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Old 4th February 2003, 11:41 AM   #1
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Question Transformer problem

Am in the process of making a bench PS. (linear regulated)
at the main time am using LM3x7 to do the regulation until
ican find others with more current handling (eg. lm137,lm350..)

my problem is that i cant find a suitable say 24-0-24 3A trans.
so i can get about +36 , 0 , -36 outputs from the 2 regulators.
ques. is can i use 2 x 12-0-12 3A trans. in somehow to achieve
2x 24 rail inputs??

thanks.
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Old 4th February 2003, 11:56 AM   #2
Bobken is offline Bobken  United Kingdom
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Hi.

Yes with dual secondary transformers, you have the choice of connecting the secondaries in parallel(which doubles the output current), or to connect them in series, which doubles their voltage.

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Old 4th February 2003, 12:01 PM   #3
subwo1 is offline subwo1  United States
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Use one transformer for the positive supply, and the other for the negative. leave the center tap of each open.
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Old 4th February 2003, 12:26 PM   #4
Bobken is offline Bobken  United Kingdom
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Quote:
Originally posted by subwo1
Use one transformer for the positive supply, and the other for the negative. leave the center tap of each open.
Hi,

When you say "open" the centre-tap secondaries must be connected together (although not grounded) to work like this, of course.


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Old 4th February 2003, 01:13 PM   #5
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ive connect 2 of the seconderies wires together, and the 2 tap
wires together and use it as the ground, then use the 2 outer
secondries together to feed the rectifier as shown,,
am i right?
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Old 4th February 2003, 01:21 PM   #6
UrSv is offline UrSv  Sweden
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Quote:
Originally posted by deepanger
ive connect 2 of the seconderies wires together, and the 2 tap
wires together and use it as the ground, then use the 2 outer
secondries together to feed the rectifier as shown,,
am i right?
Rather like this:
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Old 4th February 2003, 01:44 PM   #7
Bobken is offline Bobken  United Kingdom
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Hi,

The way I would do it is, precisely as subwo1 suggested.i.e. use one transformer for +24v and the other for -24v.

However 'terminology' varies and my understanding of secondaries being "open" means completely disconnected, which is not quite correct here.

On all dual secondary transformers (at least all those I have ever seen), there should be 4 secondary wires which will be identified by colour if they are leads, or markings if it has tags.

For each TR there will be two 'starts' to the coils and two 'ends'.

Taking each individual transformer at time, you can connect the two 'starts' together and the two 'ends' together, and you will then have the same voltage (12v, in your case) across these two leads, but the TR will permit twice the current drawn. This the parallel connection.

Alternatively (and this is what you need here) you can connect the 'end' of one winding to the 'start' of the other, which is the series connection, and across the two remaining leads this will give you twice the nominal output voltage, with the same current.

The point I tried to ensure you understood about this is that two of the secondaries (at the centre-tap) *do* need to be joined together, but they should then be insulated and not used for any other connection.

I would then use each individual transfomer (in series connection) to provide a separate 24v supply, one for the positive, and one for the negative, each with its own diode bridge to provide the rectification to DC.

This way, the only real 'join' between the pos and neg is at the ground connection.

It is the orientation of the diodes in each bridge, of course which will determine whether the supply is pos or neg.

Regrettably I have no means for graphics which would save many words, and make it quite clear to you.

If you can re-draw what you think I have just described, I will willingly check it for you.

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Old 4th February 2003, 01:48 PM   #8
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how connecting the 2 x 12v secondries make the ground?!
got confused in this
why cant i use the original wiring i sent 1st?
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Old 4th February 2003, 01:58 PM   #9
Bobken is offline Bobken  United Kingdom
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Quote:
Originally posted by deepanger
how connecting the 2 x 12v secondries make the ground?!
got confused in this
why cant i use the original wiring i sent 1st?

Because you are grounding the two centre taps which is not what you wish to do.

Look at UrSv's diagram (which wasn't posted when I started my last reply) which is also fine as the centre taps are not grounded.

This is another way of achieving what you want with only using one bridge, but personally, as bench power supplies are subject to a lot of abuse, and can sometimes fail, I would rather spend the small additional sum on the second bridge, and have two completely independent supplies.

This way, if one goes down the other is still OK, but it is simply a matter of choice.

Also, using two bridges, each one is loaded less this way, so it will be more robust.

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Old 4th February 2003, 02:09 PM   #10
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itried to draw the wiring but icouldnt get the bridge inputs
neither the ground!
can u complete it if u dont mind?
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