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Old 20th September 2007, 03:46 PM   #1
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Default Positive regulator to produce negative voltage?

Hi warm folks!

Can we use a positive regulator to produce a negative voltage?

I'm looking to build a +-15v (adjustable) using 1 or 2 rectifier bridge, and use 2 identical positive regulator (LM1086, LT1086, LT1963, etc.).

I've found a schematics but they use 2 bridges. Can we use this with only one bridge, and remove the first stage of the schematics (darlington and opamp).

Possible ?

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Old 20th September 2007, 06:06 PM   #2
KSTR is offline KSTR  Germany
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One bridge and center-tapped xformer: NO
Two bridges, isolated secondaries (and basic topology as shown): YES
- Klaus
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Old 20th September 2007, 08:01 PM   #3
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Quote:
Originally posted by KSTR
One bridge and center-tapped xformer: NO
Two bridges, isolated secondaries (and basic topology as shown): YES
- Klaus
Thanks.

I tried with only one bridge. It was funny. Boom.

If I follow you, I can have for each:

15v secondary + bridge + caps + LM + caps

And I connect the ground at output (or one ground plane for both ?)

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Old 20th September 2007, 08:09 PM   #4
AndrewT is offline AndrewT  Scotland
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Hi,
a dual positive supply can be series connected AT THE OUTPUT to give a dual polarity supply.
But you cannot use a centre tapped nor a single bridge to create the two completely separate supplies needed for the commoned output dual polarity.
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Old 20th September 2007, 08:46 PM   #5
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If fact, if you already have the right transformer, the cost is only one bridge more and more work to design the pcb?

For the ground plane, on the PCB. Can we build it using all the component in red or we MUST build 2 separated ground planes et connect them at the exit (green).

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Old 21st September 2007, 06:46 AM   #6
AndrewT is offline AndrewT  Scotland
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Hi,
the sensing resistors feeding the regulator control circuits MUST SENSE the OUTPUT voltage, not some arbitrary point on a ground plane.
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Old 21st September 2007, 07:39 AM   #7
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Quote:
Originally posted by AndrewT
Hi,
the sensing resistors feeding the regulator control circuits MUST SENSE the OUTPUT voltage, not some arbitrary point on a ground plane.
Like this?

The objective is to produce someting better than LM317/LM337 couple with the minimum component and lower input voltage.

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Old 21st September 2007, 09:07 AM   #8
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Forgot the first PDF. I did a mistake. I not used the right pinout for the LT1965.

I've still some problems with grounding...

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Old 21st September 2007, 04:34 PM   #9
AndrewT is offline AndrewT  Scotland
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Hi Stef,
you need to do more reading on basic circuits before you try to make them more complex.
Fuses F1 & F2 might blow on first switch on. You can either make them bigger (less protection) or place them after the main smoothing caps.
The upper bridge rectifier is back to front.
The 10VA will give 410mAac maximum continuous current.
After rectifying and smoothing the maximum continuous DC current is ~200mA. A 150mA transformer (3.6VA) will not do.
If you want 15Vdc then you need a 15Vac transformer.
10VA will now give 165mA @15Vdc.
The inductor before the first cap has not been designed to make this a choke regulated PSU.
It will still act as a capacitor input PSU.
the inductor in the lower 0v lead should be on the lower -ve lead.
The sense resistors should be connected to the output.
the ground in the lower reg should be connected to the negative supply line for this regulator (just like the upper reg is connected to it's negative supply line.
Are these regulator stable with a low ESR cap on the output?
where should C7 & C17 be connected?

Sorry to appear so hard, but you would be better building a simple PSU first and understand what you have achieved, before you go regulated.
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Old 21st September 2007, 06:58 PM   #10
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Quote:
Originally posted by AndrewT
Hi Stef,
you need to do more reading on basic circuits before you try to make them more complex.
Sorry but yesterday night I worked as a pig on this drawing. Too fast and with not edited copy/paste. This not my first psu (go to see my last one, the dedicated DEQ/DCX psu on the forum). I uploaded a corrected version of the drawing. Most of your comments was already corrected.

The fuses are not fuse but polyswitch. The LT1965 is a new low dropout regulator with low noise from Linear. It works with low esr caps. This is an example. We can replace it with others as the LT1086, LT1763 or others from National.

The idea is to build something different of the regular 317/337 couple or the more detailed Sulzer type psu (too many component in my case, no space).

Quote:
Originally posted by AndrewT
The inductor before the first cap has not been designed to make this a choke regulated PSU. It will still act as a capacitor input PSU.
Interesting. I use VK200 for their low price and small size since years. May be a bad habit. What is this story of "input psu capacitor"? Do you prefer other type of ferrite bead?

Quote:
Originally posted by AndrewT

If you want 15Vdc then you need a 15Vac transformer.
The LT1965 is a 290mV dropout. We can may be try 13 or 14v at input to get 15v.

Quote:
Originally posted by AndrewT

Sorry to appear so hard, but you would be better building a simple PSU first and understand what you have achieved, before you go regulated.
No problem at all. If I ask for advice, it's because I have no problem to be corrected when wrong. I'm looking for that.

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