Power over Ethernet

Having both power and data over the same cable is so useful!

Background

For electronics designers, the decision about how to power the project is a necessary part of the design.

  • Plug in?
    • Then is it AC or wall-wart or USB?
  • Battery?
    • Disposable or rechargeable?
  • The issue with batteries is that the voltage drops over time, which has to be factored in.

But there’s another option that’s available, and is especially useful when the project also uses Ethernet: Power over Ethernet (PoE). The problem with PoE is that the cable delivers around 48V—which is great for a reasonable amount of power, therefore current, but unfortunate since the project now needs a down-converter, as well as having to abide by the IEE802.3af (and later) PoE standards.

Silvertel make a range of PoE module voltage converters, to supply 3, 5, 12 or 24V to the project. Their smallest module has two different form-factors: the through-hole ‑LP “low profile” series, which gives slightly less current and doesn’t have thermal protection; and the surface-mount ‑M and ‑MT series—the latter has thermal protection. Although it only requires a minimal number of external components, for EMI/EMC reasons more components are recommended.

Projects

So I’ve started playing around with these modules, and have a number of projects with them in mind. Not all the projects will have Ethernet—I’m just using it for the raw power.

Curtains!

The first is a project to power a curtain motor. It needs 12V at 0.833 amps, which is perfect for PoE. The problem is that the motor is controlled by a 433 MHz “garage door opener”, so the EMI coming from the module needs to be carefully suppressed.

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