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Old 20th August 2003, 10:47 PM   #11
jkeny is offline jkeny  Ireland
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I used Pedja's circuit for a + & - regulated supply using LM317 for both and it blew the LM317 on the neg supply and a 470 uF cerafine cap after the regulator. Did I do something wrong? It was connected exactly as per diagram in http://www.geocities.com/react_71/au.../gainclone.htm
but using LM317 for a 12 V supply (low ampage)
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Old 21st August 2003, 12:26 AM   #12
EJ is offline EJ
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Quote:
Originally posted by paulb

It won't work if you need both positive and negative supplies, and they are coming from the same transformer secondary winding with the centre tap used for ground. In other words, if ground needs to be common between input and output. Otherwise it will work.
anybody knows why?

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Old 21st August 2003, 12:29 AM   #13
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Because if you are using your centre tap as a ground reference, you need to be able to regulate both +ve and -ve relative to the centre tap. +ve regulators can only regulate +ve relative to some other point.
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Old 21st August 2003, 12:56 AM   #14
EJ is offline EJ
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Originally posted by AudioFreak
Because if you are using your centre tap as a ground reference, you need to be able to regulate both +ve and -ve relative to the centre tap. +ve regulators can only regulate +ve relative to some other point.

Are you meaning that if we need a 0-12vdc-24vdc power supply and our transformer does not have separate secandary windings, we can only use a +12V setup plus a +24V setup, instead of two cascoded +12v setup?
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Old 21st August 2003, 01:06 AM   #15
Pedja is offline Pedja  Serbia
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Quote:
Originally posted by jkeny
I used Pedja's circuit for a + & - regulated supply using LM317 for both and it blew the LM317 on the neg supply and a 470 uF cerafine cap after the regulator. Did I do something wrong?
I’d say you did. Generally it should work with any positive regulator.

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Old 21st August 2003, 12:02 PM   #16
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Quote:
Originally posted by EJ



Are you meaning that if we need a 0-12vdc-24vdc power supply and our transformer does not have separate secandary windings, we can only use a +12V setup plus a +24V setup, instead of two cascoded +12v setup?

No that will work fine.
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Old 21st August 2003, 05:35 PM   #17
EJ is offline EJ
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Quote:
Originally posted by AudioFreak



No that will work fine.
So what's the difference between a 0-12v-24v setup and a -12v-0-12v setup? I still don't get it.
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Old 21st August 2003, 09:28 PM   #18
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In fact there is no difference between your two examples.
It just depends where you put the Com probe of your voltmeter.

However:

Some transformers (like most toriodal) have two entirely separate windings which for example both can supply 24v. (mine does).

There are thus four wires coming out:
- blue / green from one winding
- red / yellow from another winding.

When you connect blue / yellow (or green / red) and measure the AC voltage across the unconnected wires : 0V. Due to cancellation of the sinus wave. You've connected both windings 'antiphase'

When you connect blue / red (or green / yellow) and measure the AC voltage across the unconnected wires : 48V.

In the latter case this transformer is behaving exaclty like a transformer with a center tap.
When you have separate windings you can rectify both windings, put both DC's through a regulator and after regulation connect one positive output to the other negative output. BUT YOU DO NOT CONNECT THE AC-WINDINGS.

Now draw out the schematic when you only have a centre tap and you use two bridges and two positive regulators .

Note that when you connect one of the regulated positive output's to the other negative a closed circuit does exists (through the transformer and connection at the bridges) which imho does look quite tricky. As the the regulator is trying to impose a voltage difference in this loop (the voltage across the regulator ) you're asking for problems.
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Old 22nd August 2003, 06:35 AM   #19
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Quote:
Originally posted by EJ


So what's the difference between a 0-12v-24v setup and a -12v-0-12v setup? I still don't get it.
It can be two things:

0-12-24 _can_ be designed so you MUST connect the load between 0-12 or 0-24 which means that current can't flow into the 12 volt terminal, just out. I think it's quite confusing to write 0-12-24 if you mean -12 - 0 - (+12). I wouldn't do that but this is an american thing I suppose.

Or two true voltage sources which are connected in parallel.

Conclusion: If you are an american there's probably no differnce but surely if you are an european.,
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Old 22nd August 2003, 12:38 PM   #20
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You can put two 12V regulators in series to get 0/+12V/+24V and you can draw power from all 3 terminals without problems as long as you realise that the upper regulator will carry the current for both outputs and so will need to be higher power than the lower regulator which will just carry the current required for the 12V output. As said earlier though, current will only be able to flow out of +12V and +24V.
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