9v Isolated Powersupply - Parts?

[Gnome]

Ok, I want to biuld a 9v powersupply like the Pedal Power II. Not to complex, a big trafo which goes from 220 to 9v, then a rectifier and an isolation trafo per output. So I have 7 separate, isolated 9v outputs to drive pedals etc.

Anyway, I have searched Elfa (local), Mouser, Smallbearelec, and I have still not found a 1:1 10w isolation transformer. It does not have to be exactly 10w, 5w is a minimum. So, if anyone knows where I can find these, I would be a happy man.

Cheers, Emil

[SoundTech]

Umm, you're going to have a hard time finding a 1:1 DC isolation transformer.  I think you should head over to GEO and look at the The Spyder.  This is what you're trying to build:  http://www.geofex.com/Article_Folders/Spyder/spyder.htm

[Jason Stout]

Yes, check the spyder at GEO.

[R.G.]

The Spyder and its ilk are the simplest way.

There is another. You can use a switching circuit to make two out-of-phase square waves at higher-than-audio frequencies, then couple those through capacitors and rectify/filter them to 9V. It does break the ground loop. Using CMOS CD4049's can do this, but the primo way seems to be with CMOS gate drivers for MOSFETs. I have a couple of the TI gate driver chips to try this with as soon as I get down to that layer on the workbench.

[dosmun]

http://www.generalguitargadgets.com/v2/index.php?option=displaypage&Itemid=107&op=page&SubMenu=

[aaronkessman]

Just built the Spyder (Thanks RG!) and it's great. It was a little annoying to build since the work is pretty repetitive, but it was easy. Highly recommend.

[R.G.]

One other method. I messed around for quite a while with the idea of using audio isolation transformers as power isolation transformers. Mouser sells quite a selection of audio transformers, and you could easily take 9V, chop it with a couple of MOSFET switch transistors at an audio rate, and then rectify/filter/regulate the output.

Like all impressive sounding ointments, there are some flies in this one too. First, you have to have enough power through the transformer. Mouser sells audio devices up to 400mW, and that is undoubtedly 400mW at 300Hz, which is the lowest design frequency for these communications transformers. 400mW at 12V (so you have some voltage headroom to regulate down) is 33.3ma, which is enough for most pedals, but won't power some current hog pedals.

You can get around that by driving it at higher frequencies. Power handling is proportional to frequency in transformers, so if you need 300ma, you can just run it at 3kHz. Of course, 3kHz is a really, really irritating scream at one of the sensitive points in the human ear's hearing spectrum. You'd have to do a really good job of suppressing feedthrough and noise from the power supply.

Hey! We can use 30kHz, way over human hearing! That would make 3A!
Not quite. Those transformers have a hard time getting over 20kHz. They will, but barely. And iron cores lose a lot of power at those frequencies. And leakage inductance gets to be a huge problem.

The final straw that made me turn away from the audio-transformer isolator is that Mouser charges about $2.00 for the audio isolators. They also sell the 12V output transformer I recommend in the Spyder article for $2.20 in 10's.  So ... 10% more cost per transformer versus no circuitry to make and debug... the easier to make line frequency stuff won on a practicality basis.

The audio power isolator is a usable trick for some places, and could be made to work here. I just decided that the average Joe making a Spyder power supply would be better off using a slew of little line transformers than a slew of circuits to be debugged.