[sdiy] Fwd: transformer question . . .

[Bob Weigel]

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
>  
>