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Old 9th June 2008, 03:14 PM   #1
Dxvideo is offline Dxvideo  Turkey
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Question Problem with LT1083 PSU...

Hi all,

I've made a (combined) regulated symmetrical power supply with two LT1083 + LM317 + LM337.. Which is;
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I use a toroidal trafo with two independent 30v AC outputs (225VA) and two seperated 35A rectifiers plus 10,000u main reservoirs before this circuit. And the V+/V- would be 30v each.
However, I cannot read any voltage on main negative rail (V-) while main positive rail (V+ to GND = 30v) and input of that negative rail (DC2+ to DC2- = 42v) is exist. And the second thing is the bottom LT1083 gets warm a bit while the other was still cold!
Anyway,
Do you see any unsuitability on that circuit design? May LM337 causes that kind of fault? Because I cannot imagine any other suspect..

Thanks in advance for the replies.
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Old 10th June 2008, 11:41 AM   #2
juma is offline juma  Serbia
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Your PSU design is absolutely OK. I made it that way a couple of times, everything works like a charm. You probably made an error in wiring or on PCB or you have a faulty component - double check everything, step by step, piece by piece - don't give up - it must work !
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Old 12th June 2008, 10:57 AM   #3
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On the first view i also couldn't find any bug in your schematic, but could you explain me how much current you do expect with this circuit ? Low drop out indeed, but two regs in series ? one 7.5 A and then 1.5 A ? hmm ... I don't think you'll need the diodes D1,2 ... As far as i remember the LT1083 has an internal protection ...

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Old 12th June 2008, 12:16 PM   #4
Dxvideo is offline Dxvideo  Turkey
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Ok guys..
I found the problem and already fixed. The LM337 was fake!
Now it works fine..
By the way,
The first stage is for two LM3875 gainclone and the second is for their buffers..
And... LM317/337 pair is warming a bit.. May be 50° or more.. Do you think it causes any problem by time?
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Old 12th June 2008, 12:32 PM   #5
AndrewT is offline AndrewT  Scotland
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add a pair of 20C/W sinks.
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Old 12th June 2008, 01:39 PM   #6
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Hi Dxvideo,

You are using a trafo that outputs 30 Vac, rectified raw, should put out more than 40Vdc, as you measured @ 42Vdc. per winding, feeding; Whew! The LT1083, a fixed +ve reggie, which, assuming is a 12Vdc output, it's recommended input is 25Vdc. Max 30Vdc. Aren't you stretching the LT1083 a fair bit here. Then it feeds the LM reggie. Even if everything works, I don't see how you are going to get +/- 15Vdc output from a +/- 12 Vdc input.

Then, you have to consider that a +ve reggie, although it may be utilised to regulate and produce -ve voltage, but it may only work in a single stand alone loop only. In your circuit, you have 2 loops; ie; a +ve loop and -ve loop combined. May not work!

Try this, remove the LT1083 on the -ve DC2 side and jumper wire in it's place. Power up and see if you can obtain a -ve voltage at output of LM337. If yes, then it is confirmed that LT1083 cannot be used here.

Usually 1 reggie per loop is sufficient. You don't need 2 in series for your circuit.

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Gerard
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Old 12th June 2008, 01:48 PM   #7
AndrewT is offline AndrewT  Scotland
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Hi V,
one can use two positive regulators to generate a dual polarity supply if there are separate secondaries and separate rectifiers for each positive regulator.
Once these are tied together to give a zero volt line the following regulators must be pos+neg to give lower voltage dual polarity supplies.

He has been told to buy the correct voltage transformer, but he's made his mind up.
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