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Selecting an AC-DC Power Adapter

How to choose the AC to DC power adapter you need. A brief explanation of voltage unregulated and regulated power adapters.
Author: David Kolanowski

How do I choose the right AC to DC power adapter for my needs?
Many people considering a power adapter purchase are unsure about how to choose the power adapter they need because they're unfamiliar with a few basic concepts about power adapters. This article attempts to explain some of the basic differences between the most common types of power adapters available, and guide you in selecting a power adapter that best fits your requirement.

Typical AC to DC power adapter

Typical unregulated AC to DC "wall wart" power adapter

Obviously, not all power adapters are the same. They come in different voltage outputs, current outputs, power ratings, and input-output connector types and sizes. In fact, some power adapters have strictly AC voltage outputs, while others have DC voltage outputs (regulated DC or UN-regulated DC). A few of the concepts to follow are shared by both AC and DC voltage output power adapters, but in this article, we focus mainly on DC voltage output power adapters and how to use this information to select the right one for your needs.

Unregulated DC Power Adapters

Regulated or un-regulated: what does it mean?
A voltage regulator is a device (usually an electronic circuit) that controls or maintains the constant voltage output of a power supply. Some (more expensive) DC voltage output power adapters have voltage regulators built in, while most do not.

Unregulated DC output power adapters are still the most common type of power adapter in use today. Unregulated, or "non-regulated" simply means the power adapter is not subject to the control or authority of a voltage regulator. With unregulated power adapters, or any other unregulated DC power supply, the DC output voltage is dependent upon an internal voltage reduction transformer, and related to the amount of current used by the electrical load. (The load is the device receiving power from the adapter).

The first important concept to understand about unregulated DC voltage output power adapters is that they are designed and rated to produce a particular voltage at a particular maximum output load current. The output voltage of an unregulated power adapter will decrease as the current provided by the adapter output to the load increases. The power adapter selected should be matched as closely as possible to the voltage and current power requirements of the load.

Example: A given unregulated power adapter has an output rating of 9 volts DC at 500ma (500 milliamps = 0.5 Amps).
This rating means that the unregulated voltage output of the power adapter will be 9 volts DC when it is under the full rated load of 500ma. Many people are surprised when they plug an unregulated power adapter into a wall outlet and measure the DC output voltage of the power adapter with a volt meter. While the adapter is providing no power to a load device, the measured output voltage is several volts higher than the rated voltage of the power adapter. Often, people jump to the conclusion that something is wrong with the power adapter, while the simple explanation is that the power adapter has no voltage regulator, and this is normal. But if the same unregulated power adapter is connected to a load which draws 500ma of current, the measured output voltage of the power adapter would drop to the rated 9 volt DC output level as it should.

The second important concept to understand about any power adapter, whether regulated or unregulated, is that the rated output current of the adapter should not be exceeded by the maximum current requirement of the load.

Example:  If a power adapter has an output current rating of 500ma, it should never be connected to a load that will draw more than 500ma of current from the adapter. Drawing more than the rated current output from an unregulated power adapter will certainly cause the actual output voltage to drop below the rated value, and overloading any power adapter in this way will lead to overheating and premature failure of the adapter. Overloading a power adapter may cause a fire hazard and depending on the power sensitivities of the load, may cause damage to the load as well. Most power adapters made today, have built in overload protection (essentially an internal fuse to prevent the adapter from going up in a puff of smoke), but overloading the adapter may still cause it permanent damage. Appropriate external fuse protection can be added if the extra precaution is warranted.

A third concept about unregulated power adapters which for most applications is less important, but still helpful for people to understand, is that unregulated DC output power adapters do not produce a pure and "clean" DC voltage. But if the power adapter selected is closely matched to the power requirements of the load, the purity of the DC voltage produced is more than adequate for many jobs (with few exceptions).

