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I understand that a transformer can step up or step down voltages when it is fed in AC. However, I am wondering why can't we use a non constant DC source as input.

For example, if I rectify the AC voltage using a full wave rectifier, I still get a sinusoidally varying voltage which can generate a emf and thus technically be stepped up or down. What is wrong with this method?

Thanks.

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  • $\begingroup$ You would lose any DC component of your signal this way. $\endgroup$ – probably_someone Mar 23 '18 at 22:02
  • $\begingroup$ I wish to rectify the AC voltage and feed it in the transformer continuously rather than directly feeding the AC voltage. Why would there be losses here? And I am not looking at the feasibility, but rather the possibility of stepping up/down DC voltage. Thanks. $\endgroup$ – Alpha7200 Mar 23 '18 at 22:07
  • $\begingroup$ A non-constant DC source is either a pure AC source (in which case you can use a transformer, but you might as well just call it a pure AC source then), or it has some zero-frequency DC component (which will be lost if you use a transformer). $\endgroup$ – probably_someone Mar 23 '18 at 22:11
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    $\begingroup$ The DC component will tend to cause the transformer to saturate and will be wasted just to make the transformer hot. What you need to do is feed the DC into an oscillator that is then fed into the transformer. These are called switching power supplies. There switching frequency is much higher than 50-60Hz, which lets you use a smaller transformer. $\endgroup$ – zeta-band Mar 23 '18 at 22:13
  • $\begingroup$ I think what you're trying to ask doesn't come through in the question. The question you posted asks if you can put a "non constant DC source" through a transformer. However, your comment posted under the question makes it sound like you want to step the voltage of a DC signal, i.e. go from 10 Volts to 5 Volts. If you can please edit the original post to make what you're asking more clear, I would be happy to write an answer. $\endgroup$ – DanielSank Mar 23 '18 at 22:19
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Voltage in a non constant DC source, but not reversing polarity, is also known as fluctuating voltage. It is indeed possible to step it up or down. The primary coil current (and thus voltage) in a transformer induces an emf in the secondary coils as it changes magnitude (this is electromagnetic induction).

Now, a fully rectified (but not smoothed by capacitance) voltage has rounded peaks but sharp troughs (wave minima) as shown in the figure below, bottom diagram, which is for diode bridge, from this reference. This asymmetrical wave form will be maintained after you transform your fluctuating current source. The zero point is at half the waveform amplitude by default. Full wave rectification

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