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Old 20th August 2003, 02:39 PM   #1
EJ is offline EJ
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Default why do we need negative voltage regulator ICs?

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?
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Old 20th August 2003, 04:10 PM   #2
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Default 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.
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Old 20th August 2003, 04:52 PM   #3
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Quote:
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?
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.
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Old 20th August 2003, 06:17 PM   #4
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Quote:
Originally posted by analog_sa


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.
....well, I still don't see why it obviously won't work.
could you please explain in more detail?
thanks!
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Old 20th August 2003, 06:24 PM   #5
bocka is offline bocka  Germany
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Default 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.
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Old 20th August 2003, 06:42 PM   #6
paulb is offline paulb  Canada
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Quote:
Originally posted by analog_sa


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.
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.
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Old 20th August 2003, 07:42 PM   #7
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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.
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Old 20th August 2003, 08:25 PM   #8
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http://www.geocities.com/react_71/au.../gainclone.htm
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Old 20th August 2003, 09:25 PM   #9
sam9 is offline sam9  United States
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I reversed polarity on the NTE equivalant of the LM317 once. It started a fire.
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Old 20th August 2003, 11:21 PM   #10
bocka is offline bocka  Germany
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paulb:

completely correct, a dual secondary winding is needed, I implied this was the question (and I alyway do so).
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