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Old 14th May 2007, 06:40 AM   #1
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Default Question about regulated PSU by CarlosFM.

Hi guys, I have a PSU which has only 30-0-30 (3 terminals), is it possible to build the Carlos' regulated PSU? if yes, what is the schematic?? and one more question, how many uf of capacitance is necessary for 3886GC before lm338?? Thx!!
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Old 14th May 2007, 11:45 PM   #2
gootee is offline gootee  United States
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Default Re: Question about regulated PSU by CarlosFM.

Quote:
Originally posted by blackboy
Hi guys, I have a PSU which has only 30-0-30 (3 terminals), is it possible to build the Carlos' regulated PSU? if yes, what is the schematic?? and one more question, how many uf of capacitance is necessary for 3886GC before lm338?? Thx!!
I don't think it's possible, without two separate transformers.

- Tom Gootee

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Old 2nd July 2007, 10:36 AM   #3
Archwn is offline Archwn  United Kingdom
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use only 4 rectifier diodes instead of 8 and you will get what you want.

hint: a ground is a ground.
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Old 2nd July 2007, 10:51 AM   #4
AndrewT is offline AndrewT  Scotland
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Quote:
hint: a ground is a ground.
that is just the start of more confusion.
Most "grounds" do different jobs and we could champion the cause of clarity if we used different names for each different use.
eg, safety earth,
return,
zero volt reference,
PSU power zero volts.
common,
etc.

Often, these are combined, but that combination should be the result of deliberate thought by the designer and not an accident because everyone called them "ground".
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Old 2nd July 2007, 10:55 AM   #5
AndrewT is offline AndrewT  Scotland
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Quote:
I have a PSU which has only 30-0-30 (3 terminals)
Quote:
build the Carlos' regulated PSU?
the Carlos' power regulator uses positive chip regs. This requires a pair of isolated inputs to each regulator. The zero volt line after the regulators becomes the power ground. The four inputs must ALL be isolated from the power ground and from each other.

A three input PSU cannot meet this requirement.
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Old 2nd July 2007, 12:03 PM   #6
neeraj is offline neeraj  India
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Hi,
i have 2 nos Toroidl trafos 35-0-35 , 225 VA rating.
Can i use these trafos with chip LM 3886?
What are the necessary requirements to bring down the out put voltage to 30-0-30 volt after rectification?
Regulated power supply is another option which is also suitable for the same.
Plz guide.
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Old 2nd July 2007, 12:35 PM   #7
AndrewT is offline AndrewT  Scotland
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Hi Neeraj,
the voltage is too high.
The DC will be in excess of 50V.
Dropping that to +-35Vdc (for 8ohm) results in 15V*2.2A= 33Watts of maximum dissipation for each polarity.
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Old 2nd July 2007, 12:35 PM   #8
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Quote:
Originally posted by neeraj
Hi,
i have 2 nos Toroidl trafos 35-0-35 , 225 VA rating.
Can i use these trafos with chip LM 3886?
By itself, no -- the max operating voltage of the LM3875 is 42 volts each side for an 8 ohm load. If you have a 4 ohm load it is much lower.

Don't forget that you can unwind a few turns of the transformer to attain lower voltage.
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Old 2nd July 2007, 01:01 PM   #9
AndrewT is offline AndrewT  Scotland
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Quote:
you can unwind a few turns of the transformer to attain lower voltage
but that reduces the VA rating in direct proportion to the voltage reduction.
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Old 2nd July 2007, 01:11 PM   #10
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Quote:
Originally posted by AndrewT
but that reduces the VA rating in direct proportion to the voltage reduction.
Isn't the current a function of the BH character of the core?
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