Re: Calculating DC Output Current From Unregulated AC Transformer

On Jul 9, 10:57 pm, "Phil Allison" <philalli...@xxxxxxxxxx> wrote:
<emailaddr...@xxxxxxxxxxxxx>
"Phil Allison" >
** This is your error.

This what?  I was looking for details.

** Read the whole post before making silly replies.

** Phil, isn't this a bit of irony since I clearly asked for something
specific and you replied throughout your entire prior post with
anything except what was asked?



The 1.67 amp ( ie 20VA) rating of the transformer is only for AC output -
not some derived DC voltage.

Ok, but I'm not asking what it's not, I'm asking what it is.

**  Read the whole post before making silly replies.

See "** Phil" above.
I recall you're always a bit cranky but that's why I wrote what I did
two lines up, because I'm "asking what it is", for an equation, which
makes it seem as though you either choose not to provide one, or you
don't know. If you don't know it's not a problem, but isn't it
important to clarify what I am looking for?



what I
am asking is what is the like-equivalent DC it should be capable of
based on the AC rating.

**  Read the whole post before making silly replies.

Because if I do, I'll see the equation(s) I was asking about?



Is 20VA really the answer, since we're
talking about winding ratios?  It would seem not, since we do see
things like peak output around 16V instead of ( picking some number
out of  air) 40V @ 0.5A for 20VA.

**  Read the whole post before making silly replies.

See above.


It is also a " free air " rating - meaning
there must be plenty of ventilation and no heat sources next to the
transformer.

Yes, I am looking only for a free air rating.  It all starts there.

**  Don't complain when you get free, good advice -  fuckhead.

Phil, how do I put it so you understand? If you really want to build
this entire project for me then go right ahead, I'll email you the
parameters. Until then, all I needed was what was asked, not for you
to second-guess something that was even a first-guess since you have
no idea what current I plan to run from any particular transformer,
you only know that I asked how to convert a spec mathematically and
used a few numbers as random examples of a starting point.


I am not interested in what overloads it,

** You need to be  -  for safety and functional reasons.

This is not a fixed plan for a one-off design Phil, this is a generic
question about transformers. The numbers were just random, forget the
numbers because they have nothing to do with the topic. I am not
interested because I have no trouble building within margins once I
know what those are - which requires having the equations.

Because I am not dead set on using any particular transformer for any
particular thing, it is valid and correct to say I am not interested
in what overloads any one particular transformer, only in how to
mathematically determine the free air DC ratings from the free air AC
ratings.



  Read the whole post before making silly replies.

I am interested in what
current it can supply at 12.3V or what voltage it would be at 1.67A
DC, what the math is to arrive at that free air rating for DC current
and DC volts give either or the other would be a variable.

** There is no possible math based only on the figures you gave.

False. Given all the parameters it can be solved. Some of those
paramaters I do not have like transformer core saturation values,
others can be resolved experimentally but that can still be expressed
in an equation.


The whole
topic is about ACTUAL temp rise of the windings of the particular
transformer under particular conditions.

This is getting amusing. The whole topic is not about building one
thing with one transformer and what that one transformer's rating
should be. The whole topic which I started (least you forget?) was
asking for equations. If you don't know those, join the club- neither
do I. I could guess, have some theories but it would be better to
hear from others who can stay focused on the topic.

Normally, one applies a de-rating figure to the AC current rating of a
transforemer when it is feeding a bridge rectifier and capacitor filter..
The figure depends somewhat on the design of the particular transformer (
toroidal, E-core ) but is in the range of 0.5 to 0.7.

So, if you really do need 1.67 amps DC at 16 volts - then pick a 40 VA
transformer.

As mentioned I need to know the transformer's capability, not 1.67A at
16V.  Those were example figures, I need the actual equations for any
transformer, even the 40VA one must have that.

