Re: gEDA-user: HPS announce

[John Sheahan <jrsheahan@optushome.com.au>]

47,000uf  ? thats huge.
lets think. 10ms period (50hz full wave)
1A (for this reg)
i = c dv/dt => 
c = i * dt / dv
  = ,01/2
  = 5000uf

I suspect you have a kind of a lot of significant figures 
for the accuracy of supply voltage too :)

an extra couple of volts  from the transformer would help the cap size
and hurt power.  Thats linear supplies  for you.


john




Robert Riemer wrote:

> After a few quick calculations regarding the power supply filter capacitance, 
> I think you should spec a different transformer.  With the transformer you 
> have specified:  Vfl = 16.968Vp, I used this for my calculations even though 
> you are not using full rated current draw for the transformer.  Vout = 12.0V.  
> According to National Semiconductor the voltage accros the regulator could be 
> 2.5V worse case.  I used Vf = 0.7V  for the diode.  Subtract all the drops 
> from the transformer voltage, this leaves you with the maxinum ripple voltage 
> accros the capacitor.  dV = 1.768V.  Assuming a full load current the filter 
> capacitor for the 12 volt supply should be >47000 mfd.  The 5V supply will 
> require an even larger capacitor.
>
> I may be under estimating the transformer ratings, and over estimating the 
> dropout voltage, but is that what a good designer should do?
>
>
>
>
>
> On October 29, 2004 06:16 pm, Robert Riemer wrote:
>  
> > Some technical tips:
> >
> > 1)  Use fewer 500 mfd capacitors.  There may be too much capacitance.  If 
> >    
> not, 
>  
> > use fewer larger value capacitors.  Probabaly the same price and size 
> > approx..  The value of capacitance depends on the transformer voltage, 
> >    
> amount 
>  
> > of allowable output ripple, and current drawn from the supply.  If the 
> > capacitance is too large, the regulator could dissapate too much power.  If 
> > it is too small the output ripple may be large, our the regulator can drop 
> > out of regulation.
> >
> > 2)  The transformer is potentially unevenly loaded.  There is a split 12v 
> > supply but not 5v.
> >
> > 3) The terminal labled +Ureg (unregulated) should be as variable, not 
> > unregulated.
> >
> >
> >