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LM317 dual rail power supply

post #1 of 33
3/21/10 at 3:06pm
Thread Starter 
I'm designing a +18v -18v power supply with two lm317. I have made a schematic with eagle. It's my first project so any tips are welcome. It will be used to power an unbalanced to balanced converter based on the THAT1646, it will draw about 100 mA per channel. For now im going for 2 channels but it might be extended to 8 or 12 in the near future. Have I made any big errors, or are there things that are better done differently?



Thanks for your help

Steven
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post #2 of 33
3/21/10 at 3:19pm
Why not used fixed regulators?

And you may want to check maximum supply voltages for the THAT1646
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post #3 of 33
3/21/10 at 3:35pm
Thread Starter 
Quote:
Originally Posted by cobaltmute View Post
Why not used fixed regulators?
LM317 are easily available and it makes it a more modular kind of power supply.


Quote:
Originally Posted by cobaltmute View Post
And you may want to check maximum supply voltages for the THAT1646
My bad, It should have been +18 and -18. I will edit it on the first post.
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post #4 of 33
3/21/10 at 3:55pm
This is from the datasheet for the LT1033. It shows a way of dual tracking two adjustable regs from one pot.



I learned about this from a thread at diyAudio where someone had done the same with LM337/LM317. I've sim'med it with the LM317/337 regs and it works in sim, but I've never built it.

May be of use to you.
LL
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post #5 of 33
3/21/10 at 4:31pm
Thread Starter 
Very interesting, but it seems to be more for a benchtop supply. It is ajustable with just one pot while mine is adjustable with the 2 trimmers. like a variable voltage vs a set-once-and-then-just-use-it-like-that supply.
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post #6 of 33
3/21/10 at 4:32pm
And Tangent's TREAD would work for you as well
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post #7 of 33
3/21/10 at 5:07pm
Quote:
Originally Posted by steven2992 View Post
I'm designing a +18v -18v power supply with two lm317. I have made a schematic with eagle. It's my first project so any tips are welcome. It will be used to power an unbalanced to balanced converter based on the THAT1646, it will draw about 100 mA per channel. For now im going for 2 channels but it might be extended to 8 or 12 in the near future. Have I made any big errors, or are there things that are better done differently?



Thanks for your help

Steven
It looks good but you should add some caps across the pot's. Please consult the datasheet for LM317/337.
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post #8 of 33
3/21/10 at 6:43pm
Thread Starter 
http://www.national.com/ds/LM/LM117.pdf

The spec sheet says the caps across the pots should be 10 uF. It also says that in some cases protection diodes are needed. The protection diodes are in case the input is shorted and to prevent the caps discharging across the lm317. Are they absolutely necesary or can I leave the out? page 9-10 drawing 906307 on the datasheet.
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post #9 of 33
3/21/10 at 6:58pm
Thread Starter 
I've updated the schematic with the extra caps and optional protection diodes. The diodes are 1N4002 or equivalent.

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post #10 of 33
3/21/10 at 8:19pm
if you're really planning to trim the V to closer than the tolerance of the reg internal ref then you should use 2 fixed 1% metal film resistors for the divider and arrange the pot in parallel with R ratio or series R to trim ~5%, always use potentiometer mode when possible to minimize wiper contact influence on setpoint stability

Bob Pease recommends single turn pots as being more settable/stable than multiturns - the screw drive reduction mechanism in multiturns always has backlash
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post #11 of 33
3/22/10 at 7:50am
I have something like this:


Bought it off fleabay for $24. I think it's an ok implementation.
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post #12 of 33
3/23/10 at 1:55pm
Thread Starter 




I've updated the schematic with some of your advise and some stuff from the schematic posted by devast. I'm going for "comercial" quality and not for "audiophile", so trimming to within the regs tolerance isn't necessary.
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post #13 of 33
3/23/10 at 2:13pm
The fixed resistors across the trimpost is 2 resistors and would easily fit your board layout with little to no modification.

I would also suggest the use of a top ground plane, with the rest of the traces on the bottom. This would make your routing easier, and is still doable for home etching, if that was your goal.
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post #14 of 33
3/23/10 at 3:37pm
Quote:
Originally Posted by steven2992 View Post
... I'm going for "comercial" quality and not for "audiophile", so trimming to within the regs tolerance isn't necessary.
then leave the pots out altogether - just use 15 cent 1% metal film reisitors in a fixed divider
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post #15 of 33
3/24/10 at 9:59am
Thread Starter 
I've put in the extra resistor and i've replaced the multi turn with a single turn and i've redone the board layout





I've just noticed that c10 is the wrong way round, I have already fixed it.
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