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

 

[sardar]

An AC voltage converter works by using the changing magnetic field created by alternating current to induce a voltage in the secondary coil of the transformer. This changing magnetic field is what allows for the transformation of voltage from high to low or vice versa.

In DC, however, there is no changing magnetic field as the current flows in only one direction. This means that there is no alternating current to induce a voltage in the secondary coil of the transformer. Therefore, a DC voltage converter cannot work in the same way as an AC converter.

Additionally, the emf of a DC source is not constant over time. It may seem constant when the voltage and current are steady, but in reality, the emf can fluctuate due to factors such as resistance, temperature, and external influences. This means that the transformation of voltage in a DC circuit would not be consistent and reliable.

In order to convert DC voltage, a different method such as a DC-DC converter or a rectifier circuit must be used. These methods involve changing the circuit components to manipulate the voltage and create a different output. It is important to understand the fundamental differences between AC and DC in order to properly design and use voltage converters.