Building a transformer

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

NT

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

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.

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.

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

In message , dated Tue,

Not direct current but DC magnetic flux.

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.

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 (

Mark Fortune wrote:

NT

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