Wall->Bi-polar 12V... What transformers/circuit?

[ExpAnonColin]

So say I want to take a direct plug from the wall, bring it to my pedal, and use an internal transformer to make a +12V and a -12V.  Does anyone have any good schems/ideas for this?  I'm not very familiar with transformers in this application, or the AC/DC conversion in general.

-Colin

[ExpAnonColin]

(Edited per comments of puretube)

[ExpAnonColin]

With this circuit: http://www.epanorama.net/counter.php?url=http://www.psyber.com/~dibsed/CIRCATS/opamp-ps.pdf

Could I just change the voltage regulators to 7812's and get +/-12V?

-Colin

[Michael Allen]

yeah you can use both a + and - 12 volt regulator. Connect the grounds of both regulators to the diodes, and the output of the regulators will give you + and - voltage respectively.

[Peter Snowberg]

With this circuit: http://www.epanorama.net/counter.php?url=http://www.psyber.com/~dibsed/CIRCATS/opamp-ps.pdf

Could I just change the voltage regulators to 7812's and get +/-12V?

-Colin

Yes you could. That's a really well developed classic 78xx/79xx supply. Most of the time the additional diodes are left off and only half the caps are used. Your transformer would need to be 24VCT at the least to give your regulators excess volts to work from to get a +/-12V supply. Somebody please double check my math...

24VCT @ 2A or 48VA

Into a bridge rectifier (*1.4141414) that  = ~34VDC - Diode loss = ~32.8V with 30V required for good regulation. Good match.

If you want more than one amp, you have to start working with TO-3 packaged devices or you need pass transistors. it gets cumbersome quickly so I would recommend looking for high current regulators rather than using big pass transistors.

Also if you're using more thean one amp you'll probaly want to switch to a bridge rectifier module with a higher current rating rather than use individual diodes.

Take care,
-Peter

[puretube]

THIS CIRCUIT drawn by a certain roger maxwell WAS A SURE CAP-BLOWER! (from ...edu.wash... nota bene...)

the 2 leftmost diodes apply the full positive voltage to the negative
pole of the lower left electrolytic, whilst it`s pos. pole is grounded.
After the cap exploded, the LED will hardly light up, due to the (too)
big 470k series-resistor, and the small differential voltage it is hooked
to. (12V+ rippled, and 12V+ smoothed).
Anyway, it will probably not light up at all, since the low internal
impedance of the exploding cap (why doesn`t the english language
have a nice short name for these things, like the German "elco")
will have shot the diodes, too, and maybe even set the "trafo"
(another nice short name), on fire...

And, anyway: you`ll probably never be able to see any LED light up anymore, unless you used special safety-glasses to watch the
spectaculum...


I`m not so sure, who`s insurance will turn up for the damage...

[ExpAnonColin]

Peter-Thanks.  I think I'll order some parts this weekend.

puretube-Makes sense.  I edited mine, now you edit yours

-Colin

[puretube]

 hey Colin: I didn`t wanna criticise you in any way;
just wanted to make sure, nobody would do a quick breadbording of
that faulty schem that was depicted...
(a 2200uF/50C can be a chemical weapon)

[RDV]

I've often wondered about the old exploding cap trick. I haven't had a cap explode, but I did have a MPSA18 in a wah-wah go off like a firecracker when I plugged a cheap unregulated wrong-polarity wall-wart into a Dunlop Jimi Hendrix Fuzz-Wah. I'd just bought it(Fuzz-Wah) off eBay a couple of days before and was pretty upset. I replaced the transistor with a Radio Shack cheapie and it fired right up and worked fine! It's strange to use a transistor as a reverse-polarity protection diode(seems like a diode would be cheaper), but that's certainly how they did it.

Of course I hated the Fuzz-Wah and built a Vox circuit to replace it almost immediately. I got the fully functional Fuzz-Wah guts if anybody wants to do a trade of some sort. :wink:

Regards

RDV

[ExpAnonColin]

 hey Colin: I didn`t wanna criticise you in any way;
just wanted to make sure, nobody would do a quick breadbording of
that faulty schem that was depicted...
(a 2200uF/50C can be a chemical weapon)

No offense taken

So does that mean that Bush is going to send troops into all of our houses in search of our 2200uF stockpiles?

-Colin

[puretube]

no, he will search the houses for "biological weapons":
i.e.: the DIYers that mis-polarise diodes for cap-blowing purposes...  :wink:

[R.G.]

Colin -

In messing with effects, a good reading of the info at GEO is a really useful start. I certainly don't think it's all there, but about half the time I spend typing here is to direct people to look for the answer that's already written down over at GEO.

What you're looking for is hidden under the topic "Power Supply Basics" where it talks about using a single AC wall wart to make two half wave rectified supplies which can then be regulated.

[ExpAnonColin]

Thanks RG for the recommendation,

All the information asked for in this thread is not only available at Geofex, but also available from many sources from google, as the power supply cirucits I had posted suggest (all were google searches).

I just wanted to see who else had designed a working bipolar supply from the wall, as there are many many articles and circuits on this topic on the net.

One reason I rarely ask questions is because many of the answers are out there on Google-I only really ask when I'm dealing with something dangerous or almost completely unfamiliar-in this case, the wall and popping caps, and transformers, which I have a decent understanding of but have never used.

Geofex is certainly a good source of info, but so are the other sources whose URLs I have posted in this thread, those are just the ones that came up on google first.

I was surprised to see that you think I am currently looking for a "good start" and "messing around with effects"-is that the impression I give?

-Colin

[mattv]

I think we're all here to mess with effects.

Geofex is a 'useful start' that many people ignore or find too late. I spend much time there reading things I know I've read before, but had forgotten, and I'll do that many more times in the coming years. A reminder that Geo exists is never a bad thing.

[Peter Snowberg]

I hope we're all here to mess around with effects.

Electronics is a broad field with lots of specialties. Linear power supplies is one of those "special" areas. Designing an RF mixer won't really help you with designing a linear supply and vice versa. If you don't have experience with a given area, then a "good start" is a great thing to have somebody hand you. To have it be R.G. Keen is even better still. That's the sort of thing you should be seeking out!

When it comes to RF, I might as well be a plumber. If I do ever get my feet wetter than a crystal set, I hope somebody can give me a good start.

Take care,
-Peter

PS: After this it becomes your duty to pass along a good start to somebody else down the road. It might be 25 years from now. You'll find it's really rewarding to watch the impact it can have over the course of years.