Calculating DC Output Current From Unregulated AC Transformer

[[email protected]]

Go tohttp://www.hammondmfg.com/5cpwr.htmand download their Power
Transformer Selection Guide - it is a one page .pdf that shows various
circuit configurations and resulting voltage/current relations.

--
Peter Bennett, VE7CEI  
peterbb4 (at) interchange.ubc.ca  
GPS and NMEA info:http://vancouver-webpages.com/peter
Vancouver Power Squadron:http://vancouver.powersquadron.ca

Thank you. This is a lot closer to what I was looking for. While it
may not resolve all variables it begins to explain the discrepancies
between what a certain someone insisted and what has been observed
about existing designs field proven to work acceptibly.

 

[Phil Allison]

Apparently not, because plenty of products out there use wall warts,
are you suggesting they all have to build prototypes for even a simple
AC-DC wall wart?

** They do exactly that.

But they also use computer models ( produced in house) to create new
designs PLUS use all their accumulated knowledge from all their previous
ones.

Such custom engineered transformers are NOT available as off the shelf
items from the usual dealers.

You gave a figure of 0.5 to 0.7 times the rating but
clearly darn near every wart here (quite a big box full) don't adhere
to this,

** Complete bollocks !!

No wall wart tranny has its VA rating supplied to the user - so YOU are
BULLSHITTING !!!

All wall wart trannies run comfortably within safe temp limits - so the
(unknown) VA rating is not being exceeded.

Nearly all wall wart trannys have INTERNAL thermal fuse protection in case
of overload - a must for them to pass compulsory safety regulation in most
countries. So it they are even slightly overloaded for too long - they go
dead and do not become a hazard to users.

they're all using transformers smaller than your suggestion
would imply

** Nonsense.

I flushed the rest of your spew inducing, puerile verbal diarrhoea where it
belongs.

Go drop dead

- you VILE nascissticic prick .



....... Phil

 

[Hammy]

[snip]

Please I'm asking to learn how to calculate this myself instead of
only the numerical answer.

Thanks,
JC

The info you need can be found in any electronic text, or in numerous
sources on the web. If I understand what you want transformer sizing
and bulk filter cap "unregulated supply".

Here is just a couple sources.

Onsemis "Linear & Switching Voltage Regulator Handbook" see SECTION 8
" DESIGNING THE INPUT SUPPLY".

http://www.onsemi.com/pub_link/Collateral/HB206-D.PDF

Other free HB from onsemi

http://www.onsemi.com/PowerSolutions/supportDoc.do?type=manuals

Midcom also has an extensive collection of tech notes.

http://www.midcom-inc.com/Tech/TechNotes.asp

More transformer info

http://www.atc-frost.com/products/design/

Similar to onsemis

http://elearning.algonquincollege.com/coursemat/saurioc/ELE-3/ELE-3-NOTES/VOLT-REG.pdf

 

[Baron]

[email protected] Inscribed thus:

I agree, and understand what Phil was saying, but that's a generic
guesstimation that I'm looking to move beyond. Given enough time most
generic guesstimations can be resolved mathematically and there are
plenty of products out there that don't go to overkill Phil suggests
is necessary. Yes there are unresolved variables but this is a
science after all, and equations can have variables in them and be
valid expressions. Remember I asked for an equation, if Phil can't
provide one that's not something to be ashamed of, neither can I, so I
asked hoping for someone who knows rather than someone who thinks I
don't need to know.

I would openly admit that I couldn't give an equation either ! I would
fall back to first principles and go from there... I hate math but
thats my curse.

If Phil insists he is right and the rest of the world is wrong -
having produced working electronics that seem an aweful lot like
evidence, it would be nice to have some of the pixie dust they must be
sprinking on these things to keep them working.

There HAS to be a better way!

I think that the Chinese engineers have learnt how to work up against
the tightest of margins to produce some of the products we see today !
Probably born of the culture / nature of their country.

 

[ehsjr]

Ok, but this should be resolvable to a reasonable level given the
example context of it being a typical E-core transformer and basic
silicon bridge rectified and capacitive filtered circuit. The output
at that point is the question. Even if some formula has an unknown
variable, or two or one hundred, the start would be to resolve those
which requires a working equation in which to place those variables.





