budgie wrote:
On Sun, 17 Aug 2003 03:54:19 +0100, "zindazenda"
wrote:

thanks for info peoples.....

2x12v computer psu is interesting but takes up too much space....
now i have more info i will try using the parts from a computer psu but with
modified/own stepdown transformer...going this way as it will be cheap..ie
ex psu costs 3gbp so if i kill it then so what!!

I assume the transformer will be similar just with twice (ish) the amount of
windings on the sec.and that i will have to add some R in the feedback crt
so that the pwm chip still sees 2.5v(ish) and prehaps remove the scr
overvoltage protection..OR i will make it unregulated.

Find out which winding is used for feedback/regulation, and leave it
alone - or consider replacing it with lighter gauge wire to obtain
more core cross-section for your 28V winding.

Now that's a good idea!

Also, if an AT type, usually the +5V winding requires a base load.

But it's less than 1A so the new wire could be quite thin, and the rest
of the core window is all yours to play in... thanks mate, that's set me
thinking...

Simon, there's also an interesting Australian radio club site that has
lots of in-depth information on modifying computer PSUs (sorry, I
thought I'd saved the URL, but can't find it - can anyone post the URL
please?).


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek

zindazenda wrote:
ok i have a pc psu or two now and have attempted to strip the transformer
but theres no way to take them apart without damaging the ferrite as they
seem to be glued!!so i am ordering some new un wound ones from rs...but the
primary winding appears to be in 2 parts..one half on top(the outer winding)
and the other on the bottom (inner.)i assume i will have to duplicate this??

I'm not an expert on SMPS design, and espcecially not on the little
tricks that make the difference between just working and working well.
Obviously the extra complication of the split winding is there for a
reason, and my guess would be that it's to improve the high-frequency
performance by minimizing leakage inductance. Anyone else know for sure?

You may also be running into trouble by ordering un-wound cores, since
you don't know the specifications of the existing ones (permeability,
Bmax etc).

which makes winding the secondary a pain as its in the middle, and i am sure
i wont get it right the first time,meaning stripping the primary down a few
times..your thoughts??
also total winding of secondary 4+3+3+4 to give 12v..8+8 for 28v???5v 12v
not needed...

The basic turns ratios are OK... the details, I'm not so sure about.

Can anyone recommend a book on SMPS design that leads you all the way
from basic theory to real-life designs - including all those little
practical details that need to be right?


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek

zindazenda wrote:
ok i have a pc psu or two now and have attempted to strip the transformer
but theres no way to take them apart without damaging the ferrite as they
seem to be glued!!so i am ordering some new un wound ones from rs...but the
primary winding appears to be in 2 parts..one half on top(the outer winding)
and the other on the bottom (inner.)i assume i will have to duplicate this??

I'm not an expert on SMPS design, and espcecially not on the little
tricks that make the difference between just working and working well.
Obviously the extra complication of the split winding is there for a
reason, and my guess would be that it's to improve the high-frequency
performance by minimizing leakage inductance. Anyone else know for sure?

You may also be running into trouble by ordering un-wound cores, since
you don't know the specifications of the existing ones (permeability,
Bmax etc).

which makes winding the secondary a pain as its in the middle, and i am sure
i wont get it right the first time,meaning stripping the primary down a few
times..your thoughts??
also total winding of secondary 4+3+3+4 to give 12v..8+8 for 28v???5v 12v
not needed...

The basic turns ratios are OK... the details, I'm not so sure about.

Can anyone recommend a book on SMPS design that leads you all the way
from basic theory to real-life designs - including all those little
practical details that need to be right?


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek

yes already found a problem..and thats i cant get the same core.
SO.....i have been led to believe that just about any toroid (that works at
HF) will work..has anybody wound a SMPS transformer on a toroid..input
voltage cir.
240v ac in rectified etc...28v out...
Since i dont intend to use the original pcb space is not a problem..

thanks simon g0zen
"Ian White, G3SEK" wrote in message
...
zindazenda wrote:
ok i have a pc psu or two now and have attempted to strip the transformer
but theres no way to take them apart without damaging the ferrite as they
seem to be glued!!so i am ordering some new un wound ones from rs...but
the
primary winding appears to be in 2 parts..one half on top(the outer
winding)
and the other on the bottom (inner.)i assume i will have to duplicate
this??

