enter image description here

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  • \$\begingroup\$ useless -1 voters must validate their objection \$\endgroup\$ – Sunnyskyguy EE75 Mar 15 at 19:40
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    \$\begingroup\$ Definitely no DC within a pair! Connecting that to Gigabit Ethernet port would blow up the transformer of that pair! If DC must be inserted without PoE then it should be on different pairs. \$\endgroup\$ – Justme Mar 15 at 21:10
  • \$\begingroup\$ You would not use it on a 1GHz port as it is already used for this so if 100MHz port then there is NC . But he has not stated interface. @DanDavis agrees \$\endgroup\$ – Sunnyskyguy EE75 Mar 15 at 21:11
  • \$\begingroup\$ @Justme do you agree? \$\endgroup\$ – Sunnyskyguy EE75 Mar 15 at 21:23
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    \$\begingroup\$ But even if it is only Fast Ethernet interface, who knows if it still uses Gigabit "MagJack" or transformers. There are also other interfaces with RJ45 connectors - which may not expect power on the middle pair. Some Fast Ethernet interfaces (MagJacks) short out pins 4&5. Power should better be one terminal on both pins of a pair (4&5) and the other terminal on other pair (7&8). Even then a MagJack might have a 150 ohm DC termination path between those pairs, so this is why PoE has a identification resistance so DC is not applied unless the receiver is determined to be compliant PoE receiver. \$\endgroup\$ – Justme Mar 15 at 21:59

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