This seems to counter the majority of small transformer examples out
there, does it not? Consider products powering just about anything
that uses a wall wart.
Consider a sub-20VA transformer rated in a wall wart for 12VDC, 1A
output. Granted, we could call that a little over 1/2 the rating but
just how much or little is the crucial issue, what variables determine
how much or little and how to express those mathematically.





Thank you for the link, though since I am not mass producing equipment
and so not overly concerned about small differences in component cost
or size, I'm essentially going to consider ripple current a constant
resolved later as would apply to any one design, while I am asking
about a general formula for conversion without regard to variables
that would change in different designs beyond the basic assumption of
a capacitor large enough to achieve acceptibly low ripple for example.




This is what the example circuit would need at a bare minimum by
calculating similarly to what you have above plus other drops in the
circuit @ expected current levels, to still retain the necessary
minimum input voltage, plus or minus a margin of error as median
datasheet values where used.

However, I am not concerned about the entire circuit, not about drop
over a regulator, I am only concerned about resolving how the AC
transformer rating relates to DC output before anything further in a
circuit beyond a typical bridge rectifier and capacitor sufficient to
smooth to a hypothetical 0V ripple, and I am also accounting for the
(Often negligable) different in forward voltage over the rectifier(s)
at different current levels, but this too can be expressed
mathematically.

Essentially, I don't want information more applicable to one project
than to another. Only what remains true mathetically for all projects
which employ AC spec'd transformers of typical design and through
bridge rectification by the most common silicon diodes. In other
words, as you'd see in the vast majority of electronics already if
they're not using switching PSU.

I don't want to argue - that seems your intent.

I know you want a magic formula - you have made that
clear. There is no single formula for you. Those who
have replied have made that clear.

We've tried to show you some of the factors that are
involved. You insist on ignoring them. So, we can't
help you, no matter how hard we try. Sorry.


Ed

 

[Hammy]

---
There's also Motorola's Silicon Rectifier Manual, an excellent
reference and cheap at:

http://www.amazon.com/gp/offer-listing/B000HVEWGK/ref=dp_olp_3/103-7647257-4204638

JF

The hanbook from onsemi as well as the link I posted to algonquin
(volt-reg) provide work examples using Schade curves.

Buying a book for future refrence is a good idea though. Hell for five
bucks I might get that John. We used to have a shelf full of those
motorola data books and manuals in school.

 

[[email protected]]

Why can't you just used the raw unfiltered DC and current limit the
input to the battery?

I am not designing a charging circuit. The charging circuit is fixed,
done years ago, works fine. It's just being mated with a different
supply because the need for it has changed. I am looking for an
equation to qualify transformers per their AC spec for DC supply when
used with a basic bridge rectifier following them.

 

[[email protected]]

I don't want to argue - that seems your intent.
I know you want a magic formula - you have made that
clear.  There is no single formula for you.  Those who
have replied have made that clear.

We've tried to show you some of the factors that are
involved.  You insist on ignoring them.  So, we can't
help you, no matter how hard we try.  Sorry.

A formula yes, but no it's not magic. It simply requires considering
all the significant variables which I'd hoped others would assist
with, but obviously nobody else wants to do more than argue instead of
putting thought into what such an equation would look like. If you
say "it depends", then that should be in an equation. If you say some
other thing depends too, then that too can be put into the equation.

There is an equation that could be made. How accurate the answer from
it would depend on how complete it was. I came here looking to make
as complete an equation as possible but it seems everyone else is
apathetic about the idea and only wants to tell me I don't "need" to
know or that it can't be done.

It's not magic. A caveman looking at a bic lighter would think that
is magic but is it? Your great great grandfather, if he were alive
today, might think a computer is magic, but is it? Just because
someone doesn't know something it doesn't become magic. It would just
require someone bothering to do so, which I was attempting but it
seems I'm going it alone because others think it's too hard or they're
too lazy or whatever the reason. That's fine, nobody is compelled to
do anything but if they had no assistance (some replying did!) then as
always they should've just moved on to the next thread.

 

[Phil Allison]

<[email protected]>


I am not designing a charging circuit.


** You are NOT designing ANYTHING AT ALL

YOU STINKING CRIMINAL LIAR !!!!!!!!!!


FOAD - you anonymous pile of shit.





