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How to Select the Right External Power Adapter


Whether you need an external power adapter for a new product or as a replacement for an existing product, finding the right one can be tricky. There are a few details that are important to consider when selecting an external power adapter, including its power and voltage specifications, which if selected appropriately will prevent certain issues that can lead to product failure.

How do you choose an external power adapter? This guide covers several parameters to consider when selecting the right external power adapter for your device or system.

What Are External Power Adapters?

External power adapters are a type of power supply that plugs into a wall outlet and convert alternating current (AC) to a direct current (DC) that powers certain electronic devices. They may also be called AC adapters, AC-DC adapters, AC-DC converters, DC power supplies, external adapter power supplies, or simply power adapters. 

External power adapters are used in a variety of applications, from sophisticated medical devices like circulatory pumps, ventilators, dialysis equipment, and telemedicine carts, to commercial or industrial manufacturing equipment like barcode scanners, weighing scales, and assembly line monitors, to consumer devices like laptop computers, computer monitors and printers. 

You can think of an external power adapter as a “black box,” with the AC input on one side and the DC output on the other side. It is typically offered in one of two styles, depending on the application requirements: wall mount or desktop. A desktop power adapter is designed to sit on the floor or a desktop and is typically shaped like a brick and has a power cord sticking out of one or opposite ends. A wall mount power adapter has only an output cord at one end and includes power blades that protrude from the body of its case to plug directly into an AC wall socket.

A wall mount power adapter eliminates the need for an AC input cord but is limited to being used with the style of AC wall socket specific to a particular country unless it has an option for interchangeable AC plugs. A desktop adapter may include a captive AC cord and plug, which would have a similar limitation to a wall mount adapter, but most use a standard AC inlet that can accept different AC cords with plugs specific to whichever country in which they will be used. 

Most external power adapters available today – both wall-mount and desktop – can accept a “universal” AC input voltage of 90 to 264 volts AC (VAC). This voltage range will support the vast majority of countries, with the need to match only the AC input cord or wall plugs for a specific country; the power adapter itself can be used universally. This feature results in supply chain efficiency, and may even result in reduced product costs by leveraging volume manufacturing.

The main reasons for a product engineer to use an external power adapter over an internal (“embedded”) power supply in their system are size reduction and ease of replacement. By using an external adapter, the product itself can be made smaller (although the power adapter itself adds size back to the complete system – no free lunches!) And an external adapter can easily be replaced by a user, without the need for support from an experienced technician. The user simply needs to ensure that they have selected an adapter that will work with their product.

How Do I Know Which External Adapter to Use?

When choosing an external power adapter for any application, one must consider a few critical parameters to ensure that the adapter will perform effectively and safely:

AC Input Voltage

As previously mentioned, most external power adapters available today can accept a “universal” 90 to 264 VAC input voltage, which can support the vast majority of worldwide applications. In North America, domestic outlets supply 110-120 VAC. In Europe, they supply 220-240 VAC. Portable medical applications may require an AC input voltage as low as 85 VAC, and lighting applications often use a 277-205 VAC source. Note that some industrial and many infrastructure applications use a much higher AC voltage range, which would require different types of power converters.

DC Output Voltage

A product or device that is powered by an external power adapter requires a particular voltage, usually in a range from as low as 3.3 VDC to as high as 56 VDC, to operate correctly. An adapter must be selected with the appropriate matching DC output voltage, which will be marked on its label. Most adapters provide a single DC output, but some adapters provide two or three different output voltages (each with its power rating.) A selection of adapter models with standard output voltage increments are generally available, and custom models with non-standard output voltages can be provided from certain vendors, including Astrodyne TDI, as well.

Power Rating

One of the most important external power adapter specifications that is often overlooked is its output power rating. An adapter’s power rating is marked on its label, typically specified in Watts (W). This rating depicts the maximum power that can be reliably delivered from the adapter to the product being powered. The power rating of the adapter must always be equal to or greater than the power that will be consumed by the product being powered. Otherwise, undesirable results, such as reduced adapter lifetime, limited performance, or complete loss of functionality could occur.

It is important to note that an adapter's power rating is generally defined for use at room temperature. Operation at temperatures well above that - typically above 40-50 deg C (105 to 120 deg F) – may not support the full power rating. The datasheet for the adapter should include a power derate over-temperature graph, which details the maximum power that can be delivered at given operating temperatures.

Efficiency Rating

Recent commercial and industry regulations mandate minimum levels of power efficiency for external adapter power supplies. For example, in the United States, the Department of Energy instituted a requirement that beginning in 2016, all external adapter power supplies introduced into the U.S. market (with a few exceptions, including adapters used to power FDA-approved medical devices) must meet Level VI efficiency requirements. Similar mandates exist in the European Union. 

Adapters that meet these U.S. requirements are usually marked with the roman numeral VI enclosed in a circle. Older designs marked with roman numerals IV or V are no longer approved for U.S. market introduction. Many external power adapter manufacturers still provide solutions that do not meet the new requirements, so it is important to confirm what is needed for a particular application and whether the power supply being considered conforms to the applicable mandates.

Connectors

Connectors that are appropriate for each application will need to be selected for both the AC input and the DC output of the external adapter power supply. For the AC input, a common practice for desktop adapters is to use an IEC320-C14 or -C8 inlet, to which standard power cords with wall socket plug ends suitable for use in specific countries can be separately attached. Alternately, a captive AC input cord may be used, with a wall socket plug specific to a particular country or countries. Wall mount adapters will require fixed blades specific to the country of use, or a selection of interchangeable blades that can support a variety of countries.

A great diversity of DC power connectors are required by the innumerable applications that can use external power adapters. System designers tend to make their own decisions on what connector they prefer, and it is usually straightforward to accommodate almost most requests. The most common standard DC connectors are female barrel plugs, which are two-conductor (output voltage and return) solutions that are also available in a variety of sizes.

The four main parameters that need to be specified for a barrel plug are inner diameter, outer diameter, barrel length, and polarity - whether the center contact is the output voltage (standard configuration) or the return. Other barrel plug features include whether there is a “locking” mechanism, such as a screw-on collet, to secure the plug to the jack on the device being powered. Other types of common connectors include multi-pin DINs and “Molex” style pin/socket receptacles.

Safety Certifications

Finally, external power adapters will require safety certifications specific to the applications and countries in which they will be used. There are different certification standards for medical applications than there are for commercial/industrial applications, and each country or region requires its certifications as well. Astrodyne TDI offers adapters with either medical or safety certifications (and sometimes both), for standard use in the U.S., Canada, and the European Union. Additional country safety certifications, and certification to other standards, can be obtained upon request.

Contact Us With Any Questions

Choosing an external power adapter for any specific application requires an understanding of the electrical performance, mechanical and regulatory requirements needed for that application. The many considerations involved can make adapter selection tricky, but Astrodyne TDI can help you sort through these considerations and find the right adapter for your needs. At Astrodyne TDI, our experts understand the nuances of external power adapters, and web design and manufacture solutions for a broad range of medical and industrial applications. Reach out to us if you have any questions about finding the right adapter for your system.

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