Dual 12vdc rails to +/-12v?

[stelleg151]

Title pretty much says it. Is it possible to say get +/- 12vdc (with near full and equal power on +/-) from the rails on a dual rail ATX PSU? Im pretty new to EE, (CS major), and would like to learn more about how to manipulate power, so if anyone would rather point me to a good source than answer the question directly that would be great to.

Thanks

 

[Bassyhead]

What would you need the +/- 12VDC for? And what do you mean by "manipulate power"? I believe the -12VDC supply was used for legacy serial controllers and is currently being phased out as there's really no use for it. The -12VDC supply can only handle a tiny amount of current relative to the +12VDC supply, even more so if you are talking dual +12VDC rails. If you look at the ratings for your power supply, for example (Seasonic 430w S12), there are two +12VDC rails that supply 14A and 15A while the -12VDC rail can only source 0.8A. So in terms of power (you do mean watts, right?) then no you're not going to get equal amounts of power from an ATX PSU.

 

[Mark R]

You need to look for a PSU designed for dual supply use. ATX PSUs do not fall into that category.

Many audio amplifier PSUs will be very similar to what you want - but these are usually linear, rather than switching, supplies - so are big, heavy and inefficient.

If you're a wizard with switched mode supplies you can make a negative voltage converter - which can take +12 V in and produce -12 V. But this is certainly not a beginner's project (unless you only want about 0.4 A).

 

[PsYcHoCoW]

For one thing, don't try to wire it like you'd wire batteries in series... it'd short 12v to ground

With one power supply with dual rails, you'd need a negative switching regulator since the grounds are the same. It doesn't matter that there are two rails in this case...

 

[GalvanizedYankee]

Use thier search function to find what you seek http://www.smpstech.com/
You may not find a direct answer here but it is one of the better resources for SMPSs.

...Galvanized

 

[stelleg151]

Thanks guys. So it sounds like you would basically need to create another smps just to invert the +12v power of one of the rails to -12v, so basically not worth it.

Yeah what I hoping for a cheap alternative to expensive SMPS for audio. I dont really like the idea of buying a huge 30 pound transformer for the amp Im gonna mess around with, but I guess I will have to. Too bad, cause at least for my Seasonic the noise/ripple from the specs I saw seemed theoretically low enough for audio use.

Oh well, was worth a shot, thanks guys.

 

[bobsmith1492]

http://sound.westhost.com/project89.htm

Here's a "switching" power supply you can use to create any rails you want from 10-15V input. I made one myself, and it works, though I haven't tried it with much of a load yet. It's dirt cheap, too; I spent a few bucks for screws and wire and used scrap parts. You can get a toroid from wilcocorp.com by asking for samples, and wind the transformer with that (what I did.)

It's a bit of work, but quite cheap. I put up a bunch of pictures of mine in progress, if you'd like to see them.

 

[tex1138]

You didn't state exactly what your audio project entails, so here is another suggestion: Depending on what exactly you have in mind, it may be possible to use just the +12VDC supply and AC-couple the audio signal, biasing it up to +6VDC. This approach is definitely effective for small-signal operation (such as op-amps that require dual supplies), but I'm not sure it would work with a power amplifier. The primary disadvantage is that you need quite large capacitors if you want a good low-frequency response.

 

[stelleg151]

Thanks for the replies guys. Welcome to the forum tex . Yeah I was unclear about what I was going to use it for: a power amp (Gainclone to be exact).

I know I would need big caps for low freq response, but I will need those if I build a linear supply anyways.

Ill look into biasing the signal to +6vdc, but when I asked around at DIYAudio they recommended that I just give in and buy the huge chunk of metal.

Ill also look into that SMPS, any idea if it has low enough noise/ripple for audio use?

 

[bobsmith1492]

Yeah, it is made for audio use. It switches fast enough that you don't need huge supply caps either (also the ripple is higher frequency than you can hear.) They're tricky from what I hear, though.

By the way, how would you use a transformer if you're converting from 12 VDC?

 

[stelleg151]

Thanks for the diagram, I need to learn more about SMPS chips. Is that chip recieving +12v dc and outputting +/-12vac?

I would love to see photos, mostly to gauge the size of the thing.

Thanks a lot.

 

[bobsmith1492]

It takes 10-15VDC in and can output whatever voltages you want (DC), depending on how you make your transformer. I made mine trying to get a lot of power out of it (350W ++), so it is a lot bigger than necessary, especially the heatsink - just something I had lying around (old CPU heatsink.) I'll look for the pics.

Here we go: http://sound.westhost.com/phpBB2/viewtopic.php?t=677

 

[stelleg151]

Awesome, thanks man. I think I just might try that out and then maybe eventually move it to my car if it works, and then I think Ill take apart the PSU that is powering it and try to make one to run off mains.

Thanks for your help. And as to your question a couple posts above, I would use a transformer if I was running off the mains in replacement of a SMPS, which is what I would like to avoid.

Edit: Oh and a bit late but when I said "manipulate" power I basically meant learn more about power converters as that is the more appropriate name for power supplies IMO.

 

[bobsmith1492]

I learned a bit from this site about switching supplies: http://www.bcae1.com/trnsfrmr.htm
Note: a friend of mine who works for Rockford and has worked on car amps for forever tells me those are technically NOT "SMPSs." Switching supplies are actually those in computers, which work differently. But anyway, minor details; I still call 'em switchers.

Another good site, which I already linked to: http://sound.westhost.com/index2.html

He's got a bunch of power supply stuff, as well as amplifiers. I built one of them, good for probably 100W+. Good luck man! Let me know if you want any tips, as I'll tell you what I can.

 

[Colt45]

Originally posted by: bobsmith1492
I learned a bit from this site about switching supplies: http://www.bcae1.com/trnsfrmr.htm
Note: a friend of mine who works for Rockford and has worked on car amps for forever tells me those are technically NOT "SMPSs." Switching supplies are actually those in computers, which work differently. But anyway, minor details; I still call 'em switchers.

Another good site, which I already linked to: http://sound.westhost.com/index2.html

He's got a bunch of power supply stuff, as well as amplifiers. I built one of them, good for probably 100W+. Good luck man! Let me know if you want any tips, as I'll tell you what I can.

the chopper deal on that site is still a SMPS, just no regulation and such. the heart of the power supply is the same... switch the primary of a transformer fast. regulation etc is optional

i dont care much for SMPS's for audio though because. well. i guess im oldschool

 

[stelleg151]

Originally posted by: Colt45

Originally posted by: bobsmith1492
I learned a bit from this site about switching supplies: http://www.bcae1.com/trnsfrmr.htm
Note: a friend of mine who works for Rockford and has worked on car amps for forever tells me those are technically NOT "SMPSs." Switching supplies are actually those in computers, which work differently. But anyway, minor details; I still call 'em switchers.

Another good site, which I already linked to: http://sound.westhost.com/index2.html

He's got a bunch of power supply stuff, as well as amplifiers. I built one of them, good for probably 100W+. Good luck man! Let me know if you want any tips, as I'll tell you what I can.

the chopper deal on that site is still a SMPS, just no regulation and such. the heart of the power supply is the same... switch the primary of a transformer fast. regulation etc is optional

i dont care much for SMPS's for audio though because. well. i guess im oldschool

Yeah I think I can understand the desire for big meaty power supplies, but I tend to need to move my stuff around pretty often and would like to avoid a large heavy transformer if I can. I will probably make a linear psu once I dont need to move around so much.

@ bob: thanks a lot man, those sites are really helpful, and I finalllly figured out what the center tap is. That answers a few questions .

 

[krotchy]

If you really want to learn about high speed switching power supplies, get Fundamentals of Power Electronics by Bob Erikson and Dragan Macksimovic. Or you can skim over their lecture notes which are very nice PDFs that summarize each chapter in the book.

http://ece.colorado.edu/~ecen5797/notes.html (PWM Switching Converters)
http://ece.colorado.edu/~ecen5807/lectures.html (Modeling and Control)
http://ece.colorado.edu/~ecen5817/schedule.html (Resonant converters)

Heres a link to a copy amazon.com sells, but I must warn you that this version has many typos as its the first edition, and the second edition was released almost 2 years ago. http://www.amazon.com/gp/product/041208.../002-3529841-2705661?v=glance&n=283155

This was my power textbook when I took Power Electronics Theory from Dragan. However, I found out later that many many people in industry use this book as a general reference, and I must say its quite comprehensive in discussing everything from design, to implementation and analysis of all switching power converters (DC/DC,AC/AC, AC/DC, DC/AC and even Resonant converters) If your curious, or really interested, this book is awesome, and the only textbook I can see using frequently at my jobs in the future.

This morning I was actually finishing up my Power Lab final project, taking a 120V AC line input, using a DCM Flyback Transformer and outputting 12V converter with a peak power of around 80W (closed loop compensator regulated). It may go higher, but I dont like things blowing up, plus with a hand-wound transformer, leakage inductance is a serious issue. Building a switching transformer like this is no small feat I must say, and due to what turned out to be snubber/transformer design flaws, we ended up nuking about 6 diode bridges, 10 fuses, 18 mosfets and 15 power diodes. But now I learned my lesson


p.s. Power diodes become shorts when they nuke, and most normal diodes become open circuits when they explode. Power diodes breaking is a good way to blow fuses.