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Bipolar +/- 12v PSU, what caps?


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Hi

 

I was looking for a straight up BOM for the +/- 12v PSU for the AOUT board, but i can't seem to find any, so i am trying to gather the components for myself based on the center-tap transformer mode http://www.midibox.org/dokuwiki/doku.php?id=bipolar_12v_psu#center_tap_transformer

 

I was thinking this setup:

Transformer: http://www.reichelt.de/Transformers-PCB-5-0-8-0-VA/EI-42-14-8-215/3/index.html?&ACTION=3&LA=5&ARTICLE=27411&GROUPID=3316&artnr=EI+42%2F14%2C8+215 

Rectifier: http://www.reichelt.de/Rectifiers/B40C800DIP/3/index.html?&ACTION=3&LA=5&ARTICLE=4639&GROUPID=2998&artnr=B40C800DIP

7812 and 7912 (pos/neg 12v)

 

But the caps i am a little confused about - in the guide it says a general rule is 470 uF per 0.1 ampere - the transformer will provide 2x167mA - Meaning something lik 2x470uF on both sides, so around 1000 uF caps - and here i need 1 electrolytic and 1 cheramic for pos and neg side before and after the 7812/7912 - correct? To sum it up:

4 x http://www.reichelt.de/Electrolyte-Capacitors-105-C-1000-5000h/RAD-FC-1-200-16/3/index.html?&ACTION=3&LA=2&ARTICLE=84701&GROUPID=4000&artnr=RAD+FC+1.200%2F16

 

and then 4 x cheramic/film non-polarity? I can't find any on reichelt with the right specs, they are all in pF which seems like to little (http://www.reichelt.de/Ceramic-Capacitors/2/index.html?&ACTION=2&LA=2&PROFID=256&GROUPID=3159&SHOW=1).. 

 

Hope it makes sense - i am just a little confused with the cap values actually? 

 

Thanks! 

 

:smile:

Edited by istel
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It definitely depends on what you are planning to drive with this PSU. MFOS' design is not a bad choice, especially if you want to go for a small footprint and don't need a lot of power. However, it's a bit like sucking every last mV out of the wall wart to produce +/-12V, hence the huge cap bank and the feedback diods.

Also make sure you have an old school transformer in your wall wart and not a switched PSU, or directly go for a transformer. This design is suitable for basic functionalities without critical VCOs or circuits which need an absolute rock solid voltage source or a lot of power.

 

If you can afford the cost, space and weight, I would go for a +/-15V center tapped transformer, as this will leave you with enough head room for mains noise, drops and ripple. You can also go more for the 2200uF-3300uF range than the 9900uF tank bank, which cuts cost on the other side. However, that's the opinion of an analog synth builder who needs a stable +/-12V source with about 1.2A. 

 

You're talking about 167mA transformers. One thing to consider is, that the AOUT_NG alone sucks about 40mA without the core, any other module, display or subsequent circuits attached. So keep power consumption in mind.

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^

Disagree 100%, an "old school" linear supply will be 10x bigger, 5x more expensive, and will get much hotter than a modern decent switching one which will also be 5x more efficient, just search the forums and see how many people here are using Meanwell PSUs without any issues. There is a reason that no one uses big linear supplies any more.  For a $40 Meanwell you get a 5A 5V rail with +/-12V.  How much would just a linear transformer cost to do 5A? Buchla Music Easels use SMPS for a reason.

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Disagree 100%, an "old school" linear supply will be 10x bigger, 5x more expensive, and will get much hotter than a modern decent switching one which will also be 5x more efficient, just search the forums and see how many people here are using Meanwell PSUs without any issues. There is a reason that no one uses big linear supplies any more.  For a $40 Meanwell you get a 5A 5V rail with +/-12V.  How much would just a linear transformer cost to do 5A? Buchla Music Easels use SMPS for a reason.

 

Sorry, I think this is a misunderstanding. I didn't want to say that a switching supply won't be a good choice at all. What I mean is, that a switching wall wart will likely cause troubles with the MFOS design, as the caps as well as the backward diodes are specifically designed for old school transformers with a 50Hz cycle. I just mentioned this, as nowadays it is not always clear what you get if you have a wall wart. However, it is very unlikely that there are many switching wall warts on the market which produce AC.

 

But I think this input about using a switching PSU instead of a conventional one has definitely to be considered in the first hand. 

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Didn't see all the replys on this topic - i was planning to build this just for the AOUT, but maybe i could switch out the transformer to get more current and tap out 5v DC for the core also? Only having one power supply would be nice :) 

 

I ordered all the stuff, so now i am just going for this, but next time i'll check out the Meanwell.

 

Thanks!

 

Isak

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