Simple 3 Amp Step Down Switchmode Regulator

I bought mine from Farnell for $7 for 10

But a google search shows there are few places around 1 off retail price is $13.95

Jaycar
Electus Distribution etc...

This is not indented to tell people how to suck eggs, but it is a good question for people doing POE systems generally.

So I’ll give some information on how to calculate this.

The regulator has an approximate 2V overhead required to achieve the desired voltage eg: 14V for a 12V output or 7V for a 5V output.

So we only need to calculate the resistance of the Ethernet run to determine the minimum injection voltage.

Cat5e I believe is 9 ohms per 100metres, so a 10 metre run would be about 0.9 ohm.

At 2 amps (a typical current draw) The expected drop would be calculated as:

V = I x R so this would be 2 x 0.9 = 1.8 volts.

So the minimum ejected voltage required on a ten metre run at 2 amps would be:

1.8v + 2v + 12v = 15.8 volts.

However, as the regulator has quite a high input voltage e.g. max 45V the POE injector can be anything between 15.8 and 45V. So as 24V power supplies are more common than 16V, I would tend to use a 24V power supply.

Just a side note, a switchmode regulator has similar power characteristics to that of a transformer (the current is inversely proportional to the voltage) , (e.g. more voltage less current) so you don’t need to buy a 2 amp 24V supply in this scenario.

To calculate the minimum current at 24V, convert the minimum requirement into power.

P = E x I or 15.8 x 2 = 31.6W

So the current draw at 24v will be:

P/E = I or 31.6 / 24 = 1.32 Amps

So the power supply that would be most cost effective for this scenario would be:

24V at 1.5Amps or 36VA.