Re: gEDA-user: HPS announce

[Robert Riemer <rob-riemer@shaw.ca>]

True, more transformer voltage = more Pd.  And worst case the 12V regulator is 
dissipating 3.4 watts already.  With out doing the heatsink calculations, the 
spec sheet suggests  a 10C/watt heatsink with an ambient of 35 C and Pd of 
7.5W.


On October 29, 2004 07:24 pm, John Sheahan wrote:
> 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.
> >>
> >>
> >>    
> >>
> 
>