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 Forum index » DIY Hardware and Software
Difference between wall wart trafo
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capicoso



Joined: Nov 19, 2012
Posts: 112
Location: Argentina

PostPosted: Tue Jan 29, 2013 10:09 pm    Post subject: Difference between wall wart trafo
Subject description: and trafo inside cabinet
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Hello. I'm going to power my modular with a mfos wall wart +- supply with 7815 7915 regulators. I've been told that they aren't very good. Until 500mA everything is fine, then the performance is degraded...
But my first cabinet will consume about 300mA so i'm sticking with that supply.
Thinking about a bigger cabinet or replace this supply, i have to buy a 25.6vac trafo, for the 317 power supply. SO i've seen everyone has their trafo inside their cabinet and just use a simple cable to connect it. Is there any difference by using a wall wart or a plain trafo inside the cabinet? Apart from making the cabinet heavier Razz
thank you
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prgdeltablues



Joined: Sep 25, 2006
Posts: 162
Location: UK
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PostPosted: Wed Jan 30, 2013 4:56 am    Post subject: Reply with quote  Mark this post and the followings unread

I've stuck with 1A linear AC wall-warts, with a MFOS power supply circuit in each cabinet. but using 317/337, as these are better spec'd than 78xx's. Sure it means a row of wall-warts, but they're not that expensive, and with 10-12 modules per cabinet I've got plenty of power headroom. Also means I can turn each cabinet on in sequence, so no real need to worry about inrush current, etc.

Just my 2p

I can't think of any reason you couldn't build a higher power traffo power supply as a separate unit, with +/- DC lines into each cabinet. Of course, the box would need to be well ventilated and well insulated electrically.


Peter
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Osal



Joined: Aug 16, 2011
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PostPosted: Wed Jan 30, 2013 9:40 am    Post subject: Re: Difference between wall wart trafo
Subject description: and trafo inside cabinet
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capicoso wrote:
Is there any difference by using a wall wart or a plain trafo inside the cabinet? Apart from making the cabinet heavier Razz
thank you

Hola capicoso,
I wouldn't use these wall wart transformers. A properly transformer inside the case would do a better job. The power supply is a very important part of the synthesizer.
This could be interesting for you: http://electro-music.com/forum/topic-51694.html

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prgdeltablues



Joined: Sep 25, 2006
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PostPosted: Wed Jan 30, 2013 10:19 am    Post subject: Reply with quote  Mark this post and the followings unread

Osal wrote
Quote:
I wouldn't use these wall wart transformers. A properly transformer inside the case would do a better job. The power supply is a very important part of the synthesizer.


Could you say why not? I'm not talking about switch-mode, but linear i.e. transformer-based, with a built-in fuse, AC-AC (so 230V to 12V or 15V unregulated for the UK). For any-one who doesn't feel comfortable working with the mains supply, they seem a good alternative. I've not encountered any problems with stability or noise: the only down-side has been the one-shot fuse, meaning when I accidentally shorted one out on the bench, it died.

Peter
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capicoso



Joined: Nov 19, 2012
Posts: 112
Location: Argentina

PostPosted: Wed Jan 30, 2013 11:30 am    Post subject: Reply with quote  Mark this post and the followings unread

Hey, thanks for the info guys.

At the beginning i opted for the wall wart because i THOUGHT they were cheaper, but yesterday checking in e-store from my country, i saw some guys that make the transformers on demand. And way cheaper. A 15VAC wallwart cost me 105AR$, and a plain transfomer is about 60AR$... almost half yeah. I guess i'll end testing both( since i'll have both types). But yeah, watching those huge trafos inside other people's modulars looked better.

I'll read that post
thanks both!
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bubzy



Joined: Oct 27, 2010
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PostPosted: Wed Jan 30, 2013 1:12 pm    Post subject: Reply with quote  Mark this post and the followings unread

im no expert on power supplies, but i believe that to get the rgulators working nicely, you have to supply about 3v more than they deliver, so 15v regulator works best with 18v supply.

theres probably some maths to go with ti, but im just a dummy Smile

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Agent86



Joined: Jan 17, 2013
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Location: The Cone of Silence

PostPosted: Wed Jan 30, 2013 1:15 pm    Post subject: Reply with quote  Mark this post and the followings unread

Yes, 3V is a good rule of thumb for the 78xx and 79xx series regulators. You can get low dropout regulators that operate with a smaller voltage differential though...
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Osal



Joined: Aug 16, 2011
Posts: 145
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PostPosted: Wed Jan 30, 2013 2:01 pm    Post subject: Reply with quote  Mark this post and the followings unread

prgdeltablues wrote:

Could you say why not?


Peter,

Looking at the schematics, there are two half-wave rectifiers, one for each rail. This means that the current that the transformer provides must feed the two rails. I bread-boarded it (toroidal 18VAC secondary and 15V regulators) and measured 1A flowing in the secondary when in the output of the power supply it is flowing 260mA.

Maybe it is useful for a small portable device. But for a common power supply it is advisable a dual split or dual full wave rectifier configurations with a properly transformer.

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