PoE Minimum Load | 🧨 Engineering Schematic Solutions
Intro
Now even cheap Ethernet switch could support PoE feature, which allows to power devices using single CAT5 cable.
Special PoE IC like SI3404 give a chance to build your own PD, so if you're going to use it in your design take a look at the unobvious thing.
🔺 Issue Description
| If you have PoE and measure the voltage with no load you will see situation like that (T=2.5s, f=0.39Hz) | If you take a look at the switch LED PoE status you will see periodic blinking |
 |  |
❔ Causing
Minimum current is important here — if the PD draws too little, the PSE might disconnect power assuming the PD is removed or faulty.
For 60 ms every 400 ms |
Up to 4-pair delivery |
🔧 Issue Fix
Additional Load Resistor
| DC-DC | DC-DC 5V |
| Minimum Load Current | 10 mA |
| Load Resistance at 12V Rated Voltage | 1000 Ohm |
I = 12V / 1000 Ohm = 12 mA
P = I * R = 0.144W
✔ Fixed
| After adding resistor for constant load the voltage is always stable: |  |
Conclusions
- It's important to have always-on PoE when you have 2 or more power inputs since when another power input disconnects it should instantly switch to the PoE which should be already available for usage without delays:
