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Old 10th April 2004, 03:10 AM   #1
zobsky is offline zobsky  India
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Default simple power supply question

given that i have a transformer with dual 14V AC secondaries, .. i want to obtain +/- 15 V DC , and also employ some kind of regulation..

question, .. if i use the PS schematic here (http://sound.westhost.com/project05.htm), can i safely ground the center tap.

else, . . would it make more sense to use a circuit such as http://www.elfrad.net/Pics/pwrsup.gif


thanks
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Old 10th April 2004, 03:22 AM   #2
amb is offline amb  United States
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Default Re: simple power supply question

Since you have a transformer with two secondaries, it's better to use the second circuit from the elfrad.com site. The first circuit is really only "half-wave" because it employs only two rectifier diodes, and you will have more ripple before the regulator. It should be used only when you have a single-primary transformer without a center tap.

-Ti
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Old 10th April 2004, 03:32 AM   #3
HDTVman is offline HDTVman  United States
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Default Re: simple power supply question

Quote:
Originally posted by zobsky


http://www.elfrad.net/Pics/pwrsup.gif


thanks
Just do this one. Use 15 volt regulators of course and it will work just fine for a low current power supply up to about .5 amps.

Later BZ
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Old 10th April 2004, 03:35 AM   #4
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secondaries on your xformer need to be more than 15 to achieve a stable 15 v from a 15v vreg..... and you'll lose some in your filter
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Old 10th April 2004, 03:37 AM   #5
zobsky is offline zobsky  India
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thanks for the posts so far.

Quote:
Originally posted by koolscooby
secondaries on your xformer need to be more than 15 to achieve a stable 15 v from a 15v vreg..... and you'll lose some in your filter



good point, .. i was wondering about that , .. but wouldn't 14V AC rectified yield more than 14V DC to give me some headroom??


OTOH, ..this is for a line stage suply , .. and my opamps can also handle 12V, .. so I could just plug in the appropriate 7812/7912 regulators instead.
thanks
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Old 10th April 2004, 03:42 AM   #6
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BR1 (Bridge Rectifier) can drop up from .7 to 1.2 volts... so watch that

make sure you have a stable power supply (like 16 volts for a LM7812 etc...) to get a clean power out of the reg.
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Old 10th April 2004, 03:44 AM   #7
zobsky is offline zobsky  India
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Quote:
Originally posted by koolscooby
BR1 (Bridge Rectifier) can drop up from .7 to 1.2 volts... so watch that

make sure you have a stable power supply (like 16 volts for a LM7812 etc...) to get a clean power out of the reg.


thanks, .. that makes sense
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Old 10th April 2004, 03:49 AM   #8
HDTVman is offline HDTVman  United States
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The 14 vac secondarys will give you 14 * 1.414 = 19.8 vdc at no load into the regulator. The regulator will need about 3 volts or so to work. Ya, it's a little close. If you can work at 12 vdc I'd do it. That will be OK.

It realy depends on how much current you need. If only a few milliamps are needed then 15 volts might be OK. Besides the chip regulators are cheep and easy to change if you want to try it.

Later BZ
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Old 10th April 2004, 03:50 AM   #9
amb is offline amb  United States
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Quote:
Originally posted by koolscooby
BR1 (Bridge Rectifier) can drop up from .7 to 1.2 volts... so watch that

make sure you have a stable power supply (like 16 volts for a LM7812 etc...) to get a clean power out of the reg.
Assuming a 14-0-14 transformer secondary, each regulator is going to see in excess of 18V DC at the input (with diode drops taken into account), so there is adequete "headroom" for a 15V regulator, and certainly more than enough for 12V. In my experience most transformers put out slightly more voltage than spec'ed without load, so that adds even more to the margins.

For example, the Telema/Amveco toroidal transformer I used for my recent headphone amp project is spec'ed for 15-0-15 output at 117V input, but
I measured over 22.5V DC after the rectifier with only 115V AC.

-Ti
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Old 10th April 2004, 04:20 AM   #10
zobsky is offline zobsky  India
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FYI, .. application notes

this rectifier module built to accept secondary input from a variety of trannys, perhaps via a molex or XLR connector.

sometime in the near or distant future, .. i'll be using the setup to try out a bunch of projects such as
1. buffered (BUF634) preamp
2. buffered (BUF634) preamp driving a DRV-134 line driver to split phase on my tube amp
3. BUF634 driving edcor line trannys to split phase on my push pull tube amp
4. maybe a gainclone, .. although those may require different values of filter caps for optimum performance.

right now, all i have in my parts bins are a huge EI 15V dual secondary and a cute dual 14V x 400mA EI transformer (which is what I wanted to use for a line amp PS)

I'll play around and hopefully something will work out
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