Calculating DC Output Current From Unregulated AC Transformer

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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

You should go to the library and make a copy of the paper:

"Analysis of Rectifier Operation", O. H. Schade, Proceedings of the IRE, July 1943

This will give you an idea of what's involved in a detailed analysis of the sort you're asking for.

--
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

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

 
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.

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.

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 am not designing a charging circuit.

** You are NOT designing ANYTHING AT ALL

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

FOAD - you anonymous pile of shit.

..... Phil

Having an unusually bad day, Phil? You see ever so slightly more crazed than normal.

John

--
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

"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."

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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

:-) 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

--
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

"John Fields"

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

..... Phil

--
Yeah, good one. :-)

JF

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

JF

Ah. Thanks.

Ed

Thanks! That example makes the concept clear.

Ed

e m

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.