DC transformer

[NoHoper]

Should a transformer really pass on smoothed DC like it in the experiment I designed.

I thought transformers only pass on fluctuating currents?

[OStep]

Some background:

Generally a transformer can couple any signal, including DC, among its windings.

What gives rises to the coupling phenomena is the magnetic flux, which is produced by the magentic field (similarly to the way current is produced by voltage potential). This magentic field is directly proportional to the current in a winding. If an applied voltage signal has a non-zero DC component, it will cause the current to rise indefinitely. This is just like applying a DC source to an inductor. So, your magnetic field rises indefinitely. However, the magnetic flux in a real magnetic material cannot rise indefinitely - it saturates at some value of applied magnetic field and you no longer get the coupling effect.

Your case:

In the model you're using, the core is actually simulated using two ideal coupled inductors, which do not saturate. However, note that the current in the primary winding is ramping up VERY quickly.

In the real word:

After a short amount of time, the tranformer would saturate and appear as a short ,or a very small inductor. This would cause even higher currents to flow and probably result in some damage.

Thanks,

[yyao]

To paraphrase what Max said:

You are assuming that the current is not fluctuating on the primary coil which is not true. Place a probe on the primary loop and simulate:

You will notice that the current through the loop rises at an alarming rate. So, the current through the transformer fluctuates a lot!

In your circuit you are using ideal transformers. Ideal transformers are modeled as 2 coupled inductors. The voltage across an inductor is proportional to the time derivative of the current. Alternatively, the current through an inductor is proportional to the time integral of the voltage across it. If you apply a constant voltage, then the current will increase linearly over time. This fluctuation in current is coupled to the secondary coil which generates the voltage which you see.

Please think about your circuit theory when considering these problems. Multisim is a great tool and can help you test your hypotheses, but in the end, it is up to you to understand it.

[Rocky_LP]

Hi, Yi! Your discussion is really helpful. But now I come into another problem which I want you help me with, as below.

If I replace the "Coupled-Inductors" with "1P1S" in the Transformer part, the phenomenon that current rises disappears. Do you know why?

Thanks!

Rocky

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