Can a passive POE splitter be used with an active POE connection (802.3af or 802.3at)?

For example: an active POE switch provides power through the ethernet cable and on the other side a passive POE splitter is used. Would this scenario work? Are there any problems that might appear?

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IEEE-PoE devices (PSE or PD) require a compatible counterpart. They do not regularly work with "passive" devices. There may be occasion when some combinations work but this can't be taken for granted.

The difference on the PD/splitter side is whether it has an MDI signature that tells the PSE to turn on the power. If it doesn't in your case just swap the splitter for an IEEE-compatible one.

Even IEEE-compatible splitters don't require much electronics and you could call them "passive". Essentially, they require magnetics with middle contacts and a signature resistor. Most also sport a PSU for converting down the 48-54 V from PSE to e.g. 5 or 12 V.

Generally, it's a bad idea to mix IEEE-PoE with non-standard devices. The latter can really break things.

  • 802.3af also allows power polarity to be inverted or delivered on different pairs according to the choice of the PSE; also correct me if I'm wrong but the signature resistance has to be removed by the PD in normal operation. Are you sure there are IEEE-compatible splitters with only passive eletronics (ie no logic circuits)? – jonathanjo Aug 24 '18 at 12:48
  • The detection resistor is 27 kOhm and can be left in parallel, for proper class 0 you need a very few components, e.g. m.eet.com/media/1053791/PowerInt_POE_Fig5.gif And of course, the polarity/pairing problem may need to be handled (it's required for proper conformity). I don't know if extremely basic splitters for ca. 48 V actually exist but you could build one that way. – Zac67 Aug 24 '18 at 16:41
  • thanks @Zac67, very interesting, I'll consider myself corrected. Please could you post link to doc of that diagram? Thanks. – jonathanjo Aug 24 '18 at 17:16
  • Sure - that tutorial is very useful: eetimes.com/document.asp?doc_id=1273086 – Zac67 Aug 24 '18 at 17:22

So-called "passive PoE" is really just power-over-cat5 and really isn't compatible at all with 802.3af PoE.

Even the cheaper supposedly conforming PoE splitters can have issues with isolation and grounding -- which I've seen cause (literal) sparks on the receiving equipment.

Most non-standard PoE equipment only works with mode B power, and usually only with a given polarity. Given these are choices of the power-supplying equipment, even if your setup works, you might find it only works with some switches and cables.

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