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

[Andreas Gaunitz]

Thank you for all the suggestions! It seems the most popular solution 
is an Off the shelf AC transformer combined with a rectifier like Ken 
Stone's: http://www.cgs.synth.net/modules/cgs14_psu.html

Best regards
-Andreas

--------------------------------------------------------
2005 jan 05  kl. 03.44 skrev 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)
>>> > >
>