Why doesn't an AC voltage converter work for DC?

[Strangeline]

I understand the difference between AC and DC, and i also understand how an AC transformer works... but I guess not completely. My question is why can't a DC use its emf to create a lower voltage current like AC can?

Does the DC's emf accrue over time thus making its transformation short lived? If so, i have no idea how that works, since i think the DC's emf should be constant as long as the voltage/current is constant.

 

[tiny-tim]

Welcome to PF!

Hi Strangeline! Welcome to PF!

Does the DC's emf accrue over time thus making its transformation short lived? If so, i have no idea how that works, since i think the DC's emf should be constant as long as the voltage/current is constant.

Yes, you're right.

A transformer does work with direct current, but the core very quickly reaches saturation (ie, all the little magnetic domains are lined up with the applied field), so the magnetic flux stays constant, and there's no more induction.

See http://en.wikipedia.org/wiki/Transformer#Effect_of_frequency" …

The time-derivative term in Faraday's Law shows that the flux in the core is the integral with respect to time of the applied voltage.[36] Hypothetically an ideal transformer would work with direct-current excitation, with the core flux increasing linearly with time.[37] In practice, the flux would rise to the point where magnetic saturation of the core occurs, causing a huge increase in the magnetizing current and overheating the transformer. All practical transformers must therefore operate with alternating (or pulsed) current.[37]​

and also http://en.wikipedia.org/wiki/Saturation_(magnetic)#Introduction" …

Different materials have different saturation levels. For example, high permeability iron alloys used in transformers reach magnetic saturation at 1.6 - 2.2 teslas (T), whereas ferrites saturate at 0.2 - 0.5 T. Some amorphous alloys saturate at 1.2-1.3 T. …​

 

[sophiecentaur]

What comes out of a transformer depends on the input volts changing constantly so DC no good if you want a sustained output..

It is pretty easy to change DC voltages up and down nowadays, using 'switch-mode' techniques. This always involves 'chopping' the DC in some way, though, so you are, in effect, making your own AC in the conversion box.

 

[Strangeline]

Great! thanks for clearing it up, this was driving me crazy