How to Select the Right External Power Adapter

Whether you need an external power adapter for a new product or to replace an existing one, choosing the correct model can be challenging. Several key factors—such as power output and voltage specifications—must be evaluated carefully, as selecting the wrong specifications can cause issues that may lead to device failure.

So, how do you choose the right external power adapter? This guide outlines the essential parameters to consider when selecting an external power adapter for your device or system.

What Are External Power Adapters?
External power adapters are power supplies that plug into a wall outlet and convert alternating current (AC) into direct current (DC) to operate electronic devices. They are also referred to as AC adapters, AC-DC adapters, AC-DC converters, DC power supplies, external adapter power supplies, or simply power adapters.

These adapters are used across a wide range of applications, including advanced medical equipment such as circulatory pumps, ventilators, dialysis machines, and telemedicine carts; commercial and industrial systems like barcode scanners, weighing scales, and assembly line monitors; and everyday consumer electronics such as laptops, monitors, and printers.

An external power adapter essentially functions as a “black box” with an AC input on one side and a DC output on the other. It generally comes in one of two styles—wall mount or desktop—depending on the needs of the application. A desktop adapter typically rests on the floor or a desk, resembles a brick, and includes a power cord extending from one or both ends. A wall-mount adapter features only an output cord and has built-in power blades that plug directly into an AC outlet.

Wall-mount adapters eliminate the need for an AC input cord but are limited to use with the wall-socket style of a specific country unless equipped with interchangeable AC plugs. Desktop adapters may come with a fixed AC cord and plug, which creates a similar limitation, but most use a standard AC inlet that can be paired with different country-specific AC cords.

Most modern external adapters—both desktop and wall-mount—support a universal AC input range of 90 to 264 volts AC (VAC). This input range covers nearly all global power systems, requiring only a country-specific AC plug or cord. This flexibility supports supply chain efficiency and may help reduce product costs through high-volume manufacturing.

Engineers often choose an external power adapter instead of an internal (embedded) power supply to reduce the size of the main product and simplify replacement. Although the adapter adds size to the overall system, it allows the primary device to be more compact. Additionally, an external adapter can be replaced easily by the user without assistance from a technician, provided the correct adapter is selected for the device.

How Do I Know Which External Adapter to Use?
When selecting an external power adapter for any application, several key parameters must be evaluated to ensure proper performance, compatibility, and safety:

AC Input Voltage
As noted earlier, most modern external power adapters are designed to accept a universal AC input voltage range of 90 to 264 VAC, making them suitable for use in most regions worldwide. In North America, standard household outlets supply 110–120 VAC, while European outlets typically provide 220–240 VAC. Some portable medical devices require an AC input as low as 85 VAC, and certain lighting applications may rely on a 205–277 VAC source. Industrial or infrastructure systems may operate at even higher voltage levels, requiring specialized power converters not covered by typical external adapters.

DC Output Voltage
Any device powered by an external adapter requires a specific DC voltage—commonly anywhere from 3.3 VDC to 56 VDC—to function properly. The adapter you choose must match the device’s required DC output voltage, which is clearly indicated on the adapter’s label. Most external adapters supply a single DC voltage, though some models offer two or three outputs, each with its own power rating. Standard voltage options are widely available, but certain manufacturers, such as Astrodyne TDI, can provide custom adapters with non-standard output voltages when needed.

Power Rating
One critical specification that is sometimes overlooked when selecting an external power adapter is its output power rating. This rating, shown on the adapter’s label and typically expressed in watts (W), indicates the maximum amount of power the adapter can reliably supply to the device. The adapter’s power rating must always be equal to or greater than the power required by the device. If the adapter is under-rated, it may lead to diminished performance, reduced lifespan, or complete failure of the adapter.

It is also important to recognize that an adapter’s power rating is usually defined at room temperature. At elevated temperatures—generally above 40–50°C (105–120°F)—the adapter may not be able to operate at its full rated power. The product datasheet should include a power-derating curve that indicates how much power the adapter can safely deliver at various operating temperatures.

Efficiency Rating
Recent regulations in both commercial and industrial markets require external power adapters to meet minimum efficiency standards. In the United States, the Department of Energy requires that, beginning in 2016, external power supplies introduced into the U.S. market must meet Level VI efficiency requirements, with limited exceptions such as adapters used for FDA-approved medical devices. Similar regulations apply in the European Union.

Adapters that comply with U.S. efficiency requirements are typically labeled with a Roman numeral “VI” inside a circle. Older designs labeled with “IV” or “V” are no longer approved for introduction into the U.S. market. Because some manufacturers still offer non-compliant products, it is important to verify which efficiency level is required for your application and confirm that the adapter meets the appropriate standards.

Connectors
Choosing the proper AC and DC connectors for an external power adapter is essential. For AC input, desktop adapters commonly use IEC320-C14 or C8 inlets, which allow users to pair the adapter with region-specific power cords. Some adapters use a captive AC input cord with a fixed plug designed for a specific country. Wall-mount adapters, by contrast, require integrated AC blades suitable for the country of use or interchangeable blades that support multiple regions.

DC output connectors vary widely to meet the needs of numerous applications. Designers often specify their preferred connector style, and most requirements can be accommodated. The most common DC connector type is the female barrel plug, a two-conductor connector available in many size combinations.

Four parameters must be defined for barrel plugs: inner diameter, outer diameter, barrel length, and polarity—whether the center pin carries the output voltage (the standard configuration) or the return. Additional features may include locking mechanisms, such as threaded collars, to prevent accidental disconnection. Other common DC connector options include multi-pin DIN connectors and Molex-style pin/socket assemblies.

Safety Certifications
External power adapters must meet safety certification requirements for the intended application and the regions where they will be used. Medical devices follow different safety standards than commercial and industrial equipment, and each country or region enforces its own compliance rules. Astrodyne TDI offers external adapters with medical and/or industrial safety certifications for use in the U.S., Canada, and the European Union. Additional country-specific certifications or compliance with alternative standards can be provided upon request.