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Old 10th January 2005, 07:41 PM   #1
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Default Regulator choices...

I am building some power supplies... Specifically a +/- 15VDC and most likely a thirty something. I have ordered some regaltors... fixed 15+ and fixed 15- and a pair of variables in + and - also.

Is there some regualtor out there that can do both the positive and the negative?

What are your favorite regulators?
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Old 11th January 2005, 08:43 PM   #2
maylar is offline maylar  United States
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When I designed power supplies back in the 70's there were lots of linear regulators available. Motorola had a chip that did +/- 15V that could be expanded for additional current. Today they're only interested in low voltage stuff for cell phones and PC's.

You won't find much except 3 terminal devices for 15V. Fixed LM7815 / 7915 or variable LM317 / 337 etc.

My favorite is still the venerable LM723 adjustable with series pass transistors.
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Old 15th January 2005, 01:20 PM   #3
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I see that the Linear Technologies LT1083, although listed as a positive voltage regualor, is used on the negative rails. How does this work? Not how well, but how?

When can a positive regulator be used as a negative regualtor?
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Old 15th January 2005, 01:57 PM   #4
SY is offline SY  United States
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Quote:
Originally posted by chipco3434
I see that the Linear Technologies LT1083, although listed as a positive voltage regualor, is used on the negative rails. How does this work? Not how well, but how?

When can a positive regulator be used as a negative regualtor?
You use it in the return leg. A simple way to think about it is to imagine your supply completely isolated from ground. It's a two terminal voltage source. It doesn't matter which terminal you end up grounding- "ground" is arbitrary.
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Old 15th January 2005, 03:03 PM   #5
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Do you mean to say that IN/OUT/Grnd on a three pin regulator are relative?

My question refers to Pedja's or Monster's regulated supply.

pedja's supply

Monster's

So the LT1083 and LM338 will regulate negative voltage if supplied with negative voltage????

My current project is using some 7815/7915 regulators. Why should one bother with complementary parts when it could be handled with a single part on both sides?
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Old 15th January 2005, 05:43 PM   #6
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Quote:
Do you mean to say that IN/OUT/Grnd on a three pin regulator are relative?
Don't be silly. Think of a positive regulator/rectifier/transformer as a black box with two output terminals. Which one you ground is up to you so you can have either positive or neg output.

In the above setup you need two rectifiers and two independent transformer secondaries. It will be quite obvious if you actually look at Pedja's circuit in your link.
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Old 15th January 2005, 06:07 PM   #7
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So that would not be possible with a single transformer/rectifier setup then...???

In Pedja's circuit, the grounded center circuit is regulated to +24V when referenced to the negative rail????
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Old 15th January 2005, 06:11 PM   #8
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You got it.
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Old 15th January 2005, 06:18 PM   #9
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Quote:
You got it.

Not yet... I'll "get it" after I build it.

However, this doesn't answer the question, "why would somebody buy a complimentary pair of regualators?" Is the above solution only valid when running fully separate trans/rect setups?
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Old 16th January 2005, 03:47 AM   #10
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Burned two LM7815's. Evidently, the polarity of the rails does not change when you turn the board upside down for soldering! How's that for stupid!

Turns out that the negative rail is the perfect spot for a 7915!

A lesson bought is better than a lesson taught!
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