Why do we need negative voltage regulator ICs? Is there any significant difference between positive and negative regulators? Has anyone tried a "reversed" 7812 as your negative voltage source, which is to use the output pin of 7812 as ground and the ground pin as your negative voltage source?
Negative regulator
As long as your have seperate windings or
separate transformers for the unregulated supplies, you can use positive regulators. The positive output for one regulator is connected to the ground of the regulator for the positive supply. The negative supply is now the ground of positive regulator being used as a "negative" regulator.
The performance of the LM317 and LT1086
are better than the coresponding negtive regulators. For seperate secondaries it will only cost you another diode bridge. This is often done and it works very well.
As long as your have seperate windings or
separate transformers for the unregulated supplies, you can use positive regulators. The positive output for one regulator is connected to the ground of the regulator for the positive supply. The negative supply is now the ground of positive regulator being used as a "negative" regulator.
The performance of the LM317 and LT1086
are better than the coresponding negtive regulators. For seperate secondaries it will only cost you another diode bridge. This is often done and it works very well.
Haven't tried. As it obviously won't work as a regulator. But it may be fun. Listen to the doctor's advice. He knows what he's talking about.
analog_sa said:
....well, I still don't see why it obviously won't work.
could you please explain in more detail?
thanks!
It works
Lay a mesh around the regulator, IC and bridge/trafo. It will work because current always flow out of the regulator. On the other side I would not use a 78xx regulator there are much better parts/circuits available.
Lay a mesh around the regulator, IC and bridge/trafo. It will work because current always flow out of the regulator. On the other side I would not use a 78xx regulator there are much better parts/circuits available.
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.analog_sa said:
You can use positive regulator for positive voltage and the negative voltage can be derived by voltage power amplifier with gain of -1. Of course the later must be supplied from voltage higher than output stabilized one.
paulb:
completely correct, a dual secondary winding is needed, I implied this was the question (and I alyway do so).
completely correct, a dual secondary winding is needed, I implied this was the question (and I alyway do so).
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/audio/gainclone/gainclone.htm
but using LM317 for a 12 V supply (low ampage)
but using LM317 for a 12 V supply (low ampage)
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.
AudioFreak said:
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?
AudioFreak said:
So what's the difference between a 0-12v-24v setup and a -12v-0-12v setup? I still don't get it.
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.
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.
It can be two things:EJ said:
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.,
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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