I'm not an expert on SMPS design, and espcecially not on the little
tricks that make the difference between just working and working well.
Obviously the extra complication of the split winding is there for a
reason, and my guess would be that it's to improve the high-frequency
performance by minimizing leakage inductance. Anyone else know for sure?

You may also be running into trouble by ordering un-wound cores, since
you don't know the specifications of the existing ones (permeability,
Bmax etc).

which makes winding the secondary a pain as its in the middle, and i am
sure
i wont get it right the first time,meaning stripping the primary down a
few
times..your thoughts??
also total winding of secondary 4+3+3+4 to give 12v..8+8 for 28v???5v 12v
not needed...

The basic turns ratios are OK... the details, I'm not so sure about.

Can anyone recommend a book on SMPS design that leads you all the way
from basic theory to real-life designs - including all those little
practical details that need to be right?


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek

yes already found a problem..and thats i cant get the same core.
SO.....i have been led to believe that just about any toroid (that works at
HF) will work..has anybody wound a SMPS transformer on a toroid..input
voltage cir.
240v ac in rectified etc...28v out...
Since i dont intend to use the original pcb space is not a problem..

thanks simon g0zen
"Ian White, G3SEK" wrote in message
...
zindazenda wrote:
ok i have a pc psu or two now and have attempted to strip the transformer
but theres no way to take them apart without damaging the ferrite as they
seem to be glued!!so i am ordering some new un wound ones from rs...but
the
primary winding appears to be in 2 parts..one half on top(the outer
winding)
and the other on the bottom (inner.)i assume i will have to duplicate
this??

I'm not an expert on SMPS design, and espcecially not on the little
tricks that make the difference between just working and working well.
Obviously the extra complication of the split winding is there for a
reason, and my guess would be that it's to improve the high-frequency
performance by minimizing leakage inductance. Anyone else know for sure?

You may also be running into trouble by ordering un-wound cores, since
you don't know the specifications of the existing ones (permeability,
Bmax etc).

which makes winding the secondary a pain as its in the middle, and i am
sure
i wont get it right the first time,meaning stripping the primary down a
few
times..your thoughts??
also total winding of secondary 4+3+3+4 to give 12v..8+8 for 28v???5v 12v
not needed...

The basic turns ratios are OK... the details, I'm not so sure about.

Can anyone recommend a book on SMPS design that leads you all the way
from basic theory to real-life designs - including all those little
practical details that need to be right?


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek

I had no trouble getting a SMPS transformer apart when I put it in a
sealed can with some MEK solvent and left it for 24 hours. I put some
scewed up wire mesh in the bottom of a small can to elevate the core,
so that it was only exposed to the vapour, and not immersed in the
MEK.

The E-cores separated with minimal force and with no damage after this
treatment, and the MEK did not affect the bobbin.

Make sure the can is properly sealed. I left the can outside to
minimise any exposure to the MEK fumes.

MikeN ZL1BNB



On Tue, 19 Aug 2003 19:57:02 +0100, "zindazenda"
wrote:

yes already found a problem..and thats i cant get the same core.
SO.....i have been led to believe that just about any toroid (that works at
HF) will work..has anybody wound a SMPS transformer on a toroid..input
voltage cir.
240v ac in rectified etc...28v out...
Since i dont intend to use the original pcb space is not a problem..

thanks simon g0zen
"Ian White, G3SEK" wrote in message
...
zindazenda wrote:
ok i have a pc psu or two now and have attempted to strip the transformer
but theres no way to take them apart without damaging the ferrite as they
seem to be glued!!so i am ordering some new un wound ones from rs...but
the
primary winding appears to be in 2 parts..one half on top(the outer
winding)
and the other on the bottom (inner.)i assume i will have to duplicate
this??

