Building a transformer

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I took a look at the ieeta handbook. Yipee, this is something i want. You are right about the heavy physics, not for everybody. Thanks

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 JosephKK
 Gegen dummheit kampfen die Gotter Selbst, vergebens.  
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joseph2k
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2 diodes, half wave rectification. That way the OP can get 5A 50v or 2.5A +/- 50v. Which may or may not be what he wanted, and almost certainly isnt what he now wants, or needs.

NT

Reply to
meow2222

see below

Thats what you get under full load, no load V will be higher. Transformers are rated at voltage under full load. Offload their V_out can be well above for tiddlers.

yes, there is that, which I didnt take into account, and that will lower the real world Vout under load.

I was referring to small enclosed transformers I used (typ 6VA). Even without that 1.6 factor they barely got warm.

I once wanted a very small high power device so experimentally I ran a

50VA tf way over its ratings on a heatsink, attaching it to the exterior of the case so it saw good ventilation. This confirmed my suspicion that trannies tend to be conservatively rated. I cant remember for sure, but I think I may have loaded it somewhere around 2.5x its rating for a couple of hours at a time, and it lasted long term. Ran very hot, but survived. V_out sagged of course.

If the OP wanted a beefy supply but also wanted portability, and its only for personal hobby use, pushing a smaller tf is a workable option. SMPS is a better option technically, but then someone that needs to learn about linear supplies wouldnt learn a lot, as the smps would be rather over their head - and a tough project for such person.

Whats good for hobby and whats good for pro design are 2 different things. People play it too safe when it comes to hobby stuff :)

NT

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meow2222

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John Fields
Professional Circuit Designer
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John Fields

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I don't think that's true.  (View in Courier)


Consider a conventional full wave bridge:
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Reply to
John Fields

Youre not the only person to say this, and I find it very puzzling, as the means to get +/- 50v is elementary:

+-----|----+ | | +-----|>|------- V+ P||S R||E I||C | | MAINS>----+ | | +--------------- 0V

NT

Reply to
meow2222

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Aarghhhh!!! Big OOPS...


                    +-----[CR1>]--+--->V+
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John Fields

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Well, I can't argue with that, LOL, but by doing it that way you've
just made the smoothing caps twice as big as they have to be since
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Reply to
John Fields

Secondary is twice the current, half the V. Higher diode i, half the number of diodes.

Smoothing caps would need to be a little over twice the capacitance irl, as i is flowing out of the caps for more than twice the length of time between topups. When youre building from salvage the choice comes down to what you happen to find.

NT

Reply to
meow2222

It's a very convenient way of getting + and - rails from an AC wall-wart for low-current op-amp applications, where the larger filter caps aren't TOO large.

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2006 is YMMVI- Your mileage may vary immensely.
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Reply to
John Woodgate

In addition to the filter and secondary current comments: If you don't load the two outputs equally, there will be Dc in the transformer core, causing it to saturate at the end of alternating half cycles. It will hum and get hot.

Reply to
John Popelish

both go negatve or positive at the same time.

to an op-amp connected accross the rails it looks like the earth is moving,

with a full-wave rectifier and centre tap the ripple ripple is symmetric and at twice the frequency, which makes it easier to filter out.

Bye. Jasen

Reply to
jasen

Thanks; that's what I thought.

If you draw out what happens (and look with a scope, of course), you find that the two ripple waveforms are unsymmetrical sawtooth waveforms, displaced in time. So there is some common-mode and some differential mode ripple.

I would use this technique only for low-current applications, where low ripple can be achieved with reasonable filter capacitor values, or for feeding regulators.

It seems to me that the op-amp doesn't 'see' the rails until its input or output voltage gets close to them. If it did, PSRR wouldn't be so high.

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OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk
2006 is YMMVI- Your mileage may vary immensely.
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John Woodgate

Hello, and direct current in the core as a result of current flow in the secondary windings? Just how would that be possible? Is the core part of some external circuit? If you mean eddy currents they would be AC and a power transformer core is laminated to minimize this effect. Either I misconstrued your comment or I must have been sleeping in my EE101 electric machinery course years ago. Sincerely,

John Wood (Code 5550) e-mail: snipped-for-privacy@itd.nrl.navy.mil Naval Research Laboratory

4555 Overlook Avenue, SW Washington, DC 20375-5337
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J. B. Wood

"J. B. Wood"

** Of course not.

Always exclude the impossible when interpreting someone's language.

In this case "DC" is an abbreviation of "DC offset " = magnetic field offset.

** WRONG.

The heat is generated by simple I squared R loss in the copper wire due to excess primary current as a result of core saturation.

The audible hum is the iron core protesting about being driven into saturation 50 or 60 times per second.

** You did.

** Maybe you were - but the behaviour of a transformer subjected to an asymmetrical current load is rarely mentioned in elementary texts.
** Hmmmm - sounds kinda serious like ......

....... Phil

Reply to
Phil Allison

In message , dated Tue,

15 Aug 2006, J. B. Wood writes

Not direct current but DC magnetic flux.

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OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk
2006 is YMMVI- Your mileage may vary immensely.
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Reply to
John Woodgate

I have been waiting for this.

I misspoke. There is no current in the core (except for the normal AC eddy current). I meant to say that there will be a net DC in the windings, which will cause the core to magnetically saturate at the end of alternating half cycles.

Reply to
John Popelish

You want 600 va ? But that means 90% effeciency ! That means you must use switch mode .

You must choose an ungapped ferrite core with 2000 or more MU ,

it will need litz wire on output ( simply use smaller wire and mo strands ) . Tiny cores will easy do 600 watts . MJE13005's are the most popular but the Cxxxx ( 2SCxxxx) equivalents are seen more cause all are built in China .

You will see all switch mode pow supplies changing to single drive , for they are just as eff as push pull , and cheaper . Thery self oscillate ! Control circuit simply pulls down the Base of bipolar Xisistor to adj output voltage . It is a myth that more power can be sent to output using push pull ! All low end switch mode PS are using single transistor and if you measure the Xformer and other parts , SURPRISE , it is same size and cheaper to build and its also an easier circuit to control ! Simpler parts . You still need a volt ref and Opto and a LM393 to sense current .

The myth started when they said the dead time would waste time but as we EE's know the dead time dont exist ! And we dont have to control it as LM494 crowd must do !

Its a simplification made in heaven , no compromises al all . You simply dbl up the Primary winding transistors ! Remember ! Transistors in class D dont worry about balancing ! If standard PCPS has push pull using 2 each C2335/or MJE13005 i simply put them in parallel ! Same power output ! Of course in dumpster diving , you will need to be clever on the 2ndaries to get same voltages

Dont be put off , this is so simple it should be used in school to teach . You will be amased at how many parts are tossed out !

BTW Free OpSys for PDA . I'll buy a ARM Eval board (

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and create a Forth like integrated OpSys , call it NewForth for now . Wanna learn to do top System software ? Follow me .....

Mark Fortune wrote:

Reply to
werty

600VA thru litz wire? no dc component on single ended drive? (--> copper losses) cant get more power thru without the dc component? News to me.

NT

Reply to
meow2222

I decided to have a look into switch mode psu's in the end, and furthermore building my own transformers seems far more complicated tham I originally anticipated. So i've looked into the switched mode psu's, and they seem fairly straightforward (famous last words), further to that i'm bugging the local computer store for any and all busted computer power supplies - and i'm not even charging them for 'disposal'. That way I can gather many transformers at no cost to me and experiment away to my hearts content. I figure if I need a lot of power I can run several of these transformers in series or parallel.

Mark

Reply to
Mark Fortune

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