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

[bmachine]

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.

Bmachine.

 

[trigger]

It is normal, you are measuring the off load voltage and it will be higher than the rated voltage.

 

[audioguru2]

Cheap wall AC-DC adapters do not have voltage regulators. They deliver their rated output voltage only when they have their rated load current.
An adapter with a higher output current rating has an output voltage that doesn't rise much without a load.

 

[allvol]

Hi, bmachine

Using an LM7809 voltage regulator and two common capacitors will produce a steady 9 volts

 

[audioguru2]

Using an LM7809 voltage regulator and two common capacitors will produce a steady 9 volts

 

[ante1]

Hi AG,

I have to agree but, there is some hope though! If the project consumes a lot less power like one tenth of what the wall wart was originally intended for it might work! 

 

[allvol]

Well, phooy.  I guess I'll have to take these that are working on my bench and scrap them.


AllVol

 

[bmachine]

Wow!  Excellent information, all.  Thank you very much.  I guess I'll go ahead and give this a shot.

BTW, Is there a simple way to simulate a load for testing purpose?

Bo.

 

[audioguru2]

Well, phooy.

 

[windoze killa]

Wow!

 

[MP1]

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.>

MP

 

[ante1]

MP,

You can’t use Ohms law here; Ohms law has no “P” for the wattage!  ;D

 

[MP1]

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

MP