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 Forum index » DIY Hardware and Software » MusicFromOuterSpace.com designs by Ray Wilson
LM317/LM337 Power supply KIT
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Stavros



Joined: Oct 30, 2007
Posts: 27
Location: Athens-Greece

PostPosted: Thu Dec 08, 2011 3:44 am    Post subject: LM317/LM337 Power supply KIT Reply with quote  Mark this post and the followings unread

Hello,

I would like to get and use the LM317/LM337 KIT to obtain +-15VDC.

I have available a transformer which provides two 37VAC, one 24VAC and one 16VAC secondaries.

Can I use any pair of these as input (by connecting a Neutral/line as a center tap), or I need a new 24-26VAC center tapped as suggested for this project?

I will very much appreciate your advise

many thanks in advance and Best Regards

Stavros
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PHOBoS



Joined: Jan 14, 2010
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PostPosted: Thu Dec 08, 2011 4:22 am    Post subject: Re: LM317/LM337 Power supply KIT Reply with quote  Mark this post and the followings unread

you can indeed use a single supply transformer for a dual power supply.
Using one output to create the positive and negative votage and the other as a GND/0V.
You can see an example of how to connect it below*:
Posted Image, might have been reduced in size. Click Image to view fullscreen.

Now looking at the outputs of your transformer: 37V and 24V are too high.
The 16V should work but you're voltage regulators could get pretty warm.
So provide some good heatsinks and/or keep the current draw low
and you got yourself a +/- 15V supply.

*this is just to get a dual voltage you still have to add capacitor's and regulators to get the supply you want.

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MadScientist



Joined: Nov 28, 2011
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Location: Denmark

PostPosted: Thu Dec 08, 2011 2:55 pm    Post subject: Reply with quote  Mark this post and the followings unread

The catch with the schematic posted by PHOBoS is that the transformer may not like it very much, if the currents in the two supply rails are not identical. In that case there will be a net DC current through the transformer secondary, which may lead to partial or total collapse of the core magnetic field. This in turn may cause transformer overheating, blown fuses, noise from stressed rectifiers, and/or low output voltage.

For a synth you generally want independent supply rails ('no strings attached'), as many modules will not have a symmetric current demand.
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MadScientist



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PostPosted: Thu Dec 08, 2011 3:29 pm    Post subject: Reply with quote  Mark this post and the followings unread

Just checked Mr. Wilson's PSU design, and for that one you*definitely* want a true center tapped transformer. His suggestion is a general purpose, adjustable design, which by definition makes it impossible to keep the currents in the rails symmetric while experimenting etc.

Also please note that his advice on transformer size is completely wrong. You will need a 24V center tapped transformer with a power capacity of roughly 150VA, IE. a 24V/6A specimen. [1] (Transformers are specified in VoltAmps, 'VA', by the way.) This assumes you actually want to be able to draw 2x 1.5A from the PSU continuously, and that your heat sinks are large enough that they will allow you to do so.

Frank.

[1] Didn't do the exact math for his design, but the answer will be in this general ball bark.

Edit: The design won't do anywhere close to +/-15V anyway. You need a higher transformer voltage for this.
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PHOBoS



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PostPosted: Thu Dec 08, 2011 5:14 pm    Post subject: Reply with quote  Mark this post and the followings unread

Quote:
The catch with the schematic posted by PHOBoS is that the transformer may not like it very much, if the currents in the two supply rails are not identical.....

ah yes off course, didn't think of that. (I've never used the design myself like this) but that makes perfect sense now you mention it. Smile

Quote:
Just checked Mr. Wilson's PSU design, and for that one you*definitely* want a true center tapped transformer.

oops, didn't notice it was in the MFOS section, yeah it won't work for that anyway.

btw. I use a 2x 18V 50VA transformer myself for a +/- 15V PSU (which is a similar LM317/337 design) here

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http://phobos.000space.com/ http://www.acidtrash.com/ Stickney Synthyards
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MadScientist



Joined: Nov 28, 2011
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Location: Denmark

PostPosted: Thu Dec 08, 2011 5:20 pm    Post subject: Reply with quote  Mark this post and the followings unread

PHOBoS wrote:
btw. I use a 2x 18V 50VA transformer myself for a +/- 15V PSU (which is a similar LM317/337 design) here

Yep, that sounds a lot more reasonable. Even 2x 15V seems questionable.
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PHOBoS



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PostPosted: Thu Dec 08, 2011 6:49 pm    Post subject: Reply with quote  Mark this post and the followings unread

MadScientist wrote:

Yep, that sounds a lot more reasonable. Even 2x 15V seems questionable.

I had actually planned on using a 2x 15V transformer, but because the
mains voltage was low the output voltage of the transformer was lower too,
and under load it dropped too far. That's why I settled for the 2x 18V.
(I'm using the other transformer for a +/- 12V PSU now)

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http://phobos.000space.com/ http://www.acidtrash.com/ Stickney Synthyards
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Stavros



Joined: Oct 30, 2007
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Location: Athens-Greece

PostPosted: Sat Dec 10, 2011 6:20 am    Post subject: Reply with quote  Mark this post and the followings unread

Many thanks for your input it was very helpful.

PHOBos your work with the PSU is realy very nice and clear.

Best Regards

Stavros
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