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 Forum index » DIY Hardware and Software » MusicFromOuterSpace.com designs by Ray Wilson
Wall Wart Power Supply capacitance
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elmegil



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PostPosted: Sat Dec 08, 2012 8:20 pm    Post subject: Wall Wart Power Supply capacitance
Subject description: really? 10mF?
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I'm looking at trying to build a SLMS+ in a cigar box, where I have about an inch of internal clearance. That's going to make life with the pots pretty interesting. Hopefully the jacks will be OK as well, but I'll crash that bridge when I hit it.

I would really like to be able to use a WWS with this rather than mess about with batteries, but of course the standard WWS layout has big ole 3300uF caps standing up proud...way taller than 1".

So two questions:

1) is 10mF (not uF!) REALLY necessary for this purpose? I've seen many other supplies (admittedly, full wave, multi tap supplies) using as little as 1mF (1000uF). Is this because wall supplies are typically switching supplies, so there's more noise to get filtered out?

2) Is there any reason I couldn't use some 2200's I have around instead of 3300's I'd have to order? Of course, if I really do need 10mF, I still don't have enough, but....

I was thinking I ought to be able to get by with 2 x 2200's on each leg of the supply, but then again I'm not Mister Power Supply and don't want to do something that will get me hurt either.

I promise not to hold anyone liable for my own stupidity, just asking if someone knows that this definitively is or is not stupid Very Happy
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Blue Hell
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PostPosted: Sat Dec 08, 2012 9:05 pm    Post subject: Reply with quote  Mark this post and the followings unread

Hard to calculate without details Laughing

The practical way out is .. build it .. if it hums it needs more capacitance.

The engineering approach is .. build it .. add 10% or 15% extra load, if it hums it needs more capacitance.

The scientists approach is .. don't build it Shocked

There are some good tutorials now BTW here on the forum for how to design a power supply, but it needs some detailed knowledge of components used to be able to put it to good use, information you often wont have when building things .. of course you can guesstimate, and with time you will get good at that!

Another practical approach would be to look at the expected load ... the PSU was designed for some load, when you stay way under that "max designed for current" you could down scale the filter capacitors.

Hard to calculate with details too

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elmegil



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PostPosted: Sat Dec 08, 2012 10:15 pm    Post subject: Reply with quote  Mark this post and the followings unread

Not sure if Ray has published current draw specs for the Soundlabs Smile I am sure I haven't seen them if he has.

I think we'll probably try the "build it and see" approach then.
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sompost



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PostPosted: Sun Dec 09, 2012 3:23 am    Post subject: Reply with quote  Mark this post and the followings unread

I believe you can get away with 2x2200uF. If I remember correctly Ray says so himself somewhere on his web site. Note that the high capacitance is not for filtering purposes but to bridge the "gap" that you get in half-wave rectification. Filtering requires less capacitance, as far as I know (which is not much).

Have you considered taking an ordinary off the shelf 9 to 12V DC (sic!) wall wart and then use a voltage converter (ICL7660 or the like) to derive the negative voltage? I used something like that for my GM-Voice and it worked well.

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Uncle Krunkus
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PostPosted: Sun Dec 09, 2012 4:07 am    Post subject: Reply with quote  Mark this post and the followings unread

As Blue Hell said, it comes down to the power draw. For you're GM voice, the split supply converter could work perfectly, because it probably only uses one op-amp on the output stage, which would draw hardly anything from the derived -ve rail.
I would has at a guess that the SL, (in any of it's incarnations) never drew much from the -ve rail, but it would be 4-8 times more than the GM voice.
But the short answer to the story is, that I'd put at least 10000uF on each rail, and see how that goes. The worse that can happen is that you don't filter out enough of the 50/60hz hum, then you just up the overall capacitance by adding extras in the extra space on Ray's PCB. IIRC.

BTW You can add as much as you like, (it will only make the power supply rails more clean) but,.... (something I also learned from Blue Hell) if it's too high, you can blow the input fuse because of the in-rush current, when you turn it on. When a cap is completely discharged, it presents as a short circuit for a very brief time. Once it charges, it's resistance goes up, and,.. of course,.. the current drops.

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prgdeltablues



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PostPosted: Mon Dec 10, 2012 2:59 am    Post subject: Reply with quote  Mark this post and the followings unread

I use Ray's WW power supply for my modular. One per cabinet, powering twelve or so modules each. Not measured the current draw, but it must be several times that of the SLMS. I used 2x4700 uF on each power rail, and everything works fine. So I guess you would be OK with quite a bit less. I am using good quality linear wall-warts though.

As has been said, you could always add more capacitance if you needed to (1000 uFs would fit your height restriction).

Peter
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