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I have seen a lot of questions and answers all over SE about using a passive POE device on an 802.3af network. But can you use an 802.3af device on a passive POE network?

In particular, I have this POE injector that outputs 24V @ 0.5A (12 W?), and a Mitel 5320e IP phone that uses 4.3 W. The phone is 802.3af-compliant, which means it is expecting 48V, but is it reasonable to assume that it would still work on 24V? Or could I expect do damage the device by undervolting it?

It sounds like some people have tried this and it worked OK, but were they just lucky?

What about for the general case: if you have a passive POE injector that outputs 48V, would most 802.3af devices still work on it?

From what I understand, the handshake between the 802.3af POE injector and the device only determines if power will be sent; the device has to adjust the voltage to its needs. Once a 802.3af injector determines the device wants POE, it sends 48V, no matter what. This makes it sounds like 802.3af is just to protect non-POE devices. If that is correct, than a passive injector is only dangerous to non-POE devices; it shouldn't be dangerous to POE devices, even if they use a different standard. Is that correct?

More details on what I'm trying to do here

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  • \$\begingroup\$ I tried it and the Mitel device (5320e) didn't accept the dumb injector. \$\endgroup\$
    – browly
    Oct 11, 2017 at 22:02

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The standard leaves that open, basically.

The regular negotiation process will fail for a compliant device, at which point the device either falls back on some builtin support for dumb injectors, or disables PoE completely, in which case it is expected to not draw any power. It may optionally retry after some time.

Most devices use a lower internal voltage anyway, so they have a step-down converter, most likely behind a set of protection diodes or a full rectifier. As long as the voltage minus the voltage drop in the cable and the forward voltage of the diodes is still enough for the regulator to work at a duty cycle within its design specifications, the device can accept the power. As the voltage drop in the cable is likely to be significant, there is usually quite some headroom.

The other question is whether the device will accept a dumb injector that doesn't speak the protocol. That is fully device-dependent, and may be different between different revisions of the same device model. The only guarantee you can get is from the device's datasheet.

That said, I'd go for a standards-compliant injector or a manageable switch with (equally standards-compliant) PoE ports for any serious installation, simply because the cautious startup sequence will protect you if broken devices are connected.

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I had specifically this issue when moving an office of telephone staff to home working in CV-19 lockdown.

I purchased some 24V POE injectors, but the Mitel 5320 phones wouldn't power up - even though the POE "on" light was showing.

On investigating the issue I learned the phones need 48V POE. Once I plugged them into a 48V POE they powered up instantly.

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