woops ,one too many 0's. Oh yah and it is 50Hz not 60Hz eh!
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. > >> > >> > >> > >> > >