I'm not an expert on SMPS design, and espcecially not on the little
tricks that make the difference between just working and working well.
Obviously the extra complication of the split winding is there for a
reason, and my guess would be that it's to improve the high-frequency
performance by minimizing leakage inductance. Anyone else know for sure?

You may also be running into trouble by ordering un-wound cores, since
you don't know the specifications of the existing ones (permeability,
Bmax etc).

which makes winding the secondary a pain as its in the middle, and i am
sure
i wont get it right the first time,meaning stripping the primary down a
few
times..your thoughts??
also total winding of secondary 4+3+3+4 to give 12v..8+8 for 28v???5v 12v
not needed...

The basic turns ratios are OK... the details, I'm not so sure about.

Can anyone recommend a book on SMPS design that leads you all the way
from basic theory to real-life designs - including all those little
practical details that need to be right?


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek

I had no trouble getting a SMPS transformer apart when I put it in a
sealed can with some MEK solvent and left it for 24 hours. I put some
scewed up wire mesh in the bottom of a small can to elevate the core,
so that it was only exposed to the vapour, and not immersed in the
MEK.

The E-cores separated with minimal force and with no damage after this
treatment, and the MEK did not affect the bobbin.

Make sure the can is properly sealed. I left the can outside to
minimise any exposure to the MEK fumes.

MikeN ZL1BNB



On Tue, 19 Aug 2003 19:57:02 +0100, "zindazenda"
wrote:

yes already found a problem..and thats i cant get the same core.
SO.....i have been led to believe that just about any toroid (that works at
HF) will work..has anybody wound a SMPS transformer on a toroid..input
voltage cir.
240v ac in rectified etc...28v out...
Since i dont intend to use the original pcb space is not a problem..

thanks simon g0zen
"Ian White, G3SEK" wrote in message
...
zindazenda wrote:
ok i have a pc psu or two now and have attempted to strip the transformer
but theres no way to take them apart without damaging the ferrite as they
seem to be glued!!so i am ordering some new un wound ones from rs...but
the
primary winding appears to be in 2 parts..one half on top(the outer
winding)
and the other on the bottom (inner.)i assume i will have to duplicate
this??

I'm not an expert on SMPS design, and espcecially not on the little
tricks that make the difference between just working and working well.
Obviously the extra complication of the split winding is there for a
reason, and my guess would be that it's to improve the high-frequency
performance by minimizing leakage inductance. Anyone else know for sure?

You may also be running into trouble by ordering un-wound cores, since
you don't know the specifications of the existing ones (permeability,
Bmax etc).

which makes winding the secondary a pain as its in the middle, and i am
sure
i wont get it right the first time,meaning stripping the primary down a
few
times..your thoughts??
also total winding of secondary 4+3+3+4 to give 12v..8+8 for 28v???5v 12v
not needed...

The basic turns ratios are OK... the details, I'm not so sure about.

Can anyone recommend a book on SMPS design that leads you all the way
from basic theory to real-life designs - including all those little
practical details that need to be right?


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek

MEK???

proberly something obvious Mike, but am very tired and so isnt obvious at
moment..
simon
"MikeN" wrote in message
...
I had no trouble getting a SMPS transformer apart when I put it in a
sealed can with some MEK solvent and left it for 24 hours. I put some
scewed up wire mesh in the bottom of a small can to elevate the core,
so that it was only exposed to the vapour, and not immersed in the
MEK.

The E-cores separated with minimal force and with no damage after this
treatment, and the MEK did not affect the bobbin.

Make sure the can is properly sealed. I left the can outside to
minimise any exposure to the MEK fumes.

MikeN ZL1BNB



On Tue, 19 Aug 2003 19:57:02 +0100, "zindazenda"
wrote:

yes already found a problem..and thats i cant get the same core.
SO.....i have been led to believe that just about any toroid (that works
at
HF) will work..has anybody wound a SMPS transformer on a toroid..input
voltage cir.
240v ac in rectified etc...28v out...
Since i dont intend to use the original pcb space is not a problem..

thanks simon g0zen
"Ian White, G3SEK" wrote in message
...
zindazenda wrote:
ok i have a pc psu or two now and have attempted to strip the
transformer
but theres no way to take them apart without damaging the ferrite as
they
seem to be glued!!so i am ordering some new un wound ones from
rs...but
the
primary winding appears to be in 2 parts..one half on top(the outer
winding)
and the other on the bottom (inner.)i assume i will have to duplicate
this??

