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Simple question about measuring output of a wall mount transformer...


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Hello, dumb question here.

I need to use one of those little wall mounted ac/dc converter for a little 555 based project I am doing.  I found an old one that is labeled to have a 9V DC output.  But I put my voltmeter to it, it reads 14volts.  I then tried a couple that were rated at 24v output and they read more like 28 or 29v.

Is this normal?  Is it because they need to be measured under some kind of load or something?  Is my transformer really going to give 9V to my project or is it going to fry everything by delivering 14 volts?

Thank you very much.


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The wall wart will give you all the informatin you need. If it states 9V@500mA, you just need to use a resistance that will cause a 500 mA load from 9 volts. Make sure your Watt rating on the resistor is high enough. You can calculate this on your own. It is easy with ohm's law, and then you learn something also.

<Time to brush off the dust on the old ohm's law book.>


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This is true. Even though we have attributed the Watts calculation with ohm's law and most of the ohm's law calculators include it, power calculation was not part of the original ohm's law. It is an extended calculation.  ;D


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