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Old 17th November 2010, 08:59 AM   #11
akis is offline akis  United Kingdom
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ok another question: can I use the *same* secondary winding to power two independent bridges with their own rectifiers? so when one is delivering many amps, the other one is mostly unaffacted and can be used to provide a more stable supply for driving the various components of the PSU which require very little power but steady voltage.
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Old 17th November 2010, 10:32 AM   #12
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Originally Posted by akis View Post
ok another question: can I use the *same* secondary winding to power two independent bridges with their own rectifiers? so when one is delivering many amps, the other one is mostly unaffacted and can be used to provide a more stable supply for driving the various components of the PSU which require very little power but steady voltage.
No.

Draw it out: one winding to 2 bridges doesn't work!

Wind another secondary? Only needs thin wire and most toroids have some "space" inside the donut (US spelling!) to allow this.

Cliff
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Old 17th November 2010, 10:40 AM   #13
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I want to use the transformer to make a PSU "30V 5A" - obviously if it can only reach 29V then it is no good
Well, unless it is a regulated supply, you will find that your magical 30V varies all over the place over the time of day, the load, the weather, etc.
And if you go for a regulated supply, the xformer voltage gets much less critical.
I think you are chasing the wrong ghost, but it's your time and money .

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Old 17th November 2010, 02:05 PM   #14
macboy is offline macboy  Canada
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Originally Posted by janneman View Post
Well, unless it is a regulated supply, you will find that your magical 30V varies all over the place over the time of day, the load, the weather, etc.
And if you go for a regulated supply, the xformer voltage gets much less critical.
I think you are chasing the wrong ghost, but it's your time and money .

jan didden
I 100% agree with Jan.
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Old 17th November 2010, 02:53 PM   #15
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Originally Posted by cliffforrest View Post
No.

Draw it out: one winding to 2 bridges doesn't work!

Wind another secondary? Only needs thin wire and most toroids have some "space" inside the donut (US spelling!) to allow this.

Cliff
It can work - http://www.passdiy.com/pdf/a75p2.pdf Used here to provide some isolation between the two channels.

That said, if you are looking to provide regulated output for the amp's front end, either a separate higher voltage transformer, a light gauge second winding or a doubler feeding the regulator (as shown in the A75 supply) would do better than just having a separate bridge.

Last edited by BobEllis; 17th November 2010 at 02:59 PM.
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Old 18th November 2010, 07:45 AM   #16
akis is offline akis  United Kingdom
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I am building a general purpose bench PSU. Money is not a consideration because if I ever build it, it will have cost more money and trouble than buying a new one from a shop. But it is a hobby.

The transformer is 25VAC, and that does not leave enough room for 30VDC at the output. A 27VAC is much better.

If I wanted to wind a few more turns on the secondary as suggested, how would I do that? There is not much free wire left, it might not even do one turn. The only solution I can think of is to solder some more enameled copper wire on to it and as smooth as possible, then put electrician's tape around the solder and wind a few more turns. Is that correct?
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Old 18th November 2010, 08:02 AM   #17
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Originally Posted by akis View Post
.....

The only solution I can think of is to solder some more enameled copper wire on to it and as smooth as possible, then put electrician's tape around the solder and wind a few more turns. Is that correct?
Why?

Just wind some more turns (do 10 and measure the voltage which will give you the volts/turn figure) and connect the windings in series with the existing ones OFF the transformer.
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Old 18th November 2010, 08:11 AM   #18
godfrey is online now godfrey  South Africa
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If I wanted to wind a few more turns on the secondary as suggested, how would I do that?
Here's a nice example with a picture.
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Old 22nd November 2010, 06:56 AM   #19
akis is offline akis  United Kingdom
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I have opened up the transformer and noticed an abundance of ordinary sellotape/scotch tape

The secondary is 1mm or 1.25mm wire and it is wound so as to form just one layer - leaving a small gap just for solder connections to be attached. Those connections are also insulated with a few turns of sellotape...

All I have to do is to get same diameter wire, solder it on where the external wires are now attached, wind it a few turns to raise the voltage from 25VAC to say 27VAC, and then attach the external wires back on it.

In addition I can also wind another layer of wire to provide a 3rd secondary, still 27VAC but much lower current. I can wind it on top of the existing secondary winding but using much thinner wire as I only need 400mA at most.
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Old 22nd November 2010, 07:19 AM   #20
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I have opened up the transformer and noticed an abundance of ordinary sellotape/scotch tape
Polyester tape, more like packing tape with no glue, old sellotape was cellophane based (modified cellulose)

Quote:

All I have to do is to get same diameter wire, solder it on where the external wires are now attached, wind it a few turns to raise the voltage from 25VAC to say 27VAC, and then attach the external wires back on it.
I never bother with details like that, I just solder a bit of hookup wire of adequate section to the end of the original lead and wind it around the core with fairly widely spaced winding to spread the winding out over the core. Test the turns per volt first by winding a couple of turns of hookup wire around the core and measuring the voltage produced in them. Test the phasing before taping up or varnishing the extra winding. You can use enamelled wire if you like, it can take more heat and is much smaller also it can be any size as long as it is the same or larger diameter than the original wire.

If you have stripped off the tape layer then you might as well use the space left at the end of the winding for the extra turns
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In addition I can also wind another layer of wire to provide a 3rd secondary, still 27VAC but much lower current. I can wind it on top of the existing secondary winding but using much thinner wire as I only need 400mA at most.
You sure can but put varnish the new winding first, wrap it in tape (polyester not cellophane) then wind the next winding. I like toroidal transformers because they are easy to fine tune the voltage to suit the job, EI are OK if there is still space in the window and the voltage needs to go up, if down I hook the required number of turns up antiphase which increases copper losses a bit, but it is much easier than stripping windings
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