Martel notes that PoE injectors are usually configured with two unmanaged RJ45 ports—data-in and data+power out. “The data-in port connects to the non-PoE LAN switch while the data+power out port connects to a device that can be positioned up to 100 meters (328 feet) away,” he says. “If there is only one device to power, a single-port PoE injector is enough. For multiple devices, the number of PoE injector ports must match with the injector. Also, a Gigabit non-PoE switch will require a Gigabit injector to maintain network speeds.”
Antaira Technologies offers the INJ-C201G-bt-100-24-T, an industrial Gigabit enhanced PoE++ injector. According to the company, this PoE injector ensures 90W PoE output for factory automation applications. It also supports Enhanced PoE mode to supply up to 100W to the powered device, as well as a Power Booster for low-voltage power inputs. The INJ-C201G-bt-100-24-T has a fan-less design, a temperature range of -40 °C to 75 °C, redundant 9 to 55V dc power inputs and an IP30 rating.
Cabling, safety and voltage
According to IEEE PoE standards, the cabling requirements for the PoE injectors vary with each standard. For example, the IEEE 802.3af standard allows a maximum of 15.4 Watts of power to be delivered, corresponding to Cat. 3, Cat. 5 or higher cables. IEEE standards of 802.3at use Cat. 5 cable, while 802.3bt Type 3 and 802.3bt Type 4 use Cat. 6 or higher cables due to increased heat and power transmissions.
Martel says all IEEE 802.3af/at/bt compliant PoE injectors are engineered to prevent damage to a device even if that device is not designed for PoE.
“Once plugged in, a handshake procedure will be initiated by the injector before it sends power to the connected device,” he says. “That procedure uses low voltage and is harmless to any connected device—PoE or non-PoE. Once the handshake is completed successfully, the PoE injector sends power and triggers the connected device to start up. If the handshake is not completed for any reason, the injector will not send power, therefore preventing damage.”
To avoid overloads or operating issues, the PoE-enabled device needs to be compliant with the voltage supplied by the PoE injector.
Active vs. passive injectors
“There are critical differences between an active PoE injector and a passive PoE injector,” says Martel. “An active PoE injector, once connected, establishes the handshake with the PoE device and, upon success, powers up the device. An active PoE injector will first determine the characteristics of the incoming current flow. After that, it matches the flow with the requirements of the device itself. If it senses any discrepancy in current flow, it simply won't flow the current to the device. This eliminates the risk of burning out the device.”
Active PoE injectors follow IEEE 802.3af, IEEE 802.3at, and IEEE 802.3bt standards, whereas passive PoE injectors do not. “A passive injector does not perform a handshake function,” says Martel. “It will send raw unnegotiated energy down Ethernet cables to connected devices, whether they are able to handle it or not. This is why Passive PoE is dubbed as ‘always on.’ To mitigate the risk of Passive PoEs, always make sure you are connecting the device to the exact voltage the device requires.”
When using a PoE injector to power devices, an active PoE injector is recommended to deliver power.