At 08:56 AM 11/9/99 +0000, Michael wrote: >Note that a 2200uF capacitor on the output is really way too much. This >amount of capacitance does not help the regulator work better, it slugs >the transient response. A single 100nF will suffice in most cases with >maybe a 10uF cap if you are expecting big load spikes. Although I'm not an EE, it's my understanding that the criteria for post-regulator capacitors are as follows: 1. The 78xx (and presumably LM317) design specs require a minimum 0.1 uF bypass capacitor for compensation of the internal feedback loop. 2. Too high a capacitor will result in overloading of the regulator at lower frequencies (i.e., too low an impedance load), resulting in too long a rise time for voltage to reach spec. 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. So the output capacitor should be no smaller than 0.1 uF and no larger than 200 uF. Usually the breakpoint is in the cost of the capacitor. Since you can buy 0.47-1.0 uF tantalum caps for the same price as 0.1 uF, using a 0.47 uF or 1.0 uF tantalum is the best choice. 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 even. There is apparently never a reason to use higher than 100 uF or lower than 0.1 uF on the regulator output. And with lower values, you don't need a discharge resistor or diode to protect the regulator. ================================================================ 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" ================================================================