In message , dated Tue, 15 Aug 2006, jasen
writes
On 2006-08-14, John Woodgate wrote:
In message , dated Mon, 14 Aug 2006, jasen
writes

You can call it two half-wave rectifiers but it's the same circuit as a
voltage doubler, and it puts a common mode ripple on the power rails

What do you mean by 'common-mode ripple'?

both go negatve or positive at the same time.

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.

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

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

John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK

In article , John Popelish
wrote:

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:
Naval Research Laboratory
4555 Overlook Avenue, SW
Washington, DC 20375-5337

"J. B. Wood"
John Popelish

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?


** 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.



If you mean eddy currents they would be AC and a
power transformer core is laminated to minimize this effect.


** 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.


Either I misconstrued your comment


** You did.

or I must have been sleeping in my EE101
electric machinery course years ago.


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



John Wood (Code 5550) e-mail:
Naval Research Laboratory
4555 Overlook Avenue, SW
Washington, DC 20375-5337



** Hmmmm - sounds kinda serious like ......



........ Phil

In message , dated Tue,
15 Aug 2006, J. B. Wood writes
Hello, and direct current in the core as a result of current flow in
the secondary windings?

Not direct current but DC magnetic flux.
--
OOO - Own Opinions Only. Try www.jmwa.demon.co.uk and www.isce.org.uk
2006 is YMMVI- Your mileage may vary immensely.

John Woodgate, J M Woodgate and Associates, Rayleigh, Essex UK

J. B. Wood wrote:
In article , John Popelish
wrote:


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,

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



Mark Fortune wrote:
Greetings to the collective.

For my next project I want to build a bench power supply and do away
with this old switch mode AT computer PSU that i'm currently using.

The design I have in mind will be pretty beefy, giving a wide range of
fixed and variable output voltages (i'm thinking from -50v up to +50)
and deliver up to 5amps of current. if my estimates are right i'll need
a 600va+ (100v * 5a + overhead) transformer to do the job well. Now i've
had a look at some transformers in this range, and they're a little out
of my price range. so now i'm considering building my own.

The specifications I need are as follows:

primary: 0-230v @ 50Hz
Secondary: 60v-0-60v
secondary output current max: 5A

I have an intermediate understanding of transformers, but have never
actually built one, hence I am seeking advice from those with experience
in this field.
So where do I start? what type of core is best for this application? - I
have a friend who's blasting off that toroids are best, but i'd like a
second opinion, and what kind of wire to use for the primary and
secondaries? what are good safety practices regarding insulation? Is
this even a feasable project to undertake (moreso on the money side than
timewise). Any other advice also greatly appreciated.

regards,
Mark

werty wrote:

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 !


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

wrote:
werty wrote:


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 !



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


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