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PoE Voltage Revisited

Monday, October 9th, 2006

I wanted to get a better picture of how changes in conditions effected the voltage inside the WRT.
The starting voltage of the OEM wall wart with no load: 17.39 Volts.

I put the router in “Client Bridge Mode” and wirelessly connected it to an access point.
The following table shows the voltages measured when powering the router through PoE.

Condition Voltage
Tip of hot Cat5 7 feet long 17.39
Hot Cat5 in normal WRT switch port 15.76
Hot Cat5 in Port 4 (jumpered to WRT DC bus) 14.73
With one PC on switch (no net traffic) 14.63
With one PC running iperf* (network traffic) 14.56

The following table shows the voltages measured when powering the router through the normal power jack.

Condition Voltage
At the PC input jack 14.78
With one PC on switch (no network traffic) 14.70
With one PC on switch running iperf* (network traffic) 14.64

*The iperf test creates network traffic between the PC behind the bridge and a PC on the other side of the access point.

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