** I just supplied the derating figures and the reason why it cannot be
precise.

You used the term "derating", are you saying it is not a derating
figure but rather a conversion figure? I ask because you have gone on
and on about derating for temp, when I am very specifically looking
for only a mathematical equation for maximum transformer capability, a
theoretical one in a model that ignores temperature because the answer
is not going to be directly used to build something!

So are you claiming that if I short circuited the rectified output of
a transformer rated for 1.67A @ 12VAC, that the output would be 20VA *
0.5 to 0.7? I do not think that is the correct answer, I vaguely
recall it should be higher than that.

If the shorted output power is not 20VA * 0.5 to 0.7V, then you have
not answered the question, because I am not asking about
implementation details for any transformer, only how to convert
mathematically to actual peak, not prudent design values. Perhaps I
did not initially describe the question well enough. If so that is my
fault, but I hope by now it is clear I am not looking to directly
apply any info directly towards choosing the mentioned transformer for
anything in particular based only on shorted power.

I can see I should not have given any details about the project
because I only want to know how to convert a AC transformer's voltage
and current rating into a DC voltage at same current, or the resulting
current at a specific DC voltage.

** Neither is possible with simple math and only the VA rating to go on.

If it's complex math so be it. If it were simple I expect I'd have
known already, which was why it was asked regardless of how complex
the answer would be, but perhaps to the wrong group, perhaps it is not
a sci.electronics.basics level question. Ultimately if I find a spare
rectifier board lying around I'll hook that up and short it to get an
answer experimentally, at least for one transformer but it still
doesn't provide the sought after equation, I'd have to speculate about
whether similar E-Cores (which the example transformer was) behave the
same and collect more data about other E-cores in the same scenario,
hoping all the while that their manufacturers rated them equivalently,
accurately, then try to find some relationship.


I have written a ( non simple) program that gets fairly close to predicting
the results ( ie the DC output voltage at any specified current )  -
providing you have much more data on the transformer like the primary and
secondary resistances, the size of the filter cap and a figure for
transformer leakage inductance.

Then the equation(s) used in that would be much closer to what I was
after. Variables unresolvable could be assumed as constants then
experimentally compared to determine the level of deviation.



You are WAY over-analysing the problem.

You only need to be sure the voltage is high enough, but not too high,  for
you charger to work and the transformer is not overloaded under any
operating condition.


Ok, back to the charger. I have too many transformers to count that
could be used there. The transformer heat is not a problem. Using a
fan would be fine, having it shut down during regular use would even
be fine through thermal cutoff circuit(s). I would rather a warmish
running transformer with a fan cooling it than a burnt out regulator,
a bigger case for a giant heatsink plus added weight, or hot project
case.

With all due respect Phil, you have no idea of the project
requirements beyond what I had told you. Three manditory ones are low
temp density, small size, light weight. I'd MUCH rather put a thermal
cutoff or two in than dump a lot of heat on a regulator. A 40VA
transformer is not going to work, unless it's one rated slightly below
12V which is an odd figure. I guess I could strip some windings off
of one but that is such an ugly solution.

Ultimately for that particular project I will most likely use a SMPS
instead, but that project was only an example not the question asked
which is what the equation is to determine AC to DC spec conversion.
I do not need an answer based on the limited specs of the transformer
info I had given, I only need the equation as complete as it can be
for future uses.

Perhaps we got off on the wrong foot Phil, but please believe someone
when they state plainly what they don't care about and focus on what
they asked. This is NOT a topic about building one project, this is
about actual vis-a-vis rating conversion. I wouldn't have ran a 12VAC
1.67A transformer at 1.67A, AC output either, I know about derating
and temperature issues already which is why I stated several times
that this was not what I asked about, not what the question was about,
not what I care about. I care only about the equation. If you have
the answer because of your computer application that was relevant.

I hope I've made things clearer. I do not care about anything except
an equation to determine exactly what I asked about. It was why I
asked that question instead of some other question.
.

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