Browse through various options to power products with a 802.3af compliant supply.
48V POE Explained
Telephone companies (telcos) have traditionally used 48V DC batteries to power the standard telephone handset over the standard 2-wire phone cable.The 48V source is at the telephone exchange.
When companies decided to use the same concept to power network devices (such as Voice-over-IP phones), they also decided to use the same 48V.
The industry has converged on a number of standards to facilitate interoperability between different manufacturers.
Read more on how to choose a 48V POE supply
The following terms (as used in the standards) will be used below.
- PSE: Power Sourcing Equipment. This is the device supplying the 48V voltage.
- PD: Powered Device. This is the device that receives the 48V.
What to look for
- Power: Different PDs have different power requirements. The standards bodies have settled on these basic classes.
- 14.5W: Known as "802.3af" (or rarely "802.3at Type 1").
- 30.0W: Known as "802.3at Type 2", or commonly just "802.3at".
- 60.0W: Known as "802.3bt Type 3"
- 100.0W: Known as "802.3bt Type 4"
- Active or Passive. A Passive ('dumb') PSE always applies 48V to the powered conductors. An Active ('smart') PSE first probes the connected PD to see if it will accept 48V before it applies the voltage. Some active PDs just respond with the equivalent of 'Yes'; other active PDs can also indicate the power they need. Passive PDs don't respond at all. See this page for more details on how the probe/response works. A Passive PSE will power both Passive and Active PDs. An Active PSE will only power Active PDs.
- Mode: This refers to the pins (conductors) on which the PSE applies the voltage:
Importantly, 802.3af/at PDs that implement only Mode A or Mode B are disallowed by the standard. It is the PSE that gets to decide the mode. For a PD to claim that it is compliant with an 802.3x POE standard, it MUST accept both Mode 1 AND Mode 2 power (not at the same time). E.g. A PD that only accepts Mode B power on pins 4,5,7,8 is not standards compliant.
- Mode A: The PSE applies voltage on pins 1,2 (+) & 3,6 (return)
- Mode B: The PSE applies voltage on pins 4,5 (+) & 7,8 (return)
- 4-Pair: The PSE applies voltage on all 8 pins. This is usually only used for the higher power standards.
How to Choose the right 48V POE supply
- Read the datasheet(s) of the PD(s) to determine.
- the power requirements
- what pins the PD(s) will accept power on
- are the PD(s) active or passive
- How many devices are you powering. If just one device, a basic POE adapter is all you need. Otherwise, POE switches come in various sizes.
- What is the Total power requirement of all PDs running off the same switch. Make sure the POE switch has sufficient power rating.