Five Quick Tips About Power over Ethernet (PoE)
As with as type of job, having the correct type of equipment is crucial. That is no different when working with Power over Ethernet devices. Understanding the equipment is the first step in making the correct equipment decision for whatever the job may be. Here are five quick informational tips for working with Power over Ethernet devices:
1. Refer to the datasheet of the device being powered. It should specify the type of PoE.
A. if it simply states, PoE, then it is likely passive voltage, i.e., the voltage does not require any “communication” between the power source and the device being powered
B. if it states 802.3af, then be sure to use an 802.3af compliant power source
C. if it states 802.3at, the device requires higher levels of power and you will need an 802at compliant power source
2. Passive PoE is used on 10/100 Mbps interfaces. It will not be used with Gigabit interfaces. It will use the two unused pairs in a four pair Ethernet cable.
3. If using a device with passive PoE, consider the voltage requirement. If it is a camera that came with a 5vdc power supply, then distance will be an issue. The voltage drop over a 20 feet cable with 24awg size wire can result in 4.5 vdc available at the camera causing it to be inoperable. For this check out the AIR802 model PSA612V which is a power supply with four selectable voltages (6, 7.5, 9 and 12vdc) to compensate for the loss. If the power supply that came with your equipment is higher voltage, for example 24 vdc or 48vdc then voltage drop will become much less of an issue.
4. Ethernet switches are almost all IEEE 802.3af or 803.at and thus will not power a passive or non-standard device.
5. IEEE 802.3af/at devices operate at 48vdc+. Passive PoE devices generally can be anywhere from 5 to 48 vdc.
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