Re: +/- 18VDC PS basic questions


"Pooh Bear" <rabbitsfriendsandrelations@xxxxxxxxxxx> wrote in message
news:436AD8B9.5FD46955@xxxxxxxxxxxxxx
>
>
>
> Remember that a common winding has DC continuity. You should be able to
> distinguish those windings that are entirely isolated from each other that
way.
>
> Independent windings way still however be combined to make a centre tapped
> supply. It's often more convenient for the transformer maker to avoid
internal
> connections. For example an 18-0-18 supply can be made from 2 x 0-18
windings
> by series connecting them.
>
>
Okay, I'll take a closer look... I know that there are two windings. I know
that one of them has four leads. If I can identify three of those leads,
call them leads (1-2-3) and two combinations give the same voltage, I can
combine for a center-tapped winding? For example, if 1-2 and 2-3 each give
18VAC, that's what I'm looking for as a usable combination? And the common
lead (2 in this case) would be my center tap, no?
>
>
> > In this regard, I have a 12.6VDC CT x-former (which I thought would not
be
> > enough voltage). I really only need 15VDC, so can I expect ~18VDC out
of my
> > rectifier if I use 12.6VAC in?
>
> You mean the transformer is 6.3-0-6.3 ?
>
> You *could* use it to power some op-amp circuitry at alowier than usual
voltage.
> Depends on your performance criteria.
>
Another light is flickering in the back of my brain. I am reading what you
wrote, above, and I am thinking that if I use the center tap of a
transformer, I would be effectively cutting the rated output voltage in
half? In my example (12.6VDC CT, 6.3-0-6.3) I thought that if I used the
outer leads to power my rectifier, I'd be getting an effective 12.6VAC input
to the rectifier. Is this not the case?

My outputs from the regulator (~18VDC), then use the center tap as their
ground. Does this cut the voltage in half? As you can tell I'm unclear on
exactly what I need to get my +/-15-18VDC out.

Here's what I want:

120VAC --> transformer --> 12VAC --> rectifier --> 18VDC --> 2 sets of
caps --> two 15-18VDC power supplies, one pos, one neg which use the CT lead
for ground (common).

Do I need to start with a 12.6-0-12.6 transformer? This puts 25VAC into my
rectifier, does it not? That's >35VAC out! It will cook any 30V regulator
UNLESS by using the center tap for gorund you are cutting the voltage in
half.

Geez, just when I was starting to think I was getting a handle on this...

Thanks again for all of your help.


.

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