...... Phil

 

[Paul E. Schoen]

I don't want to argue - that seems your intent.
I know you want a magic formula - you have made that
clear. There is no single formula for you. Those who
have replied have made that clear.

We've tried to show you some of the factors that are
involved. You insist on ignoring them. So, we can't
help you, no matter how hard we try. Sorry.

"A formula yes, but no it's not magic. It simply requires considering
all the significant variables which I'd hoped others would assist
with, but obviously nobody else wants to do more than argue instead of
putting thought into what such an equation would look like. If you
say "it depends", then that should be in an equation. If you say some
other thing depends too, then that too can be put into the equation."

"There is an equation that could be made. How accurate the answer from
it would depend on how complete it was. I came here looking to make
as complete an equation as possible but it seems everyone else is
apathetic about the idea and only wants to tell me I don't "need" to
know or that it can't be done."

"It's not magic. A caveman looking at a bic lighter would think that
is magic but is it? Your great great grandfather, if he were alive
today, might think a computer is magic, but is it? Just because
someone doesn't know something it doesn't become magic. It would just
require someone bothering to do so, which I was attempting but it
seems I'm going it alone because others think it's too hard or they're
too lazy or whatever the reason. That's fine, nobody is compelled to
do anything but if they had no assistance (some replying did!) then as
always they should've just moved on to the next thread."

=========================================================================

I find it curious that you reply only to those who either disagree with you
or do not give you a formula that automagically takes into account every
design parameter and spits out the answers you demand. And who is the lazy
one? You are asking for free advice, and you are getting a lot of bang for
your buck. Contract a design engineer at $100/hour and they will not be
"lazy".

The factors involved in your design are numerous:

Input voltage range
Frequency range
Physical size constraints
Efficiency
Costs (NRE, materials, labor)
Environmental conditions (temp, humidity, altitude, etc.)
Safety (UL, NEMA, Medical, etc.)
Isolation/insulation/leakage current
Surge withstand
EMI/RFI
Load variations
Duty cycle
Overcurrent and short circuit protection
Reverse connection protection
Charging current/voltage/time/temperature profile
Acceptable MTBF

You have constrained the parameters by saying that the charger circuit has
been designed and you are not willing or able to change that. So you are
only looking for a cheap and dirty AC to DC power supply that will meet
your needs. Your charger circuit must have specifications for what it can
accept as raw DC input, and what sort of current it will draw under all
conditions. IOW, you need to model this circuit with a "formula" or SPICE
model. Then you must decide if you want to use an off-the-shelf transformer
or design one specifically to meet your needs. An OTS solution will be
initially cheaper, but you will sacrifice something, like efficiency or
physical size that will not be optimized. You will need to choose one that
meets (but probably exceeds) your specifications.

There are many simplified formulas that have been given to you, that will
give results close enough to choose from several OTS transformers that
should meet your specs. You can also model the transformer pretty well in
SPICE. But you will, sooner or later, need to build a prototype and test
the hell out of it. After a few hours of testing, you can pretty well
characterize the transformer, and then fine tune your selection. Or you can
use the results to design a custom transformer that exactly meets your
needs. But tranny design gets rather complex when you are pushing the
limits of efficiency, cost, size, regulation, and other factors.

You have all the information you need to design, build, and simulate your
DC front-end circuit. We do not have all the information we need to model
your charger load, and even if we did, it is up to you to actually design
the thing. As I said before, SPICE is essentially a mathematical formula
that you can modify with all the parameters you want, and run step and
sweep analyses to make sure your initial design is OK. Then you can tweak
it ad nauseum to get it as close to perfect as it will ever be.

Paul

 

[ehsjr]

---
Funny, meteorologists feel the same way, and yet it seems that
detecting that last elusive flap of butterfly wings has been
perpetually beyond their grasp.

JF

Nice analogy, wonder if he'll get it.

What is interesting to me is the apparent contradiction in
the op's thinking. He chooses to lecture respondents with
his thinking, rather than attempt to understand what they
are saying. Or, if not lecture, argue with points that are
made. I've seen this a few times here - someone comes here,
states that they don't know about X, and asks a question.
But then they argue with the answer! What kind of thinking
is that? Is it the "new way of thinking" or ???

