[sdiy] MG-1 revisited, LM337 issues

[Ingo Debus]

Am Samstag, 04.06.05 um 23:43 Uhr schrieb Metrophage:

>  I connected my lame
> meter to the 317 side and easily tweaked it to +12v. Then I tried the
> -15 side, and the trimmer does not do anything. This side always
> outputs about 22 volts.

Perhaps the regulator needs some minimum load. Did you try with a load 
or with no load at all?

> I am wondering whether or not there is any special reason to use
> negative regulators for bipolar supplies. Why not just use two 317s? Or
> two 78XX? I suppose there must be reasons since I never see this done.

You can use two positive regulators if you have two separate secondary 
windings (or two separate transformers) and two separate rectifiers. 
Then you can connect the positive output of the second power supply to 
ground and call its "former ground" terminal "-15V". It was done this 
way in the old Elektor Formant (with 723 regulators).

This does not work with a center tapped transformer.

> I am definately going to get a decent meter fairly soon. Sooner than a
> scope.

If I had to choose between a crappy meter plus a scope; or a good meter 
and no scope at all, I'd sure go for the first option. I think even a 
scope but no meter is better than a meter and no scope...

Ingo