Picking the Correct Wall Adapter

Electronic devices are an ever-present part of our modern world; and one thing that all electronic devices share is a need for power. Some electronic devices are powered by batteries while others rely on wall adapters. Various devices operate on a DC voltage which can be created by using wall adapters to convert an AC voltage from a typical wall outlet into a DC voltage for a given device. There are many questions that can be used to determine which wall charger will work for a device.

Some of the electrical considerations for choosing a wall adapter are the voltage that the device will be operating at, the current requirements of the device, and the maximum output power that your adapter will need to be able to provide.

You will want to choose the exact voltage that your device is supposed to operate at. Current and power have a broader range of options when choosing your adapter. You can always go above a given requirement for current and power in an adapter, this is because the current will be a result of the given impedance of the device and voltage. Your device will draw a certain amount of current regardless of how much your adapter can supply. When thinking about output power ratings for an adapter, it is good to remember that the rating is a maximum rating. Using any power supply consistently at its maximum power output will eventually cause damage to the power supply so having an adapter that has a higher rating than you need is a good thing. Another thing to keep in mind is whether your adapter requires a positive center or a negative center. This can usually be determined by the markings on the existing adapter that will look like this:

Figure 1: Adapter markings will show whether it has a positive or negative center contact. (Image courtesy of CUI Inc.)

The electrical considerations are only half of the battle when choosing an adapter. Physical dimensions for wall adapters are not standard by any means, but there are a couple of sizes that are very common: 2.1 mm barrel plugs and 2.5 mm barrel plugs.

While the 2.1 mm and 2.5 mm barrel plugs both typically share the same 5.5 mm outer diameter, they vary can vary in length. Beyond the popular 2.1 mm and 2.5 mm plugs there is an assortment of other sizes that are less common which can be confusing and lead to picking the wrong adapter. When faced with the many options of output connectors on the DigiKey website, one can easily be overwhelmed.

There are a few different ways to pick which size your existing charger is to replace it. One way is to use a caliper; this will give you a rough idea of how large the diameter is. There is a good chance that you will not measure exactly 2.1 mm or 2.5 mm for the inner diameter. This can be due to individual tolerances in the connector. This is still a bit crude and could easily miss an odd-sized inner diameter which brings us to our best method, reading any documentation you can find on the existing adapter. A method of doing this would be taking the numbers that can be found on an existing adapter and doing a Google search. Some of these adapters will have a PDF file that the manufacturer has created detailing the specifications of the adapter. An example of this would be the international adapter SMI6-12-V-P6 from CUI Inc. This is a common 12 volt 500 mA adapter that has a 2.5 mm inner diameter and a 5.5 mm outer diameter. Plug dimensions can be seen under the product attributes on the DigiKey website, but can also be found by looking at the datasheet for this product. This can be confusing sometimes because many manufacturers will not have a specific datasheet for a specific product, but rather a general datasheet that covers a whole series of products. This datasheet covers the entire SMI6 series, so there are steps that can be taken to decipher which information is pertinent to an individual product.

Figure 2: Part numbering key for CUI’s SMI6 series power adapters. (Image courtesy of CUI Inc.)

Looking at part number SMI6-12-V-P6 with this part number key reveals everything we need to know about this adapter. We see that it is part of the SMI6 family, the 12 calls out the 12 volt output of the adapter, the V calls out the international blades that come with the charger, and the P6 calls out the physical size of the barrel connector on the end of the adapter. At the end of the datasheet on page 4 we find the output plug options. Looking at the DC plug type we see that the P calls out a positive center, and that the 6 calls out the dimensions of the barrel plug. Figure 3 below is a table from the datasheet to illustrate this point. If we examine this table, we see a column for size. Since we have a 6 for our size callout, we follow the row under size 6 to find dimensions A/B/C of the plug. Dimension A is the outer diameter and is 5.5 mm in this instance. Dimension B is the inner diameter, which happens to be 2.5 mm. Dimension C is the length of the barrel plug, which is 9.5 mm. The last set of numbers on this number scheme have been left blank which means that this is a straight plug with no custom configurations. 

Figure 3: SMI6 datasheet image showing plug configurations and sizes. (Image courtesy of CUI Inc.)

Many wall adapters have options that also call out which blades they will use. Several parts of the world use different blades, so many manufacturers build the option to change the blades out of their products, along with allowing for large input voltage ranges to accommodate different wall voltages around the world. Some wall adapters will not plug directly into a wall. Many adapters are power bricks that need an extra cable to be plugged into them. An example of this would be TE30A1803F01 from SL Power Electronics Manufacture of Condor/Ault Brands. This adapter will need an additional IEC 320-C13 input connector like the 11-00022 from Tensility International Corp. There is another type of brick adapter that is popular; an example is the PSAA15W-050L6. This adapter needs an IEC 320-C7 cable like a AC15WNA from Phihong USA.

Many consumer electronics today are powered by USB adapters which can also be found at DigiKey. USB A is a fairly common type of adapter where the power brick plugs right into the wall and there is a receptacle for a USB male plug. The PSM03A-050Q is an example of a USB A adapter. While USB A is very common for the adapter side, USB mini B and micro B are more common on the device side. Most of the items at DigiKey that have associated products, like the USB cable that plugs into the adapter, have those accessories listed at the bottom of the page under the “For Use With” tab. From the PSM03A-050Q page on the DigiKey site, we can see a couple of items listed under this tab. The IPUSB1CS is an example of USB A to USB mini B, and the IPUSB1MS is an example of USB A to USB micro B. Since these adapters will take a standard USB A male, you can safely use a USB cable from another manufacturer to suit your needs for length and whatever end you need on the other side. USB adapters are becoming more and more popular in vehicles today as well. These adapters, like the 31800 from Assmann WSW Components, plug into the cigarette lighter and have a USB A receptacle and function like the USB adapters mentioned above.

To summarize, picking the correct wall adapter can be a challenge, but if you follow a few steps it is not so daunting. Voltage levels and physical dimensions of the barrel connectors are the two things that you cannot safely deviate from because you may damage your device, or the barrel plug will not fit into the device to provide a secure electrical connection. There are several methods to find the size of your barrel connector, but the best way is by reading the documentation on it, if any exists. Current and power ratings are important in so much as you will want to be at or above your requirements so as to provide as little stress on your adapter as possible. Many adapters require accessories, like an IEC 320-C13 or IEC 320-C7 cable. Many wall adapters are meant for USB devices and typically have a standard USB A receptacle that will accept any USB A male cable. For some applications there are more considerations that will need to be taken, but for most consumer electronics this is a great way to find your adapter.