Why does wall transformers output more voltage than printed?

[Lac]

When I measure the voltage at a selection of wall transformers that I have laying around, I see that the actual voltaged measured is greater than the output voltage printed on the transformer.

ex. a +12VDC transformer outputs +18VDC, a +9VDC outputs +14,8VDC and a +4,5VDC ouputs +5,07VDC. Why does it outputs more voltage than rated? And why is it printed a lower voltage on the transformer than it actually outputs?

Cheers!
Lac.

 

[Nigel Goodwin]

Re: Why does wall transformers output more voltage than prin

When I measure the voltage at a selection of wall transformers that I have laying around, I see that the actual voltaged measured is greater than the output voltage printed on the transformer.

ex. a +12VDC transformer outputs +18VDC, a +9VDC outputs +14,8VDC and a +4,5VDC ouputs +5,07VDC. Why does it outputs more voltage than rated? And why is it printed a lower voltage on the transformer than it actually outputs?

Because they are cheap and nasty ones, unregulated - the specified voltage will be under full load, so off load it will be a great deal more (as you've found out). Either use the better quality regulated supplies, or make sure what you feed is happy with much higher voltages - and lots of mains ripple!.

A major cause of PIC programmers not working is people using cheap unregulated supplies!.

 

[Gaston]

i think what may be happening is this. The output may say 24 volts rms. then when you rectify the output and put a filtering cap on there it jumps up to mabey 34 volts. this is because you are reading peak voltage after rectification

 

[gerty]

Usually when you load the transformer, the voltage goes down closer to what it's supposed to be...

 

[Oznog]

The voltage drops as you load it, due to the magnetic and resistive impedances inherent to the device. It the ratings should basically be correct that you have 12V @ 1 amp. If you leave it unloaded, you may have 16V, if you try to pull 2 amps, you may get 8V.

Also, if we're talking about a transformer with a rectifier & caps built in, note there is no ripple as it's unloaded since the caps charge to peak voltage and there is nothing to discharge them. Once you load it, ripple will lower the DC output even if the transformer's AC output is unchanged (i.e. perfect transformer).

And note the wall voltage is not perfectly regulated, you can see anything from 110V to 125V and this will directly affect the transformer output.