– Copied from customer email, anonymized, before the documentation update –
In the Technical Specifications section of the web page for the Universal PoE Module, it notes that the input voltage range is 58–60 V for Active PoE, and 0–65 V for Passive PoE. However, in the datasheet, it notes that the minimum voltage for ‘full PoE function’ (which I assume to mean active PoE with negotiation) is 48 V.
Can you please clarify: If I power the module with 48 VDC, will I get active PoE power delivery, or passive PoE power delivery?
Thanks for flagging this. Neither 48 V nor 58 V is a universal minimum for active PoE.
The IEEE minimum voltage at the injector’s Ethernet output depends on the PoE type:
- 802.3af, Type 1: 44 V, up to 15.4 W at the injector / 12.95 W at the powered device
- 802.3at, Type 2: 50 V, up to 30 W / 25.5 W
- 802.3bt, Type 3: 50 V, up to 60 W / 51 W
- 802.3bt, Type 4: 52 V, up to 90 W / approximately 71 W
The module’s DC input needs additional headroom for voltage loss through the board. Its compact design has higher losses than a larger injector, and longer cables cause additional loss before power reaches your device.
As a practical starting point, we recommend a 4 V supply margin above the applicable minimum: 48 V for Type 1, 54 V for Types 2 and 3, and 56 V for Type 4. The final voltage needs checking with your cable and maximum load, particularly at higher power. Our published 89 W test condition required 60 V input for successful startup.
What power level and cable length are you planning to use?
The updated datasheet explains the voltage requirements.
– Customer follow-up, anonymized –
We make tethered inspection crawlers, and depending on the deployment length that the customer requires could mean they need a tether length anywhere from less than 100 m, to 1 km or more. For anything more than 100 m this would mean a fiber optic tether, which has a PoE powered fiber to Ethernet media converter on either end. The media converter only needs a few Watts and would be less than 2 ft from the injector.
My concern was more regarding short length tethers that would be a copper-based Ethernet system, so then we would end up with two PSE devices looking at each other, not powering any PD devices. I’d be concerned about whether a passive PoE injector feeding power into another PSE would cause damage.
Thanks, that clarifies the concern.
Powering PoE Module from 48 V doesn’t automatically put it into passive PoE mode. Its default configuration uses active detection and classification; passive operation requires changing the solder-jumper configuration.
For the copper tether, if both ends are normal active PSEs and neither presents a valid PD signature, neither should enable the normal PoE operating voltage.
A passive or forced-on injector can apply voltage without negotiation. We shouldn’t assume that connection is safe: active detection doesn’t itself establish protection against externally applied power. Please keep passive injection disabled in that arrangement until the power paths have been checked.
For the fiber configuration, the relevant PoE cable length is the short copper connection between each injector and its media converter. The fiber tether itself doesn’t add PoE voltage drop. If the converter is an 802.3af Type 1 PD, 48 V is our suggested starting supply voltage, with startup and operation checked using the actual converter.
Feel free to send over the media-converter model, the PSE/module models and revisions at each end, and a simple diagram showing the supply voltages and port connections. Include whether either end has passive PoE enabled, and we’ll check the setup for you.