looking for a transformer with several windings

[electronics man]

hi
im building a power supply and i am looking for a transformer with these requirements i am finding it really hard to find one:
either:
4x 12v secondary windings
or
2x 24v secondary windings each with at least one tap
i am specifically lookingfor a 1
anyone know where i could look 160VA toroidal transformer
(i can model my circuit around the transformer i have)

[mij59]

Hi,

Transformers can be custom build, e.g.

http://www.amplimo.com/

Or use two transformers 2x12 V

[electronics man]

wouldnt custom transformers e very expensive? and 2 transformers would be too big for my aplication

[mij59]

wouldnt custom transformers e very expensive? and 2 transformers would be too big for my aplication

Yes they will cost more, don't how much.

Maybe Ian Johnson can help you, he has been working on a power supply project

http://www.ianjohnston.com/

[mariush]

Another option would be to get a toroidal transformer with one single winding of 48v AC (or a bit more), unwind the secondary wire and measure the length then cut the wire in 4 separate segments and rewind the toroidal transformer.

Just a note, for a power supply you may want additional isolated small windings, for example +5v and -5v (or +/- 9v) for each meter (for example ICL7106 or ICL7107)  and most power supplies use separate windings to power the opamps and other analogue circuitry with something like +/- 12v.
But these are usually low current, less than 0.5A each.

It would also be a good learning experience to make your own transformer, there's some very good information here:

http://ludens.cl/Electron/Magnet.html
http://ludens.cl/Electron/trafos/trafos.html

BUT, it's guaranteed it would cost you more to buy the magnet wire and the laminations separately.

[electronics man]

i have considered  that. the the 3rd and 4th windings will be put in series to make 24V which will be rectified to 34VDC and regulated to 31.5V for the opamps (tl072s witch go upto 36V) i will rectify the tap at which the 2 windings are connected together to get 17Vdc then regulated to get 12V (for fans) and 5V (for digital and some other things).

[peter.mitchell]

A small-ish audio amp transformer should do, look for something like 24v 24v 15v 15v, the 15v windings will be low current (400-500ma or so), but good for the control circuitry.

[electronics man]

sorry too low current, i may just go for a 2x12v 160VA toroidal and by a small 15VA 2x12v transformer (laminated) for the control circuitry may just save the hassle 

[mariush]

i have considered  that. the the 3rd and 4th windings will be put in series to make 24V which will be rectified to 34VDC and regulated to 31.5V for the opamps (tl072s witch go upto 36V) i will rectify the tap at which the 2 windings are connected together to get 17Vdc then regulated to get 12V (for fans) and 5V (for digital and some other things).

You'll have to be careful with that. The output of transformers will vary with load. At low loads, the transformer may output 5-10% more so your 24v AC may be 26-28v AC and so on. Be sure to use at least a zener, but you're probably going to want to use a 7815 and a 7915 regulator to give you +/- 15v regulated.. Though, probably +/-12v will be safer to use an enough for the opamps.

[mc]

http://www.airlinktransformers.com/ have quite a wide selection, and also offer a custom service.

[22swg]

Did you look here  http://livinginthepast-audioweb.co.uk/index.php?p=xfrmrvt228

[Kevin.D]

Hi there ,A bit offtopic conversation , but years ago farnell used to sell mains transformer kits (see attachment) so that you could  wind you own custom tranny .They where always handy when you needed, a large  multi winding transformer with many output V's for a restoration/repair  project that you would otherwise have no chance of buying off the shelf . They disapeared from their catalogue in the 90's , I haven't seen them on sale since ,but I have seen these old RS kits pop up on ebay occasionally.  They came in various core sizes ,once you had done one of them , you could wind them in less than an hour   . They used split bobbins for mains/secondary isolation safety (unless you are a experienced profesional transformer winder with a good understanding of the crucial mains/secondary isolation safety issues involved you shouldn't  even attempt single bobbin winding transformers) . There was really nothing special or different  about these kit transformers when compared to other off the shelf split bobbin H type trannies ,other than they came in bits ,and you supplied your own winding wire/tape/varnish .
  Since these  are no longer available  ,I always keep a few various size SPLIT BOBBIN transformer's that  I  find in old equipment ,incase I ever need to wind a 'special' . Once you have had the experience of doing one or two they are pretty easy to rewind ,this is  not really an option for begginers though .

In your case, the transformer your asking for really isn't anything special (a 2*24V center tapped secondary ?) ,you should easily get one of these off the shelf . 

Regards

[SeanB]

Normally it is the winding of the few thousand turns on the primary that is the issue, especially if you do not have a coil winding machine to ease the workload by both feeding the wire off the reel and counting turns for you. I have rewound some older transformers that could be taken apart easily, and they almost always did work again after the rewinding of the secondary. I always had to check the turns ratio during testing to see the turns per volt for the winding, and guess at the regulation figure and power handling. for one application I just used a few small 1VA potted transformers that I had plenty of and used one for the mains input and 2 wired in reverse with the windings in series and the output in parallel to get a 100VAC secondary to rectify and get the 90VDC power rail for a small electrometer. Another was used to provide the 1V2 heater voltage, using a LM317 to regulate the heater. It ran barely warm.

[electronics man]

Did you look here  http://livinginthepast-audioweb.co.uk/index.php?p=xfrmrvt228

i would go for that but i need about 160VA