[sdiy] Fwd: transformer question . . .

[Harry Bissell Jr]

To add to Bob's points

Small transformers (less than a couple hundred VA)
have much less regulation.  It is not uncommon for a
small transformer to vary 30% in voltage from no-load
to full-load.

This means (to the s-diy'er) that it may be difficult
to predict the actual voltage transformer you need.
If the voltage is too low, you will run out of
headroom
(as Bob said) but it could easily be too HIGH as well,
and generate too much heat.

Using an AC voltage rating equal to the DC voltage you
want (as Bob suggested - if I understood correctly) is
likely to leave you a little on the low side. If you
use BIG caps, it can be made to work.

Commercial supplies (like the Tamura OLD-15 at
MPJA.com) are often a better choice for a one-off. 
Finding +/- 9V supplies is a 'fat-chance' it is an
uncommon voltage.

I converted all my PAiA stuff to +/-15V years ago.

Trying to find just the right transformer (in a small
VA) is a challenge. I usually buy two candidates and
test them for the best fit.

H^) harry

--- Bob Weigel <sounddoctorin at imt.net> wrote:

> VCT is volts center tapped.  So if it says 18vct
> that means the same as 
> 9v-0-9v.   And those are always rms voltages
> remember so you have to 
> select the voltages so that they will filter DC to
> the headroom voltage 
> you need for the regulators.  I'm not familiar with
> the paia circuitry 
> so I leave that to you to decide.  But in general a
> given voltage rms 
> will usually rectify and filter to something close
> to what you need for 
> the headroom on a regulator.  But you have to
> consider the current draw 
> and size of the filters and make sure that during
> the 'non-charging' 
> portions of the waveform, that the voltage on the
> cap won't sag to below 
> the headroom rating of the regulator using Q
> (charge) = C times the 
> change in voltage that is tolerable from what it
> will be when it's 
> charged to the maximum amount.  If you have a
> transformer that will 
> deliver much much more than the current demanded,
> then the capacitor 
> will usually charge to a maximum value of 1.35
> figure times the RMS 
> voltage rating figuring for some loss in the diode
> voltage drop.  So for 
> example if the circuitry is drawing 1A steadily, and
> you have a 1000 uF 
> capacitor and a full wave power supply giving 120hz
> 'bumps' of charge to 
> the cap... figure for a little headroom and just use
> 1/120th of a second 
> times 1 amp (coul/sec) so 1/120th of a coulumb of
> charge.  If you had a 
> 9V regulator and needed 11V of headroom always let's
> say, you won't make 
> it with this scenario.  Because the change in
> voltage over that period 
> will be about 8.3V as you can see.  So you'd either
> have to bump up the 
> voltage quite a bit or get a bigger filter closer to
> 10,000 uF which 
> would assuming a good transformer delivery, drop the
> ripple down below 
> 1V and that would work in that hypothetical case. 
> Anyway, hopefully 
> that will allow you to work through what you need.
> Get the spec sheet on 
> the regulators and see how much headroom they need. 
> -Bob
> 
> rkmoore at memphis.edu wrote:
> 
> >
> >
>
------------------------------------------------------------------------
> >
> > Subject:
> > transformer question . . .
> > From:
> > <rkmoore at memphis.edu>
> > Date:
> > Sun, 17 Jul 2005 10:07:12 -0500
> > To:
> > synth-diy at dropmix.xs4all.nl
> >
> > To:
> > synth-diy at dropmix.xs4all.nl
> >
> >
> >Hello again,
> >
> >I'm looking to replace the power supplies in my
> paia 4700.  Just simple
> >regulated power supplies will be fine
> >(transformer-rectifier-caps-regulator).  I am
> having difficulty
> >selecting a transformer, though.  I will have three
> voltages, +9V, -9V,
> >18V (and a ground, of course).  This seems like it
> would be easy enough
> >with a couple of center tapped transformers, one
> for the plus and minus
> >9, one for the 18.  Transformers of this type seem
> common and cheap from
> >mouser.  
> >
> >The thing that I don't understand is the units that
> they use to describe
> >the output.  They give the output in V.C.T.  What
> is this?  Is it
> >coil-turns?  I'm used to just seeing a turns ratio
> to describe
> >transformer windings and a VA or KVA rating for the
> power handling. 
> >Also, are the output votlages given in the mouser
> catalog peak, rms, or
> >something else?  
> >
> >I won't need much power.  500ma per line will
> probably be more than
> >sufficient.  I'll have to go through and add up the
> surrent draw of my
> >modules again (where did I put that piece of paper
> . . . . )
> >
> >Thanks,
> >
> >Richard Moore
> >  
> >
>