Replacing a device power adapter

We have previously discussed power supplies for desktop computers at some length at 10stripe. What we have not discussed are power supplies for other devices, including laptop computers, but also including everything from telephones to routers. Buying a power supply for such devices is, thankfully, much simpler. But there are still some things you should know. If you're impatient, go ahead and skip to the bottom line.

The basics

There are three basic types of power supply (or power adapters; we will use the two terms interchangeably).

Switching power supply

Also called a Switch-Mode Power Supply, or SMPS, these power supplies take in AC current, use a switching circuit to increase its frequency, then send that to a transformer to create the desired voltage, and then to a rectifier to turn it into DC. They are light, efficient, and can supply quite large currents. They also have very good regulation, which means that their output voltage is stable even as their output current increases. However, they are more expensive than other supplies

Laptop computers, printers, and increasingly many digital devices use switching power supplies.

Unregulated linear supply

This is about as simple as power supplies can get. The input voltage is fed to a step-down transformer to create the desired voltage, and then a rectifier to turn it into DC. Because the device has no hardware to regulate its output voltage, that voltage will vary considerably as output current changes. For devices that were designed with the expectation that they would be used with such a power supply, this is not a problem; either they don't really care what the input voltage is that much, or they have their own regulation hardware.

Low-current, inexpensive devices sometimes still use this type of supply. Most other devices now come with at least a regulated linear supply.

Regulated linear supply

This type works just like the unregulated linear supply, but with the rather unsurprising difference that it is regulated. So after the rectifier produces a DC voltage, it is passed through regulation hardware to ensure that the output voltage is consistent. This increases costs somewhat, and the device is still as heavy and inefficient as its unregulated sibling (slightly less efficient, since the regulation hardware will produce waste heat), but it gives you pretty good regulation at a moderate cost.

Middle-of-the-road devices may use this type of supply, although they are increasingly moving to switching power supplies instead.

You may be wondering if there is an easy way to tell which type of power supply you are dealing with, and there is. Switching supplies will be the lightest at any given power rating. To tell the two types of linear supply apart, you will need a multimeter (or just a voltmeter). Plug the supply into the wall with no load attached. If the voltage goes much higher than it is rated for (sometimes twice as much or higher), the supply is unregulated.

Input voltage

Wherever you are, odds are that your local power system delivers either 110 or 220 Volts to the wall, give or take. If for some reason you don't know, you might consult our map of power systems by country. If you are buying something locally (and not on the Web), this is unlikely to be a problem for you, as locally-available devices will probably be compatible with your local power system. Anyway, you need to know what voltage you are dealing with; frequency is, for most devices other than some older wall clocks that are driven directly by AC, not an issue.

Any device that you buy needs to be compatible with this voltage. Input voltages for power adapters are often listed in ranges; devices intended for use in the United States will often list something like 90 - 120 V. As long as this range covers whatever you get locally, you are fine. Many modern power adapters are "world compatible", meaning that they are compatible with essentially all power systems. These devices will list a range more like 100 - 240 V, and may also explicit indicate that they are compatible with power systems that have a frequency of either 50 or 60 Hz.

Connectors

Your power adapter will have two connectors that we need to worry about: one for input, and one for output. We will deal with each in turn.

Input connector

The input connector for the power adapter needs to match whatever you use locally. As with voltage, buying locally usually means you don't need to worry about this. If you are buying an adapter that has a detachable cord running to the wall (like the sort you often seen sold for use with laptops), you need to worry even less. That cable will have a standard IEC connector on the end that mates to the power adapter, and then a connector on the other end intended to plug into a wall somewhere on Earth. If your wall plugs don't match, you just need to replace that cable. Electronics stores, thrift stores, and hardware stores often have such cables.

Output connector

The output connector is trickier. Most power adapters have a coaxial "barrel" plug to connect to the device that they power. This is a round, hollow connector, with one conductor on the outside surface of the "barrel" and the other conductor on the inside of the "barrel". First need to make sure that the connector is the same size and shape as the adapter that you are replacing.

There are a limited number of standard sizes, but not all manufacturers use them; in particular, many laptops use strange designs where the outside of the barrel is tapered slightly. To cope with this, many third-party "universal" power adapters for laptops include a set of various common (to laptops) connectors.

But you need more than just a physical fit. You also need to make sure that the polarity of the output voltage is the same. In barrel connectors, either the outside or the inside conductor may be positive; neither style is particularly dominant. If the inside conductor is the positive voltage, then the adapter is "center positive"; if it is the negative voltage, the adapter is "center negative". On both the adapter and the device it powers, this is usually depicted with a drawing showing a small circle and a bigger partial circle around it, which represent the inside and outside conductors respectively, connected to a plus sign and a minus sign.

Output voltage

The output voltage, which is to say the voltage supplied to the device being powered by the adapter, needs to match the adapter that you are replacing. As already mentioned, the polarity needs to be the same. This assumes that what you have is a DC supply, as it most likely is (speakers sometimes use AC supplies); for an AC supply, there is no polarity to worry about. And of course, if the old power adapter was DC then you cannot use an AC replacement, and vice versa.

There is always some question over just how close the output voltage of the new supply needs to be to the output voltage of the old supply. And the answer is, it depends.

For a laptop, the laptop will have some voltage regulation hardware of its own onboard, and so can probably cope with a slightly higher voltage (0.1 V should be fine; 0.5 V is riskier but not a bad bet; 1 V is pushing your luck). However, it will mean that the voltage regulation hardware produces a bit more heat, which will make the laptop hotter. Lower-than-expected voltages may be dicier; there you are putting your faith in not just the laptop designers (who designed in some margin in the voltage their laptop can accept) but also the power supply maker (who designed in some margin in what their supply can output, knowing it would sag some under load).

For other devices, you may or may not have as much freedom. You really don't want to stray more than around 0.1 V from the original device (with one little exception noted in the next section). Luckily for you, such devices normally use one of a limited set of common voltages, so finding a replacement that matches closely should not be too hard.

Output current

This one is a little easier. The rated output current of the new supply (measured in Amps, A, or milliamps, mA) should match or exceed the rating on the old supply. This number is just a measure of what the device is capable of, and so buying something that is rated higher (meaning it is capable of supplying more current) is fine.

You may sometimes see adapters that are not rated in current, but are rated in power (either in watts, W, or milliwatts, mW). For DC devices, you can divide the power rating by the voltage rating to get a current rating (or symmetrically, you can multiply the voltage and current ratings to get a power rating). AC devices are trickier, but the same relationship mostly holds.

That's one gotcha, and there's one more. If your power supply is a linear unregulated model, you need to be more careful about the current rating. The output voltage of an unregulated supply will change significantly as output current changes. Hence, you should try to find a replacement power adapter whose current rating is within, say 10 or 20% of the rating of the old adapter. This is increasingly unimportant though, as unregulated supplies are becoming less common.

Checklist

Let's summarize, shall we? If you're buying a replacement device power adapter, you need it to meet these criteria:

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