[sdiy] Newbie warning! What is negative DC power?

[harrybissell]

I don't know if it is still in their app notes... but Signal Transformer
(signaltransformer.com) used to have some design notes about supply
design.

You are )probably) looking for

Full wave center tapped   (FWCT)   configuration.

It uses a center tapped transformer.   This is hard to find in a wallwart.

maybe google on FWCT and you will find info.  Its not hard, well known, you
could do this.
I'd buy a PowerOne supply to start, though.

The BIG problem with power supply design is choosing exactly the right
transformer.
Small transformers are notorious for very bad regulation... at low current
out their voltage
is too high, at full current its too low.  Your regulator must handle a lot
of voltage drop
at low current, and too little input at high current.

If you set up so it will work at low line voltages as well... there can be a
small window where
proper operation is achieved.   If you chose the wrong transformer, you are
hosed !

I usually buy a range of candidate transformers... maybe a 28VCT, a 30VCT, a
35VCT etc...
(assume I'm making a +/- 15VDC).   Then I choose the right one by actual
testing, and buy more
of that one.  Hard to do if someone ELSE is not paying for it :^P

H^) harry


Andreas Gaunitz wrote:

> Thanks for the info!
>
> However, if I use the +- supply during development then I will have
> problems when I put it on PCB - I will need to create a neg current by
> then! And I will (of course) connect an audio cable sooner or later, so
> I can't use a positive ground to get minus DC either.
>
> It's just strange that this is a problem... I looked for DC wall warts
> and PCB mounted transformer components that can produce -10 : gnd : +10
> volts and I had a really hard time!
>
> So how do you people make a neg DC supply on a PCB?
>
> regards
> -Andreas
>
> ---------------------------------------------------------
> 2005 jan 04  kl. 15.12 skrev Paul Perry:
>
> > From: "Andreas Gaunitz" <andreas.gaunitz at konstfack.se>
> >
> >> I'm planning to buy a DC power supply, with limited current and
> >> digital
> >> readout etc, for my upcoming projects.
> >>
> >> Now, I am choosing between two fairly cheap ones: One that has +-15
> >> volts, and the other that has 0-30 v. Now, I always thought that a
> >> negative DC power was a breeze to produce: just set the ground to a
> >> positive voltage value. Did I dream this?
> >>
> > ........you can produce a neg voltage by putting the + at earth.. but
> > only
> > if the
> > power supply and the circuit you are driving wiht it don't already
> > share the
> > earth!!!!!!!
> >
> >> I checked out the 7805/7809 component range and it seems the ones
> >> producing neg current have to be supplied with neg current - duh. So
> >> how to get the neg current in the first place?
> >
> > If you have the +-15v supply, then you have a ground (earth)
> > connection,
> > which goes to the ground connection of the 7805, and ground of your
> > circuit.
> >
> > The -15 goes to the input of the 7805.
> > Then you are in business.
> >
> > WARNING: the pins of the 7805 and 7905 are different!!!!!!!!
> >
> > WARNING: 7805 may want a load to function correctly (a 10K resistor
> > from
> > output
> > to ground will do).
> >
> > It is worth readign the National Semiconductor data sheets (on the web,
> > just search for LM7805 data)
> >>
> >> I'm getting more and more confused the more I read...
> >
> > ..that is a good sign, because at least it means you are starting to
> > know
> > what you don't
> > know. Unless you know that, you can't learn!!
> >
> > best, paul perry (self-taught electronic manufacturer)
> >