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Q: Like many would be geeks, I have an accumulation of power cord adapters from old components that I no longer own. They vary in size, shape, and ratings. Can you please help me by explaining the differences in the ratings such as amps, hertz, watts, allowable differences, etc.? How can I know which can I use on which other components? Thank you. Your column is very informative.

– Tom G.
Navarre, Florida

A: Well, Tom, I’m happy to hear from a reader who knows that technology includes more than just PCs and their operating systems. It would be my pleasure to give you a primer on power adapters. I think you’ll find there is a lot more to know than you ever thought there was.

Modern devices are very power hungry. Many of them plug-in to the wall, but even the ones that run on batteries often come with a plug-in device capable of both powering the device, and recharging the batteries. These little gizmos go by many names: AC adapter, DC converter, power adapter, power supply, power pack, charger, brick, and one of my favorites, wall wart. Regardless of what you call them, these devices serve a common purpose, which is to convert the electricity coming out of a wall outlet into a form that is compatible with a given electronic device. Most do this by using a type of electronic device called a transformer that uses coils of wire to step down the voltage from the 120 volt house wiring to a much lower voltage that’s closer to the requirements of the device that the adapter is intended to power. If the device requires direct current the electricity is passed through a rectifier to convert it to pulsating DC, then usually a filter to further smooth it out. Higher-end power supplies incorporate a voltage regulator, to keep the output within prescribed boundaries. Before I use too many of the terms that you asked me to explain, here are some definitions to ponder:

• AC – Alternating current.  Electricity that rapidly changes direction.
• DC – Direct current.  Electricity that flows in only one direction.
• Amp – Short for ampere.  A measure of electrical current, or quantity.
• Hertz – The number of times per second that AC electricity changes direction.
• Volt – The measure of electrical “pressure” in a circuit.
• Watt – A measure of electrical consumption over time.

While some electrical devices have similar power requirements, it seems like the majority of them do not. At the Geek House, we have literally dozens and dozens of various power adapters lying around – so many that sometimes it’s tough to keep track of which one goes with which device. One place to start is the size and shape of the device connector. Obviously a plug that is the wrong shape, or too large/small to fit the device is not compatible. On the contrary, just because it fits does not mean it will properly power the device, and it might even damage it. Every device and every adapter has a label on it, describing its requirements or ratings. In the United States, wall outlet power is 120 volts, alternating at 60 hertz, which is often written as “120 VAC/60 Hz”. This is the input to the adapter. The output will include the number of volts, whether it is AC or DC, and its rated maximum current. So, a 12 volt DC adapter capable of putting out half an amp would say something like “12 VDC 500 mA” (The “mA” stands for milliamp, or 1/1000^th of an amp. 500 milliamps is one half of an amp).

Beyond the shape of the connector, as far as determining which adapters can be used with which devices, the most important number is the voltage. It must closely match the device’s requirements. Too little voltage, and the device won’t power on. Too much will destroy the device’s electronics. Current is more forgiving. The device only draws as much current as it needs, so hooking up a 500 mA adapter to a device that only needs 100 mA is okay. However, if you connect a 100 mA adapter to a device that needs 500 mA, the device won’t function, since there’s not enough available current.

There’s lots more to know on this topic. For the rest, visit my website and check-out the Supplemental Web-Only Content at the bottom of this week’s column below.

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Supplemental Web-Only Content:

Adapters vs. Batteries: Many adapters are intended to be an alternative to running a device on batteries.  In such cases, it is typical to find that the adapter’s voltage is the same as the sum-total of the voltage of all the batteries.  Common D, C, AA, and AAA batteries put out around 1.5 volts each.  So, a device that uses four batteries will probably use an adapter that is rated at 6 VDC.

Connector Polarity: Besides the shape of the plug, most DC adapters also indicate the plug’s polarity, which is to say, which portion of the connector is the positive connection, and which is negative.  Look for a symbol like this: 
In this particular drawing, with the plus sign pointing toward the dot and the minus sign pointing at the surrounding arc, this symbol indicates a “tip-positive” connector (the positive connection is on the inside, and the negative connection is on the outer ring of the barrel).  This is the most common configuration, since the positive tip is better protected from accidentally shorting.  There will be a polarity marker on both the adapter and on the device, and they must match.

AC vs. DC: Once upon a time, we didn’t have electricity in our homes.  Back when the technology was new, the same thing happened that happens with new technologies today: rival factions fought against each other to try and get their idea adopted as the “standard”.  Batteries, by their very nature, put out a constant flow of DC from the positive terminal to the negative terminal, and one school of thought was that residential power should do the same.  Well-known American inventor Thomas Edison led the charge (pun intended) for residential DC power.  However, AC power can be sent over long-distance lines far more efficiently, and is easier to step-up or step-down to meet the consumer need on the distant end.  Unfortunately, it is also quite a bit more dangerous than DC power.  These rivalries set of quite a technological war, which, as you probably know, was ultimately won by the AC Power side.  If this topic is of interest to you, check out this Wikipedia Article on the so-called War of the Currents.

More on Hertz: If you live in the USA, you’re probably used to seeing 60 Hz as the number of cycles for AC power.  However, in many parts of the world, such as Europe, power runs on a 50 Hz cycle.  There isn’t a distinct advantage or disadvantage to either one, but it can be a problem when trying to use devices in multiple countries.

More on Volts and Amps: Electricity flowing through a wire is often compared to water flowing through a pipe.  In this analogy, volts are the pressure, and amps are the volume of water.  A large pipe can move a large amount of water at low pressure, where a smaller pipe requires much higher pressure to move the same amount of water.  That’s why so-called high tension wires, which carry electricity hundreds of miles from the power generation plant, are often stepped up well about 100,000 volts, to get the large amount of electricity moving through the relatively small wires. 

More on Watts: Watts are a little out of place in a discussion of power adapters.  Nevertheless, they were part of the original question, so I’ll cover them briefly.  Watts are a measure of electricity used over a period of time.  Many consumer devices such as light bulbs and hair dryers are given ratings measured in watts to give you an idea of how much power they consume.  The actual description of how this is measured is rather, shall we say, mathematical.  If you really want to learn more, I’ll refer you to this Wikipedia Article on watts.

Geek Tip: It used to be that all power adapters had a manufacturer’s label on them that more-or-less matched the product it went with.  An adapter for a Sony telephone said “Sony” right on it.  That’s no longer the case, as many device vendors get low-cost power adapters from 3rd-party manufacturers in countries such as China, and opt to keep their costs down even more by forgoing custom brand labeling.  This can make it extra difficult to identify which adapter goes with which device.  It’s gotten bad enough at my house that we have taken to using our Brother P-Touch label maker on every new power adapter that comes in the house.  I apply my own meaningful label, thereby eliminating any future guesswork.