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Does a transformer need an AC input that goes below 0V?

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cduma

Mon Jul 30 2012, 09:57PM

Registered Member #1822 Joined: Fri Nov 21 2008, 08:04PM
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If a transformer with ratio 1:10 has an incoming sine wave that has a P-P voltage of 12V but, the sine wave only goes between ~0 and 12V will that still give a nice 120V output? I dont have a specific project in mind but, this would clarify a lot of things I dont understand.

Dr. Dark Current

Mon Jul 30 2012, 10:16PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
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The DC voltage on any inductor or transformer winding is 0V (neglecting losses), so you would saturate your transformer. Put a cap in series with it, but then it will see just +6V and -6V peaks.

cduma

Mon Jul 30 2012, 10:22PM

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Thanks!

blackgrunge

Tue Jul 31 2012, 01:39PM

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Correct me if I'm wrong but technically with the correct transformer the signal you are talking about (which is essentially an AC signal but with a 6v offset) will still behave in the same way as an AC signal with 0v offset. The current in the transformer's first coil will be the same as long as the sine wave retains it's shape because the magnetic field which creates the step up or step down is dependent on the power delivered to the primary coil and the ratio of the number of turns.

I simulated your question in MultiSim and the results I got seem to back up this idea:

0v offset:

6v offset:

So it would appear that the DC offset of your AC signal does not play a role in the output current or voltage of the secondary coil.

I'm not sure if I'm missing something but it would seem as though given the right circumstance it wouldn't matter at all.

Dr. Dark Current

Tue Jul 31 2012, 03:10PM

Registered Member #152 Joined: Sun Feb 12 2006, 03:36PM
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The flux density in the core depends on the integral of the voltage applied to the winding. With a signal which has a DC offset, the flux density will have a rising tendency and saturates the transformer after a few cycles.

The rule that a DC voltage on any inductor must be 0V applies always without an exception (well except if you count in the DC ohmic resistance of the winding, which for most practical calculations is omitted). This is because any inductor is a short circuit for a DC signal.

blackgrunge

Tue Jul 31 2012, 03:17PM

Registered Member #1889 Joined: Mon Dec 29 2008, 07:36AM
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Dr. Dark Current wrote ...

The flux density in the core depends on the integral of the voltage applied to the winding. With a signal which has a DC offset, the flux density will have a rising tendency and saturates the transformer after a few cycles.

The rule that a DC voltage on any inductor must be 0V applies always without an exception (well except if you count in the DC ohmic resistance of the winding, which for most practical calculations is omitted). This is because any inductor is a short circuit for a DC signal.

After doing some calculation I found that you're absolutely correct. While the output signal may seem the same for the first few periods it will eventually rise out of control:

Here are my quick calculations for the OP if he's interested in seeing the math behind it:

(press ctrl+R to rotate it)

radiotech

Tue Jul 31 2012, 07:25PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
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The question posed should also describe the transformer because any effects of DC on
the core would be based an the amp-turns involved, so the resistance of the winding
and the size of the core and any air gaps would determine if the AC voltage on the primary
will appear on the secondary, according to the turns ratio, less the DC component.

Transformers routinely are used with AC circuits with large DC offsets in audio
amplifiers.

Steve Conner

Tue Jul 31 2012, 07:32PM

Registered Member #30 Joined: Fri Feb 03 2006, 10:52AM
Location: Glasgow, Scotland
Posts: 6706

True, but irrespective of the transformer design, the average voltage across any winding is always zero, notwithstanding voltage drops across the DC resistance. This has some unusual consequences: for instance in a single-ended vacuum tube amplifier, the plate voltage is roughly equal to the supply voltage at idle, and when a signal is applied it varies between nearly zero and nearly twice the supply voltage.

This brings us to the "Law of Fatal Saturation" which goes thusly:

In order for a transformer to be efficient, the DC winding resistances must be small compared to the load impedances.

Therefore, if the transformer is allowed to saturate, the currents that flow will be large compared to the normal operating currents.

Therefore something will go boom.

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