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

[harrybissell]

Half-wave supplies have two big drawbacks...

they are not efficient for larger currents (otoh were talking wallwart so
maybe thats 'small' current)

the ripple frequency is 60Hz (as opposed to the FWR 120Hz) so the filter
caps need to be twice as large.

I avoid halfwaves like the plague. No... I avoid BBDs like the plague.
I 'steer clear' of halfwave supplies :^P

H^) harry

jays at aracnet.com wrote:

> Assuming that you are going to regulate the voltages yourself.
>
> I've had projects where we've used wall warts and got both +/- out of them. Use one side as the common/ground. Take the other side and add two diodes doing the half wave rectifier thing. One for the plus side and one for the minus.
>
>   +-|<-  -
>   |
> -+
>   |
>   +->|-  +
>
> -> Ground
>
> Remember if you are using 3 tab voltage regulators that you will need about 3 volts of headroom (above what you need). For 10 volts you want 13 volts at the regulator inputs. Also you'll need to figure in for the half wave rectifiers and the caps droop because of that.
>
> Think I got that right, but check the polarity before you actually do something.
>
> Jay S.
>
> Harrybissell <harrybissell at prodigy.net> wrote :
>
> > 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)
> > > >