Re: gEDA-user: HPS announce

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

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.
> 
>