12v Power Supply Schem using wall wart?

ericcoleridge · Joined: Jan 16, 2007 Posts: 889 Location: NYC

Can someone point me to a ps circuit schematic that uses a standard 12v wall wart? Is there such a thing?

All of the synth ps schematics I've seen on line are for 24VAC transformers-- this is different than a wall wart, right? Or is it just a matter of adding the proper connector.

I've been building synth modules for awhile, but using a Synth.com PS. But I'd like to build some portable modules that can be used with 12V wall warts.

Zodiak · Posted: Mon Jun 04, 2007 1:21 am Post subject:

Sorry but there is no simple answer to this one.

Wallwarts / Socketpods / etc come in an almost infite number of variations so it would be extremely difficult to design a one size fits all solution eg:

- Some provide a fixed voltage, others (such as phone chargers) provide a fixed current.
- Some have a smooth DC out (such as some laptop supplies) others provide a very rough unsmoothed DC (such as nightlights)
- They have dozens of connector types
- The polarity on the plug varies from design to design
- The current available varies
- Some have a thermal cutout that resets if it gets too hot, but most have a thin thermal strip that melts making the supply useless
- Some come apart relatively easily, most are moulded and fall to bits if you force them.
- Most are fixed voltage a few are adjustable.

Once you are aware of that the rest becomes mores straightforward and I have used them on most of my projects, but only those that require a single DC voltage, not +/- 9V or whatever.

The first thing to do is avoid anything that says "charger" they are just bad medicine for this purpose, you need something that provides a straight DC voltage at the very least 3 volts more than your circuit, and 50% more current that you need.

After this you need to add a regulator, if you have been around the block a few times then you will recognise the 78** series regulators, see http://www.fairchildsemi.com/pf/LM/LM7808.html for details (Note Output Voltages of 5, 6, 8, 9, 10, 12, 15, 18, 24 are available)

These are ideal, just a couple of capacitors, maybe some heatsinking and you are away! You will need to fit some kind of socket, avoid temptation and swap it for a jack plug as eventually one night when the moon is full you will plug it in the wrong socket and destroy something, best to stick with their plus and buy a socket to match. I tend to buy three of things like this and keep then in my "bits" drawers for future projects.

There are ways of getting +/- supplies but as I design most of my own stuff now I avoid it and only the rare occasions its needed I build in a complete supply.

zipzap · Posted: Mon Jun 04, 2007 5:26 am Post subject:

It´s funny, this subject is very popular...
I was in a discussion in the forum at ucapps (midicontroll for my synth!),
and asked to do a little wiki page. All i know about small power balanced supplys. Have a look:
http://www.midibox.org/dokuwiki/doku.php?id=bipolar_12v_psu

fonik · Posted: Mon Jun 04, 2007 5:49 am Post subject:

nice wiki, zipzap.

i wonder about the AC wallwart. there is a little contradiction to the following article concerning the power you get from a voltage doubler circuit:
http://sound.westhost.com/project05.htm

i says that using a 15V AC transformer you could get about 20V which would be sufficient for +/-15V, 12V AC transformer sufficient for +/-12V accordingly.
doepfer uses a 9V AC for +/-12V!!?? (i built it that way, which may cause the problems i experience with the midi interface powered from that PSU..., will have to troubleshoot this soon.