+- 12v supply - shouldn´t use 12v transformer wiki page up

Design Concepts
1

Hi together

I just noticed that on the project pages a 2*12v transformer is used in the schematic for the ±12v supply.

Just like to point out that this can work, but is not recomendet (and not standard).

The 78xx regulators need an input voltage that is at least 3v higher than the output to work correctly.

Normaly this is ok, since the rectification will give us a higher pulsing dc level (something with *sqr(2))

But: - The diodes will drop our voltage by 1,4v

  • Our mains supply is not stable. In germany +5%, -10% are allowed.

As soon as this is the case (propably more often than you think) it´s getting problematic.

Not that it won´t work - you will have noticed… but ripple and noise will increase, the 78xx won´t feel good.

So i suggest the use of 2*15v transformers.

A 14v transformer will work with one way rectification, i talked about it in another post, conrad sells 14v AC “Steckernetzteile” that should be sufficiant!

IF YOU THINK I MADE A MISSTAKE HERE, PLEASE TELL US!

2

(I’m currently extremely busy and therefore haven’t answered to your PM yet)

Some months ago I got a special request to specify +/- 12V instead of +/- 15V in the schematic, since transformers with +/-15V voltage range normaly output +/-18V effective voltage and higher when they are not loaded. And since (or even?) the OP amps don’t load the circuit that much, the 7812/7912 got very hot.

However, I see your point, and I think that a clear documentation is required here. Would you like to start a new Wiki page?

Also a description (and schematic) of the one way rectifier circuit would be nice.

(I never found the time to write down the details for all the variants, and I still think it makes sense that Users maintain such topics)

Best Regards, Thorsten.

3

I´ll do that, glad to contribute. Don´t know how to write a wiki page, but i´ll find out.

Or maybe some links will do as well, since i never studied this or anything, i just collect information on the web…

But on the other hand, what i like about ucapps is that it´s all in one place.

Only thing i´m a bit afraid of is writing something that just isn´t true. I often think i know what i do in electronics, only to find out that the others know better…

So if i´m not sure about something i rather check the places in the web (or public library) that i trust.

Ok, i´ll do my best, show some different alternatives for psus and see that i find an expert for review.

I once blasted an elko and a 7812, that was no fun!

It suprieses me that the 7812/7912 are getting that hot. think they shouldn´t in this case.  Are they getting hot with the 15v transformer only or with the 12v as well?

Can you still touch them? Could you meassure the current on both rails? maybe there is something wrong.

And yes, you´re right. Using a small print transformer and drawing little current is like using a higher voltage transformer - they always output much more. Sometimes even double to what´s printed on them.

Problem could be that when a really small one is used, one that delivers not much more than the power needed, the voltage will drop to the voltage rated.

Same may apply to big ringkern (english: ringcore?) transformers. If someone sees this schematic with the 12v transformer, thinks that this always works, builds a big psu for his whole setup he could get into trouble.

Also it´s important to know that one way rectification is only for small loads (although i don´t know how small)

just some links for those intersted:

http://www.powerstream.com/Wall-mount-FAQ.htm

One good descripion in german is here:

http://www.synrise.de/diy/supply/supply.htm

for more power… good info

http://sound.westhost.com/power-supplies.htm

basics

http://www.st.com/stonline/books/pdf/docs/1707.pdf

4

and here´s what we need to get ±12v out of 15vac (14v will also be enough)

http://thesquarewaveparade.com/images/ac-to-dc-bipolar.gif

5

One important thing i found out:

When transformers with no center tap are used, it´s very important that the load is rather balanced on both rails. Otherwise you get dc flowing through the transformer, it will get hot, too hot and fail.

don´t know if this is a problem here.

I was drawing some schematics and writing a description for the wiki. Could someone measure, under working conditions, how much current the modules that need ±12v draw on each rail?

6

ok, i´ve done a wiki page with all i know on the subject. Hope someone will correct if there are any misstakes.

The page looks messy, i´m not good at computerstuff…

http://www.midibox.org/dokuwiki/doku.php?id=bipolar_12v_psu

7

Thank you very much, it’s very nice to read, and a good basis for writing down new findings if required.

On my homepage, I’ve replaced the PSU links by a link to the Wiki.

I’ve also fixed some minor typos (the short name of capacitors is “cap”) - thats the big advantage of the Wiki btw., errors can be easily corrected, extensions can be easily added by anybody. :slight_smile:

Best Regards, Thorsten.

8

Yes it is! I alsways had writing problems, so i bacame a musician…

And i must say one thing:

This pages have some of the best documented diy projects i´ve ever seen and a great community. I´m happy to contibute a little to this. But i would never have done this if you hadn´t asked if i could start a wiki page. Thats very cool. Good creative comunity spirit here!

If it´s not commercial it has to be that way.

good thing :slight_smile: