Re: Transformer drawing 1 amp

Ok, Thanks again, Hopefully I'll understand more of this stuff when I
play around with transformers. I have some idea from physics but I
don't have any working knowledge and I've forgot a lot of stuff.

Bob


On Mar 15, 12:29 pm, John Popelish <jpopel...@xxxxxxxx> wrote:
Bob.Jones5...@xxxxxxxxx wrote:
On the case(of the charger, not the transformer) it says

Input: 120VAC 50/60 Hz 2.8 Amps
Output: 12VDC 15 Amps

This is why I assumed it was 15 Amps.

Now since the ratio is 10:1 that means that it would draw a max
current of 1.5 Amps at full load on the primary due to the load. Since
I'm drawing 1A without any load this is about 2.5A on the primary at
full load. So it seems to work out. I guess this transformer is a
current limiting one.

I think it is better described as a cheap transformer
(minimum copper and iron, intended to run hot) that has a
soft output regulation. In other words, you can expect a
fairly large change (say, 10%) change in secondary voltage
between zero load and full load. A true current limiting
transformer would have the secondary voltage go to almost
zero at something like full rated to twice full rated
current. This gets into what "full load rating" means.

Now if I understand a current limiting transformer it is made in such
a way as to limit the max current drawn on the primary? It does this
by sacrificing power?

Not so much. It does that by shifting the phase between
voltage and current. Current through an inductance is 90
degrees delayed with respect to the applied voltage, so that
almost half the time, the current is in the reverse
direction, compared to the voltage. During those parts of
the cycle, the energy previously stored in the magnetic
field (when the current and voltage had the same sign) are
being sent back to the power line. There is current, but
the average power (the average of the positive instantaneous
power when the magnetic field energy is increasing and the
negative instantaneous power when the magnetic field energy
is decreasing) is quite a bit less than the product of the
RMS voltage and RMS current (VA). So a 1 amp primary idling
current does not mean that the transformer is soaking up all
of 120 watts, but only some fraction of that.

So at full load of 15A the primary will be
pulling much more current than it should but if I tried to pull 20A it
would not let me?

If it was a current limiting design, the secondary voltage
would severely collapse as you passed through full rated
current. If you shorted the secondary (collapse the voltage
all the way to zero) the current would not be limited only
by the resistance of the windings, but also by the effective
inductance the design puts in series with the secondary.

Sorta like some sort of over current protection? A
"normal" transformer would not get as hot but it seems to be ok for
this one?

Thanks,
Bob


.

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