PoE 802.11af limits

Combining the power of the ATmega2560 MCU with onboard Ethernet, a microSD card slot, an efficient switchmode power supply, and a small prototyping area. [Product page]
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PoE 802.11af limits

Post by swordfishBob » Sat Jun 08, 2013 5:54 am

Does anyone know how much current I could draw through an 802.11af PoE board fitted to an Ethermega (including or excluding the mega and its ethernet)?

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Re: PoE 802.11af limits

Post by angusgr » Mon Jun 10, 2013 2:13 am

Hi Bob,

For the POE-REG-8023AF board the output current limit is 1A at 7.5V. I'll update the product page with this information as well, thanks for pointing out it was missing!

This is passed into Vin on the EtherMega, which then feeds into the EtherMega's onboard regulator for the 5V and in turn the 3.3V levels.

As I recall an EtherMega by itself draws around 250mA @ 5V (possibly higher depending on ethernet activity.) The datasheet for the EUP3476 regulator on the EtherMega doesn't give a specific efficiency curve, just "up to 95%". If we pessimistically assume 80% (it's probably much better but you'd need to measure), that's 222mA input at 7.5V.

So that leaves you something in the order of 800mA current capacity left at 7.5V, or 960mA at 5V ( 800mA * 7.5V / 5V * 80% ).

It's worth mentioning the 802.3 regulator features an overtemperature cutout, so in theory if you exceed the 1A limit then it should safely switch off to protect itself. This is likely to be the first limit reached (the EtherMega's regulator has more capacity but also features a thermal cutout.) However that in itself is not a reason to tempt fate, of course! If you think you're pushing the limits keep an eye on the temperature of both regulators. :)

- Angus

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