I'm not an expert on SMPS design, and espcecially not on the little
tricks that make the difference between just working and working well.
Obviously the extra complication of the split winding is there for a
reason, and my guess would be that it's to improve the high-frequency
performance by minimizing leakage inductance. Anyone else know for
sure?

You may also be running into trouble by ordering un-wound cores, since
you don't know the specifications of the existing ones (permeability,
Bmax etc).

which makes winding the secondary a pain as its in the middle, and i
am
sure
i wont get it right the first time,meaning stripping the primary down
a
few
times..your thoughts??
also total winding of secondary 4+3+3+4 to give 12v..8+8 for 28v???5v
12v
not needed...

The basic turns ratios are OK... the details, I'm not so sure about.

Can anyone recommend a book on SMPS design that leads you all the way
from basic theory to real-life designs - including all those little
practical details that need to be right?


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek

MEK???

proberly something obvious Mike, but am very tired and so isnt obvious at
moment..
simon
"MikeN" wrote in message
...
I had no trouble getting a SMPS transformer apart when I put it in a
sealed can with some MEK solvent and left it for 24 hours. I put some
scewed up wire mesh in the bottom of a small can to elevate the core,
so that it was only exposed to the vapour, and not immersed in the
MEK.

The E-cores separated with minimal force and with no damage after this
treatment, and the MEK did not affect the bobbin.

Make sure the can is properly sealed. I left the can outside to
minimise any exposure to the MEK fumes.

MikeN ZL1BNB



On Tue, 19 Aug 2003 19:57:02 +0100, "zindazenda"
wrote:

yes already found a problem..and thats i cant get the same core.
SO.....i have been led to believe that just about any toroid (that works
at
HF) will work..has anybody wound a SMPS transformer on a toroid..input
voltage cir.
240v ac in rectified etc...28v out...
Since i dont intend to use the original pcb space is not a problem..

thanks simon g0zen
"Ian White, G3SEK" wrote in message
...
zindazenda wrote:
ok i have a pc psu or two now and have attempted to strip the
transformer
but theres no way to take them apart without damaging the ferrite as
they
seem to be glued!!so i am ordering some new un wound ones from
rs...but
the
primary winding appears to be in 2 parts..one half on top(the outer
winding)
and the other on the bottom (inner.)i assume i will have to duplicate
this??

I'm not an expert on SMPS design, and espcecially not on the little
tricks that make the difference between just working and working well.
Obviously the extra complication of the split winding is there for a
reason, and my guess would be that it's to improve the high-frequency
performance by minimizing leakage inductance. Anyone else know for
sure?

You may also be running into trouble by ordering un-wound cores, since
you don't know the specifications of the existing ones (permeability,
Bmax etc).

which makes winding the secondary a pain as its in the middle, and i
am
sure
i wont get it right the first time,meaning stripping the primary down
a
few
times..your thoughts??
also total winding of secondary 4+3+3+4 to give 12v..8+8 for 28v???5v
12v
not needed...

The basic turns ratios are OK... the details, I'm not so sure about.

Can anyone recommend a book on SMPS design that leads you all the way
from basic theory to real-life designs - including all those little
practical details that need to be right?


--
73 from Ian G3SEK 'In Practice' columnist for RadCom (RSGB)
Editor, 'The VHF/UHF DX Book'
http://www.ifwtech.co.uk/g3sek

On Fri, 22 Aug 2003 02:22:00 +0100, "zindazenda"
wrote:

MEK???

proberly something obvious Mike, but am very tired and so isnt obvious at
moment..

methyl-ethyl ketone. definitely not a particularly user-friendly
solvent.