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Re: Here's some more clarification.

Posted by dave slagle (M) on October 25, 2006 at 08:03:58

In Reply to: Re: Here's some more clarification. posted by kurt s on October 25, 2006 at 00:55:36:

hey Kurt,

This is because the power applied by the energized core of the transformer has to go up to supply the added power that is delivered to the load since the load didn't change and the turns ratio didn't change.

I still think you are in error. Lenz Law suggests that in an ideal lossless situation a transformer of any size could deliver infinite power (current). It is my contention that this means that power delivered to the load has zero practical effect on the B-field.

But if you increase the voltage input and drop the turns ratio to maintain the same signal output then the power delivered to the same load is identical. This should maintain the same B field in the core, not decrease it.

Look at the formula for b-field, it simply states that if you increase the voltage, you increase the flux. Nowhere is there a provision for the power delivered to the load. In other word, if you believe Lenz, loading of a transformer has no effect on the B-field.

The only way to decrease the B field in the core and get the same signal output is to do one or more of three things:

i don't see where the "same signal out" comes in here, that should simply be a matter of turns ratio, and the associated impedances.

increase the core volume

yes, this is the Ac (AREAcore not Vac in the formula)

increase the number of primary turns

yes, but i would simply state increase the turns. Adding the word primary only holds for a stepdown. if it were a step up you would have to say decrease the secondary turns and then it just gets confusing.

or decrease the permeability of the core material.

the perm of the core has no effect on the flux in the core, so i'm not sure what you mean here.

that the B field can also become too small and begin a different form of distortion - the effect of stuck "domains" in the core that need enough energy to overcome it.

blasphemy, don't you know that the lower the signal, the lower the distortion :-) Seriously, i agree 100% with you on this one and have been trying to at really low level behavior of cores and lets just say it ain't easy. this is probably getting a bit too technical so lets just say i may be your only supporter in this claim.

To be complete, you can also decrease the B field by increasing the frequency. (what a can of worms that statement opens :-)

Somewhere in this design there is some optimal size and turns ratio for a given signal level, load, and core material.

absolutely, I would also include source impedance, and consider signal level and source impedance to be the two most important things to know when starting a design. Those two numbers tell you how much flux and inductance you need, then the rest needs to be fit into place.

dave

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