Ed

 

[Phil Allison]

"John Fields"

I've seen it a few times also, and I think it's a way of trying to
save face by not admitting that the gift that was given incurred
emotional indebtedness.

That is, if you give me a gift of knowledge and I dis it, I'll still
have the gift but in my mind I won't owe you anything for it.

Make sense?

** If you imagine some little a brat throwing a toy that is not the one he
wanted back at Santa.



...... Phil

 

[ehsjr]

---
I've seen it a few times also, and I think it's a way of trying to
save face by not admitting that the gift that was given incurred
emotional indebtedness.

That is, if you give me a gift of knowledge and I dis it, I'll still
have the gift but in my mind I won't owe you anything for it.

Make sense?

JF

Ah. Thanks.

Ed

 

[ehsjr]

"John Fields"





** If you imagine some little a brat throwing a toy that is not the one he
wanted back at Santa.



..... Phil

Thanks! That example makes the concept clear.

Ed

 

[[email protected]]

 Nice analogy, wonder if he'll get it.

What is interesting to me is the apparent contradiction in
the op's thinking.  He chooses to lecture respondents with
his thinking, rather than attempt to understand what they
are saying. Or, if not lecture, argue with points that are
made.  I've seen this a few times here - someone comes here,
states that they don't know about X, and asks a question.
But then they argue with the answer!  What kind of thinking
is that?  Is it the "new way of thinking" or ???

Ed- Hide quoted text -

- Show quoted text -

I'm sorry if I did not clarify this, but I do not need an introductory
course in sizing transformers. The whole point was moving beyond the
generic overestimates towards the real science in what is actually
necessary. I lecture towards the end of saying, ok, but this is
common knowlege, what else do you know? Maybe it takes a village,
maybe no one person has the entire answer but it seems to get there we
need to have some lecture, some dispelling of comfort zones and get
right down to the actual criteria necessary and have that proven
through real worl examples of success or failure, not just saying "use
a bigger hammer", until it is proven to be needed.

Yes, I will argue with an answer when I ask for an equation and
someone tells me otherwise. I have built plenty of PSU over the
years, if I needed to know what I have already done successfully then
I would have asked a different question? No offense intended, but you
need to focus on what I asked, as do others. If they are ignorant of
the answer, there is no need to reply.

 

[Phil Allison]

<[email protected]>


I'm sorry if I did not clarify this, but I do not need an introductory
course in sizing transformers. The whole point was moving beyond the
generic overestimates towards the real science in what is actually
necessary. I lecture towards the end of saying, ok, but this is
common knowlege, what else do you know? Maybe it takes a village,
maybe no one person has the entire answer but it seems to get there we
need to have some lecture, some dispelling of comfort zones and get
right down to the actual criteria necessary and have that proven
through real worl examples of success or failure, not just saying "use
a bigger hammer", until it is proven to be needed.

Yes, I will argue with an answer when I ask for an equation and
someone tells me otherwise. I have built plenty of PSU over the
years, if I needed to know what I have already done successfully then
I would have asked a different question? No offense intended, but you
need to focus on what I asked, as do others. If they are ignorant of
the answer, there is no need to reply.


** Go get fucked.

You revolting brat.




....... Phil

 

[Paul E. Schoen]

---
You may have _built_ plenty of PSU over the years, but from your
question re. transformer sizing it appears that all you were doing was
grunt work, putting together someone else's design, with no real
insight as to the "why" of the transformer's capacity needing to be
larger than what was required to drive the load.

Instead of being an annoying little ass you might try thanking those
of us who have given you good advice instead of pretending that,
somehow, we did you a disservice.

Oh, and by the way, **** you.

I usually give someone the benefit of doubt and try to come up with various
ways of looking at a problem and proposing a reasonable solution, or
pointing out the need for more information. The OP has chosen not to
comment on those posts, but only those where he feels compelled to argue
and assume a self-righteous attitude. So I second your motion. All in
favor?

Paul

 

[[email protected]]

I'm sorry if I did not clarify this, but I do not need an introductory
course in sizing transformers. The whole point was moving beyond the
generic overestimates towards the real science in what is actually
necessary. I lecture towards the end of saying, ok, but this is
common knowlege, what else do you know? Maybe it takes a village,
maybe no one person has the entire answer but it seems to get there we
need to have some lecture, some dispelling of comfort zones and get
right down to the actual criteria necessary and have that proven
through real worl examples of success or failure, not just saying "use
a bigger hammer", until it is proven to be needed.

