If the TL-RP108GE has just diodes to pick up POE power (arriving on any of 7 ports) that presumably won't work with a "proper" POE power source. I have a Linksys LGS116P switch for that.
No, I was probably wrong. Because TL-RP108GE supports PoE over GbE, it must have active negotiation logic.
Annoyingly, neither the manual or the specs say which IEEE 802.3 PoE standard it implements. It only uses the second pair of twisted pairs (that is not used for data on 10BaseT or 100BaseT), but has configurable power limits per port. It does not seem to do voltage conversion, though.
Do not confuse IEEE 802.3af active and passive PoE. Active carries data and power over the same twisted pairs, and typically requires negotiation. Passive uses the two unused twisted pairs on 10BaseT and 100BaseT, leaving the two data twisted pairs be. Again, both are standardized in the same standard, so active is no more "proper" than passive is.
Passive PoE injectors aren't "crappy", they're just IEEE 802.3af passive PoE, although they are limited to 10 and 100 Base-T. No Gigabit or better for passive.
IEEE 802.3af passive PoE injectors and extractors connect only the two data twisted pairs between input and output, and connect V+ and V- to the two twisted pairs on the powered side. They are simple and cheap, and obviously limit the connection to 100BaseT or 10BaseT. You do need to check that the voltage supplied is compatible with the downstream device. 802.3af limits the per-cable power to 15.4 W or less. PoE+, IEEE 802.3at, increased the per-cable power to 25.5W.
In the text of the standard, "A" and "C" are what we call active PoE, and "B" is what we call passive PoE. "A" uses data pairs only for power delivery, and "C" uses all four pairs; and "B" uses the two not-used-for-data pairs for 10Base-T and 100Base-T. For GbE and faster, only "C" is possible (and requires negotiation), because all four pairs are used for data and power delivery.
I really don't like it when perfectly useful, robust methods are called "crappy" just because they are looked down upon by "enterprise" folks who need to spend thousands of dollars even when a hundred-dollar device works better, more reliably, but doesn't have that prestigious "Enterprise" label on its side.