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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?
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.
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.
