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Old 9th June 2006, 03:03 PM   #1
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Default Using unregulated AC wall wart

As the topic says, I'm going to use a 16VAC wall wart to power a DAC. The DAC will have onboard regulation using LM317/337. Basically, I'm doing this to avoid dealing directly with transformers and mains power, as a safety issue. After the rectifier, is it enough to use one large electrolytic (4700 uF) to smooth the ripple before the regulators? What would you guys do in my situation?
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Old 9th June 2006, 03:27 PM   #2
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How do you plan to get +/- supplies out of a single AC winding?

What voltages do you want to end up with?

At what currents?

Are they balanced?
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Old 9th June 2006, 03:46 PM   #3
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I thought the +/- is supplied directly from the wall wart coaxial plug. It came with a schematic of the transformer, but I don't have it on me right now. I'll check tonight.

As for the voltages, I want to end up with +/- 12, +5, +3.3 for the various sections. The current draw should be well under 1 A, but I can't say for sure right now.
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Old 9th June 2006, 04:15 PM   #4
AndrewT is offline AndrewT  Scotland
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using about 2mF per A is usually enough for accepatable ripple before the reg.
Your 4m7F should easily cope.

There was a thread recently where a single AC winding was purportedly giving +-DC output. I am not sure the poster knew what he was proposing, worth searching for and try to fathom his schematic.
regards Andrew T.
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Old 9th June 2006, 04:16 PM   #5
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Old 9th June 2006, 04:27 PM   #6
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Now I understand what I proposed doesn't make sense. I just don't want to mess with transformers. Well, I guess I can try the virtual ground, but in that case, I will need a much higher AC input (~30), which would give +/- 15 pre-regulation. The highest (AC) voltage wall wart at mouser is only 24VAC, so that's not going to cut it. I suppose I will have to either a) go for an external PSU that has dual outputs (+/- 15) or b) just get over my fear of high voltage and get a center-tapped transformer.
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Old 9th June 2006, 05:48 PM   #7
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I used two half-wave rectifiers to get + and - voltage out of a wall wart. But then my load was small. Just two op-amps in a Linkwitz Transform circuit. It might not work too well if your current draw is unbalanced. I guess using a full bridge and an active virtual ground would be better in that case.
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Old 9th June 2006, 06:59 PM   #8
preiter is offline preiter  United States
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Old 9th June 2006, 07:41 PM   #9
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Brilliant! That's exactly what I was hoping for. I should be able to use LM317/337 instead of the fixed regs, right? Thanks, mucho.

Edit: Apparently, there is an update to that project (cleverly P05a), specifically, to use adj regs.
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Old 9th June 2006, 08:59 PM   #10
paulb is offline paulb  Canada
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There's an error in this schematic; C28 is upside-down.
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