• WallWarts

Wall Warts

Plug in AC power supplies

This is a listing of power supply adapters for various access points, cable modems, and other related gear. We need to know the voltage and current provided, as a bare minimum, but if you can measure the output connector shape, please add that as well:

Node devices

Laptop adapters

Explanation

Typical wall wart power supplies consist of a brick transformer with a 2 prong AC plug on one side, and a long two-wire cord with some kind of plug at the other end. These are manufactured very cheaply, and are often the first thing to fail when powering a small electronic item. They typically produce a rectified sine wave, a series of parabolic bumps at a 120Hz rate. A wall wart rated to produce 9 volts at 1 amp might produce 15 volt bumps and 0 volt minimums when observed on an oscilloscope. If the mains power line feeding the wall wart sags, or is briefly interrupted, or surges beyond 117 volts AC, then so will the output of the wall wart. Thus, they produce very cheesy power.

Because the power is "lumpy", the devices inside the adapters must tolerate a wide range of power voltage. For example, a Linksys WRT54G has a 25V tolerant input capacitor, and a 40V compatible switching power supply chip, inside the unit. Hence, it can easily tolerate a steady DC power supply between 6V and 25V, and if the capacitor is replaced with a higher voltage one, it can tolerate up to 35V or so. The worst case load on the power supply is when the device is most active; for a WRT54G, that is about 9 watts when it is broadcasting packets at a high rate. This implies that some nodes using brick transformers will work OK with light network usage, but if the pipe to the net gets bigger, and users use the higher bandwidth to download big files, the result will be to sag the supply, and possibly zombie the CPU in the access point. It can be reset, but that is annoying.

Another issue with wall warts is that they are supposed to be safe if the output is shorted. That is, they limit current to the output and don't melt or catch fire with a continuous short. The cheap adapters accomodate this by having a large internal resistance. As a result, under normal loads, they are quite inefficient and waste a lot of energy, more than doubling the wall power.

A better way to power such devices is to use a smart or "switching" AC adapter. These contain a fancy power converter and regulator, and are typically used on all laptops and some other very recent or higher power consumer electronics. Because they have a lot of electronics inside, they are more expensive, so most manufacturers don't use them if they don't need to. However, they are usually close to 90% efficient, have very low standby power drain, are more durable, and produce a steady regulated output even when the mains power sags or spikes. Using these in place of the manufacturer supplied wall wart will result in longer life, higher reliability, and less wasted power. This can be very important for a small business hosting a PTP node. A wasted watt of electrical power for a continuously-powered node is 8.8KWHr per year. A further 2 to 5KWHr/yr is needed to run the air conditioner. Saving a watt of dissipation saves more than a dollar per year, as well the potentially much larger environmental costs. Here in Greener Portland, that is a big thing.

So, when you install an access point, cable modem, USB drive, or other device with a wall wart power supply, spend a little extra for a brand that uses a switcher AC adapter that produces the correct voltage. If you can't find one, add information on the AC adapter to the list above, and we will try to locate a suitable AC adapter with the correct voltage from an obsolete laptop, add a suitable connector plug, and make the substitution in the future. If you are disposing of an ancient, broken laptop (send it to Free Geek, please!), consider donating the AC adapter to PTP to replace a wall wart. If you have a laptop or a unit that uses a wall wart and can donate neither, please at least enter the voltages and currents into the table above. Help keep Portland, and PTP, green!