Power over Ethernet (PoE) Explained: Active vs Passive PoE and Compatibility Tips

What is Power over Ethernet (PoE)?

Power over Ethernet (PoE) is one of the most convenient ways to deliver both data and power through a single Ethernet cable. It is widely used for network devices such as access points, IP cameras, VoIP phones, and wireless radios.

By combining data and power delivery, PoE simplifies installation, reduces cabling clutter, and enables flexible device placement, particularly in areas where power outlets are limited.

Active vs Passive PoE

Two main types of PoE are used in networking: Active PoE and Passive PoE. Understanding the difference is key to powering your devices safely.

Active PoE

Active PoE is the safest and most intelligent option. The power source, such as a PoE switch or injector, negotiates with the device before sending power.

• If the connected device supports PoE, power is delivered automatically.

• If it does not, the source will not send power, preventing accidental damage.

Passive PoE

Passive PoE, on the other hand, sends power without negotiation. This means you must ensure the voltage matches the device’s requirements, or you risk damaging the hardware.

Passive PoE can be used for cost-effective setups but requires careful attention to voltage and current specifications.

Active PoE Standards (IEEE 802.3)

The IEEE defines several Active PoE standards that determine how much power can safely be delivered.

Compatibility tips:

• An 802.3af device can be powered by an 802.3at or 802.3bt source.

• An 802.3at device can be powered by an 802.3bt source.

Using higher-standard PoE equipment can future-proof your network and improve reliability.

Passive PoE Explained

Unlike Active PoE, Passive PoE does not follow a universal standard. It can operate at various voltages and wattages, so extra care is needed when choosing injectors and devices.

Key considerations:

• The voltage must match the device’s input range.

• The wattage must be sufficient (volts × amps = watts).

Example: Calculating Passive PoE Power

If a device requires 12W and supports 12–30V, and you are using a 24V Passive PoE injector, you will need at least 0.5A to meet the power requirement:

24V x 0.5A = 12W

Why Some Manufacturers Use Passive PoE

• Certification costs for official PoE standards can be high.
• Lower voltage operation may reduce power consumption.

Device Examples

• Ubiquiti airMAX radios commonly use 24–28V Passive PoE.

• MikroTik and Teltonika devices often support wide voltage ranges (for example, 9–30VDC or 12–57VDC) and can operate on either Passive or Active PoE depending on the model.

Always check the device datasheet or label to confirm supported power input before connecting.

Compatibility Notes

While most modern network devices are forgiving, compatibility mismatches can still occur between PoE sources and powered devices.

Always check:

• The supported voltage range of your device.

• The pin configuration of your injector or switch.

Common Passive PoE pinout:

• + (positive) on pins 4, 5
• – (negative) on pins 7, 8

This ensures your device receives the correct power delivery without risk of short circuits or overheating.

Conclusion

Power over Ethernet simplifies network setups by combining data and power delivery, but knowing the difference between Active and Passive PoE is essential.

Active PoE offers safe, automatic compatibility, while Passive PoE provides flexibility and cost-efficiency when used carefully. Always check voltage, wattage, and pin configuration before connecting, and your PoE network will run safely and reliably.