At 09:35 AM 11/10/99 +1100, Dennis wrote: > >Since C V = i t, and the PIC requires power up within 10 ms, the maximum > >capacitor on the output of, say, the 7805 regulator would be > > > > C = (1 A) (0.01 s)/(5 V) = 2000 uF > > > >With a low-current device, this could be reduced even further to 200 uF. > > > >Been a long time, I would guess that Paul or Sean will correct me, but I >think that the formulee is wrong. >Q = IT >=> Q = 1(Assume max current) * 0.01 = 0.01 Joules I=amperes = coulombs/sec; t = sec; Q = coulombs So Q = I t gives coulombs, not joules. >EK = 1/2C(V)2 => 0.01 = 1/2C*25 = 800uF This would be correct if Q was in joules. However C V = Q in coulombs. >You have made suumptions here in that the maximum current from the REG is >1A, also if the circuit peaks at 1A for a period of 10mS, the coapacitor >would be effectily discharged (2000uF), and the regulator would make up the >other bit to hold the votage up, and rough rule of thumb the output will be >down by 63% och! This is due to the CAP taking the available current from >the reg to hold const voltage on its terminals (OK so not quite correct but >should give the purpose) This does not take into account other influences >like ESR etc. I was talking about the power-up time. ESR would only delay the power-up further, since the regulator would not supply the capacitor at full tilt. > >Larger capacitors provide no advantage at low frequency, since the output > >impedance of the regulator is very low (e.g., 1 mOhm). At high frequencies, > >distributed bypassing works better in any event. > >Only if the CAPS are suitable for high frequency stuff. There is no need to >use a large cap (Say 1uF) that has a high impedance at RF. In this case a >small cap say 100pF ceramic would be better. This might be true at very high frequencies, where mica caps are required, but my belief is that it's not true for frequencies ~ 40 MHz and below. The bigger the capacitor, the lower the impedance. (E.g., 10 uF vs. 0.1 uF) ================================================================ Robert A. LaBudde, PhD, PAS, Dpl. ACAFS e-mail: ral@lcfltd.com Least Cost Formulations, Ltd. URL: http://lcfltd.com/ 824 Timberlake Drive Tel: 757-467-0954 Virginia Beach, VA 23464-3239 Fax: 757-467-2947 "Vere scire est per causae scire" ================================================================