Yes, I will argue with an answer when I ask for an equation and
someone tells me otherwise. I have built plenty of PSU over the
years, if I needed to know what I have already done successfully then
I would have asked a different question? No offense intended, but you
need to focus on what I asked, as do others. If they are ignorant of
the answer, there is no need to reply.

http://www.despair.com/incompetence.html

 

[whit3rd]

Use Irms = 2.0 Idc and you'll always be safe .

Well, not completely, no. Consider a 1A (RMS)
rated transformer, with a diode/capacitor output.
When attached to a 1A (DC) load, the transformer
actually forward-biases the diode for a brief
time at the peak voltage, and otherwise current
from the transformer is nil. If the conduction
period is one-tenth of the full cycle period, that
means 10A current from the transformer during
the active time.

Here's where it gets mathematical: the power dissipated
in a 1A transformer with (for instance) 1 ohm winding
resistance is 1 watt, when the load is taking a simple
1A (AC) current.

Heat = 1A **2 * 1 ohm = 1 watt

When the same transformer feeds the DC load as described above, the
power is

Heat = (0.9 * 0) + (0.1 * 10A**2 * 1 ohm) = 10 watt

So a perfectly good transformer can burn up feeding a
DC rectifier and load, when a similar AC load wouldn't
bother it. That isn't always covered by a 'factor of two'
or any other rule of thumb. I've seen manufacturers offer
tables of the permissible DC output ratings for their
transformers, but you can't count on that. The AC rating
looks better, so salesmen will feed you that info first.

 

[Paul E. Schoen]

Well, not completely, no. Consider a 1A (RMS)

rated transformer, with a diode/capacitor output.
When attached to a 1A (DC) load, the transformer
actually forward-biases the diode for a brief
time at the peak voltage, and otherwise current
from the transformer is nil. If the conduction
period is one-tenth of the full cycle period, that
means 10A current from the transformer during
the active time.

Here's where it gets mathematical: the power dissipated

in a 1A transformer with (for instance) 1 ohm winding
resistance is 1 watt, when the load is taking a simple
1A (AC) current.

Heat = 1A **2 * 1 ohm = 1 watt

When the same transformer feeds the DC load as described above, the

power is

Heat = (0.9 * 0) + (0.1 * 10A**2 * 1 ohm) = 10 watt

So a perfectly good transformer can burn up feeding a

DC rectifier and load, when a similar AC load wouldn't
bother it. That isn't always covered by a 'factor of two'
or any other rule of thumb. I've seen manufacturers offer
tables of the permissible DC output ratings for their
transformers, but you can't count on that. The AC rating
looks better, so salesmen will feed you that info first.
-----------------------------------------------------------

I beg to differ with your analysis, if you are talking about an ordinary
rectifier and capacitor circuit. As an example, I simulated a FWB with a 12
VAC nominal output transformer with 1 ohm series resistance, and a load of
12 ohms, and a capacitor of 100,000 uF, which should produce the highest
possible current peaks. The simulation shows peak currents of 3.7 amps.
With 1000 uF, the peaks are 3.2 amps. Now, during the charging period, with
100,000 uF, the peaks start at 14.6 amps and then diminish to 4.3 amps at
0.5 seconds. In the first 200 mSec, the tranny is supplying 34 watts, but
then settles down to 15.8 watts when the capacitor is fully charged. At
that time, the load is essentially pure DC, and the resistor dissipates
13.9 watts. So only about 2 watts is left, and that is shared among the
rectifiers (305 mW each), and the tranny (about 0.8 watts).

If you can show me a circuit where you will get these 10 ampere peaks at
10% duty cycle, then I will agree that the tranny will be overloaded. But
you will probably need to use some sort of PWM, and there will also be a
lot more power being dumped into the load. If you are talking about AC to
DC rectifier circuits, it's a safe bet to design the circuit so that the DC
output voltage under load is about the same as the nominal RMS AC voltage
of the transformer, and in this case the RMS input current is 1.81/1.08 =
less than twice the output current. So the 2:1 ratio that John proposed is
very reasonable.

Paul

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