AC/DC Adapter showing higher than rated volts

Regulated Power Supplies

Some power suppllies are highly regulated, some not regulated at all, with everything in between.

A typical low cost DC power supply is nothing more than a transformer, bridge rectifier, an occasionally a smoothing capacitor. This style power supply will show a higher than rated voltage when there is no load. The rating indicates that at maximum load (A or mA) the voltage (V) will be at least as stated.

Place an approriate load on the power supply. Something that draws roughly the current that the power supply is rated at. Measure and you should find the voltage is correct.

For powering voltage sensitive circuits use a regulated supply (much more $$) or build a simple voltage regulator.

Here is an example of a very simple 5v regulator using the common 78xx series regulators. This will output exactly 5v at all loads.

Variable voltage AC/DC adapter

I think I have an older version of the same Radio Shack Variable multi Voltage power supply and the reason it is a little higher in voltage is because your reading a no load voltage. Find an old toy or small electrial low voltage (12v) motor and hook it to the adapter. While the motor is running measure the voltage across the terminals.

Does your Power supply have a slide switch you move to each voltage? if it does then its the same unit as mine and under a normal load the motor will draw just what it needs. I've had mine for 10 or more years and it hasn't smoked anything yet.

In the case of the Radio Shack AC/DC voltage adapter yes it's normal to have a higher voltage than rated. I think it's becauce it's a Varable voltage device and not a set voltage device that makes the voltage higher than it's suspose to be.

Dan

One more thing!

Per LK&O's suggestion, that would work...  BUT NOTE: 

when running these things from a higher voltage, they regulate the output to the desired voltage by 'dumping' excess power as Heat... sometime Lots of heat.  You can start at 30+ volts and regulate down to 3... just for example depending on Your components (for 3v, using )  and inputs.  It's not what I'd do (And I bet LK wouldn't, either)

BE CAREFUL TO USE A HEAT SINK. and carefully check how hot it gets under load... then build a safe enclosure for it... ventilate if needed.

To reduce all this concern, start with an input voltage that's close (perhaps 120% of desired output) rather than something purely withing the regulator's capability. 

78xx

Peter is correct that considerable heat will be generated by a 78xx regulator however the heat is relative to the load. At full current (.5A - 2.2A device depending) a heat sink is essential as a lot of heat is made. At no load there is no heat at all. When using a 78xx to drive low current devices such as LEDs the current demand is so low that no heat sink is required. Only as you approach the devices rated output does a heat sink become necessary. They also have built in over heat shutdown to prevent damage. A very useful data sheet is here.

I am a big fan of 78xx regulators. I have used them in a variety of applications over the years and found them to be rugged little devices requiring only a couple external components to make a reliable voltage regulator. With a RS board # 276-159  and a hot iron you can have a working regulator in under 5 minutes. I use TO220 heat sinks with 78xx regulators and it works great.

There is one other factor.

Power companies often raise the voltage to deal with excess power requirements without the need to install extra power sources at least they do here in California.  Technically your power at the wall should be 110 volts ac, but out here it is always 125 volts ac.  The step down of a transformer is always according to the ratio of windings between the primary side and the secondary (low voltage) side.  Also rectifiers (usually diode bridges) use up a little power, so a transformer might be set up to step 110 volts ac down to 15 volts ac to allow for the power loss through the rectifier and end up with 12 volts dc.  If you put 125 or 130 volts into the primary side, you may get 13 volts or even a little more out the secondary side.