Explanation:  This is due to the way AC voltage from the wall outlet is transformed into a lower DC voltage and goes beyond the scope of this article, but suffice it to say; the DC output voltage of unregulated DC power adapters have a fairly significant AC "ripple voltage" component (known as noise), that rides on top of the DC voltage component at the power adapter output. This "ripple voltage" can range from several millivolts to a volt or more, depending on the efficiency of the match of the power adapter to the load. Though it usually causes no problems, if the ripple voltage is large enough, it can lead to unexpected erratic behavior of some voltage sensitive load circuits, such as digital logic circuits that switch unexpectedly, or mechanical relays that "chatter". (most commercial appliances are designed to compensate for, or be tolerant of, ripple voltage).

If needed, the AC "ripple voltage" can be greatly reduced and largely eliminated by adding a "filter" (or smoothing) capacitor across the positive (+) and negative (-) output of the power adapter (where the power output meets the input of the load). A capacitor value of 100µF to 470µF can typically reduce the ripple voltage by 50 to 90% or more. While not providing true regulation, the filter capacitor does provide some degree of increased voltage stability, during minor short term variations in load power requirements. It should be noted that if a filter capacitor is added to smooth the output ripple voltage of the power adapter, the resultant "average" DC voltage measured at the power adapter output will generally increase by a small amount (to near the peak value of the ripple voltage), often as much as a volt or more, and this effect should be taken into consideration, as this increased cleaner DC voltage will be supplied to the load device. Care should be take to ensure that the maximum input voltage of the load device is not exceeded.

How to select an unregulated power adapter
With the concepts above considered, selecting an unregulated AC to DC power adapter for your application can be summarized as follows:

Regulated DC Power Adapters

Choosing a regulated DC output voltage power adapter leaves less to consider, because the DC output voltage of a regulated (also known as stabilized) power adapter always stays within the rated value regardless of the amount of current consumed by the load, as long as the rated output current of the power adapter is not exceeded. Why aren't all power adapters regulated? The main reason is added cost. However, if cost is not an issue (and it's really not that bad), modern regulated switching power adapters offer some other distinct advantages over their unregulated counterparts.

Regulated switching power adapter

Typical AC to DC regulated switching power adapter

While older regulated power adapters are really just like the previously described unregulated transformer type with a linear (analog) voltage regulator built in (and with many of the same limitations), today's modern "switch mode" regulated power adapters are a big step forward in technology. Gone, are the bulky, heavy, heat generating, internal voltage reduction transformers. Replaced by high speed digital switching circuitry that shrinks the overall size and weight, and greatly reduces heat generated "losses", while at the same time, greatly increasing the available output power. The result is much higher efficiency and tighter regulation control. In fact, many modern regulated switching power adapters offer multiple user selectable DC output voltages, (all regulated) and come with a range of user configurable output connectors. The user may select from a range of available output connector types and sizes, and change the output polarity of the connector as well. What's more, the permissible AC input voltage and frequency of modern switching power adapters have a range from 100 volts AC up to 240 volts AC and will operate equally well with 50 to 60 hertz (Hz) on their inputs. This makes them useful as a generic plug in "wall wart" power adapter virtually all over the world.

So now, with these modern switching power adapters, one small light weight power adapter may be all you need to charge or run your digital camera, camcorder, your cell phone, your electric shaver, cordless drill, power tools, gardening tools, car vac, flashlights, your portable printer, radio, media player, PDA, video game, portable TV, battery chargers, etc. If you tend to travel with just a few of these items, one modern switching power adapter can really lighten your load, and will likely save you money too.

How to select a regulated switching power adapter
With the many applications and advantages discussed above considered, selecting an regulated switching power adapter for your application can be summarized as follows:

Conclusion

Choosing the correct power adapter isn't difficult when the power, performance requirements, limitations, and advantages are understood. Unregulated DC output power adapters are cheap and plentiful, and available in many voltage and current output variations, though they have more limitations than regulated switching power adapters do. Unregulated DC output power adapters are better suited for dedicated applications where portability and power sensitivity are not considered a big issue. Regulated switching power adapters are the much more versatile and efficient variety. If a user can power a small number of appliances with a single regulated switching power adapter, they can be more cost effective as well.