Power for BEFW11S4 and WRT54G Access points

The Linksys BEFW11S4 V4 802.11B WAP/Router, and the WRT54G V2 802.11G WAP/Router, are designed to be powered off a 12 Volt, 1 Amp, center-conductor-positive 5mm diameter power plug.

The power tolerance for these units is quite wide, though. I did some work with a regulated bench power supply, and looked at the circuit inside, and learned that you can power these units off a *regulated* supply anywhere between 4V and 27V, with a safe area probably between 7V and 22V. Among other things, this means I can power one of these WAPs off the AC adapter for my Thinkpad laptop (16V nominal). It also means I can power one off my 130Watt-hour NCharge portable battery for a LONG time ( 20 hours for the WRT54G, 50 hours for the BEFW11S4 ).

Inside the WAPs is a circuit called a "bucking" or step-down regulator, based on the Anachip AC1501-33, capable of 40V operation. The input capacitor is a 25V device (if this was replaced with a 50V device, and there is plenty of room for it, the WAP could take 40V). The regulator is designed to provide a constant 3.3V for internal circuitry, using power from any external voltage in the above-mentioned range. This allows the unit to work with the cheezy 12V/1A wall warts provided by Linksys (which can ripple between 9V and 16V under load). In general, wall warts are pretty sloppy, and most of the numbers below assume something better controlled.

The BEFW11S4 draws about 2.4 watts idling (with ESSID broadcast) and 2.6 watts transmitting (an FTP download), while the WRT54G draws 5.4 watts idling and 5.6 watts transmitting. The current drain appears "bursty", matching the increased power into the transmitter during packet transmission, as you would expect. The buck regulator inside draws enough current at any given voltage level to supply that power, so an idling BEFW11S4 draws around 400mA at 6V and 100mA at 24V. This is called a "negative differential resistance" in electronics-speak, and can cause instabilities with the wrong sort of power source.

One typical "wrong power source" that people here might see would be a Power-Over-Ethernet kludge with too much resistance in the wire. For example, if you power the WRT54G unit from a 24V source with 25 ohms of series resistance, you can theoretically provide the needed 5.6W as 12V. However, during startup the input voltage has to move from 0V up to that 12V, and while passing through 3.3V, for example, the unit might be attempting to draw 1.7A, which is more than the 25 ohm resistance can provide. This is probably why some folks have had better luck with a regulated power source - those have lower internal resistance added onto the resistance of the interconnect wire.

My guess is that the power source will need to temporarily provide 2 amps at 3.3V to the WRT54G, and 1 amp at 3.3V to the BEFW11S4, during startup. So with a 24V source, that is a maximum resistance (including connectors and eventual corrosion) of 10 ohms for the WRT54G, and 20 ohms for the BEFW11S4 . With a 12V source, the maximum resistances are 4 ohms and 8 ohms. An RJ45 with "green scum" on it, copper corrosion pretty common in Oregon, will have higher resistance. The Linksys 12V/1A wall warts have an internal resistance of 2 to 5 ohms that adds to that external resistance. Yikes!

Well, enough calculation nonsense;

What does this all mean?

First, it means that for POE situations you may be able to use a >15V laptop AC adapter as a power source. Even the older ones have significantly lower internal resistance than Linksys wall warts. All the IBM Thinkpads since the 560 have used the same power plug, compatable with the Linksys units, and all their adapters provide more than enough power to drive a Linksys. Prepackaged and ready to go, and there are plenty on Ebay.

Second, it means that you are compatable with the supplemental external notebook batteries. My 130 Watt-hour, 3 pound NCharge battery from Valence Technologies (www.valence.com, $300 from Fry's with one adapter) can be connected to different adapters for different laptop plug/voltage combinations. So you could get one $25 adapter that works with your laptop, and if that is not suitable for the Linksys, you could get a second adapter that is Thinkpad and Linksys compatable, and interchange adapters as needed.

Third, this makes projects like Sam Churchill's Wifi bike project pretty darned easy. Sam could use 2 WRT54G's running the custom EWRT code, one device configured in client mode and connected to a high-gain antenna for the uplink, the other configured as a Personal Telco portal. A Y splitter could power both units off the same NCharge battery, and multiple batteries can be connected in tandem if Sam needed more than 10 hours.

last revision May 7, 2004