Isolated outputs power supply ?

[Basstyra]

Hello,

I have problems with my power supply, probably caused by ground loops (as I don't have it any more when I break loops). I currently have a Boss PSA, plugged in a Boss LS-2, and a daisy chain to my Muff, AX-1B and DD-6. I just can't use my muff, because of the hums. Probably because of the addition of high gain and ground loop.

So, I have in mind to build my own power supply, but to avoid any risks, I want isolated outputs.

I found that :
http://www.geofex.com/Article_Folders/Spyder/spyder.htm

Is there any other solution than a transformer for every output ?

[niftydog]

A custom wound single transformer with multiple secondaries.

I have done it, but I wouldn't recommend it for beginners.

Other than that, no suitable solutions that I know of.

[Basstyra]

I consider myself a beginner, but I'm not afraid !!    I want to learn.

You did the wiring manually ?

Considering a given transformer (I probably would take one in a broken appliance), how do you determine the needed secondary wiring (number of wire loop) to product the needed 12volts AC (for example) ?

If you have instructions or/and pictures, it would be welcome !  

[niftydog]

You did the wiring manually?

I completely rewired the secondary coil. It has 11 individual secondary windings, all of which produce around 11-12VAC from memory. Hundreds of turns to count, insane hand-cramps, headaches from trying to be neat with the fiddly, thin wire I used...

Not trying to turn you off here, just telling it how it is!

First up, it must be a suitable transformer. Some points to consider are;
- size. It must accomodate the required secondary windings
- VA rating. It must be capable of supplying the required current
- construction. You must be able to dissasemble and reassemble it without damaging or destroying it

Calculating the turns required is surprisingly easy; It's simply the ratio of turns to voltage. First you must have a fully working transformer, or at least a transformer of which you have measured the secondary voltage accurately.

Take the voltage reading of the secondary while it is without a load. Note it carefully. Now, begin disassembling the transformer. The fun begins...

Once you have the bobbin free of the iron laminations you can begin unwinding the secondary. You must count the windings as you go as this is needed in order to calculate the turns/voltage ratio.

Once you are done, you'll have a voltage and a number of turns. Divide the voltage by the turns you counted and note the answer. Now, divide your required voltage by the answer - the result is the number of turns you require to attain the voltage you desire.

eg; you have a 6.3VAC transformer - you count 100 turns on the secondary. You require 12VAC exactly.

6.3 / 100 = 0.063
now...
12 / 0.063 = 190.5

You need 190.5 turns (yes, you can have a half turn, but it won't make a huge difference so just round it up) to attain 12VAC.

This is by no means exhaustive, but there are other resources out there that might help. Choose your wire carefully, ensure it can handle the current you wish to draw from each winding.

E-I lamination transformers are the easiest to dissasemble. Sadly they also tend to be big and heavy. I used one out of an old TV. It was the heater transformer for the tubes - it was pretty beefy.

I've also considered using ones out of lead-acid battery chargers. Really old printers sometimes have useful ones in them... look for old gear as new gear tends to have welded laminations or toroidal transformers.

Good luck!  :twisted:

[niftydog]

oh yeah... almost forgot the obligatory warning;

DON'T MUCK WITH MAINS POWER UNLESS YOU ARE AWARE OF THE DANGERS!!!

Mains power can be lethal and you rarely get a warning or a second chance. Use isolation transformers or RCD protection when working on mains powered equipment.

[R.G.]

If you'll go read the Spyder article, it gives detailed directions for how to rewind a flat pack transformer to do the job.

I recommend a flat pack transformer, as the winding job gets subdivided into simpler steps, and the un-stacking of the laminations is about the same difficulty as un-stacking an E-I transformer.

The choice of a flat pack rather than an E-I transformer for the article was not random.

If you use an E-I transformer pick one with a segmented nylon bobbin, like the flat pack. Do NOT pick one with a single unsegmented bobin where you have to mess with the primary. It's not safe for an inexperienced person to do that.

When you get your "victim" transformer, before you start tearing it down, hook it up to AC and measure the AC line volts and the secondary volts out with no load. Then start tearing it down, unwinding the secondary. Unwind rather than cut off the secondary windings, and count them as you go. When you get done, you know the voltage the secondary makes and the number of turns needed to do that. You can then compute the number of turns per volt which the existing primary gives you. You can them simply multiply the turns per volt by the number of volts you want in each secondary and have the number of turns you need for each secondary.

Do NOT mess with the primary. Leave it alone, under its protective tape windings.

When you get done, test your work with a lower voltage drive on the primary. Power the "victim's" primary from, say, 12V from another transformer and measure the secondaries. They should be the same ratio less voltage as the primary. This will let you find any wiring bugs before you burn it up by connecting it to the AC line.

If you made a 120V to 11V secondary, you should for instance see 1.1Vac on the secondaries if you power the primaries from 12Vac.

[Basstyra]

Ok, thanks...  

I don't know when I would have the time (and courage) to do this, but well... Maybe during those coming vacations...  

Or I'll choose the separate transformer solution. It should be easy to pick up some 12v transformer, for instance in broken cellular charger (those charger are so fragile...   ). COuld be too big, though.

[R.G.]

I don't know where you are on the planet, but if you're in the USA, consider the transformers I recommended from Mouser. They're cheap (about US$2.50) and so even six or eight of them are only slightly more expensive than buying a single bigger transformer.

I think Farnell may have similar transformers in Europe.

And please take the warnings about AC line voltage seriously. He's right - you may not get a warning. If you touch the wrong thing, you might just be dead the first time.

[Basstyra]

I'm in France. I'll look at Farnell, thanks !

Don't worry, I know about AC lines.  :wink:  But it's smart from you to warn about it !