An even cruder, but handy approximation is 1000 uF per amp of power supply current. I have watched this work wonders in marginal power supplies. -- Lawrence Lile Russell McMahon Sent by: pic microcontroller discussion list 11/10/2003 04:07 PM Please respond to pic microcontroller discussion list To: PICLIST@MITVMA.MIT.EDU cc: Subject: Re: [EE]: Testing prototype with a battery > > ...and a low supply current device with a largish filter cap often will... > > Is a large cap necessity? What would it help solve? (I am really asking). "Large" is relative here. Large might be 1 uF or 100 uF or more depending on circumstances. As a battery gets near the end of its working life its internal resistance increases. Any current peaks created by the supplied equipment will cause an increasingly large drop in battery terminal voltage as the battery approached its endpoint. When the voltage is already low such fluctuations may be the final step in making the device stop working. Even though the battery may be easily able to provide the AVERAGE current the peaks may drop the processor etc into reset or malfunction. SO adding a suitable filter capacitor will help prevent this. The current peaks are provided from the capacitor (plus battery as much as it can manage) and the capacitor then charges from the battery until the next peak is needed. The size of the capacitor needed depends on the peak currents needed and how long they will last. The true value depends on an exponential decay of voltage, but a "rule of thumb" value can be obtained from C = T x I / V Where C = capacitance. T = time current is drawn V = drop in capacitor voltage during this period. As an example only, if the current peak lasted 100 ms and was 5 mA and you could stand a 0.15 volt voltage drop then C = 0.100 x 0.005 / 0.15 = ~ 300 uF. This assumes that the battery does not supply any current during this period and that it still has enough life to recharge the capacitor before the next peak. In this example a 100 uF capacitor would make a lot of difference. A 100uF at 3v6 can be rather compact. Russell McMahon -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details. -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.