Using unregulated AC wall wart

ezkcdude · 9th June 2006, 03:03 PM

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?

cliffforrest · 9th June 2006, 03:27 PM

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?

ezkcdude · 9th June 2006, 03:46 PM

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.

AndrewT · 9th June 2006, 04:15 PM

Hi,
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.

astouffer · 9th June 2006, 04:16 PM

http://tangentsoft.net/elec/vgrounds.html

ezkcdude · 9th June 2006, 04:27 PM

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.

megajocke · 9th June 2006, 05:48 PM

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.

preiter · 9th June 2006, 06:59 PM

Check out...

http://sound.westhost.com/project05.htm

ezkcdude · 9th June 2006, 07:41 PM

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.

http://sound.westhost.com/project05a.htm

paulb · 9th June 2006, 08:59 PM

There's an error in this schematic; C28 is upside-down.

9th June 2006, 03:03 PM	#1
ezkcdude diyAudio Member Join Date: Dec 2005 Location: Atlanta	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?

9th June 2006, 04:15 PM	#4
AndrewT diyAudio Member Join Date: Jul 2004 Location: Scottish Borders	Hi, 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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9th June 2006, 03:27 PM	#2
cliffforrest diyAudio Member Join Date: Sep 2004 Location: UK	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?

9th June 2006, 03:46 PM	#3
ezkcdude diyAudio Member Join Date: Dec 2005 Location: Atlanta	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.

9th June 2006, 04:16 PM	#5
astouffer diyAudio Member Join Date: Jun 2004 Location: Pittsburgh, crumbling wasteland	http://tangentsoft.net/elec/vgrounds.html

9th June 2006, 04:27 PM	#6
ezkcdude diyAudio Member Join Date: Dec 2005 Location: Atlanta	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.

9th June 2006, 05:48 PM	#7
megajocke diyAudio Member Join Date: Jan 2003 Location: Solna	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.

9th June 2006, 06:59 PM	#8
preiter diyAudio Member Join Date: May 2006	Check out... http://sound.westhost.com/project05.htm

9th June 2006, 07:41 PM	#9
ezkcdude diyAudio Member Join Date: Dec 2005 Location: Atlanta	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. http://sound.westhost.com/project05a.htm

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