At 11:41 AM 3/5/02 -0800, you wrote: >I was just wondering how people arrive at a choice of power supply filtering >capacitors. I'm thinking of a regulated 5V supply, around 1A (7805). > >Looking at published plans, it seems that some people routinely specify 2 x >2200. Others just put in a single 1000. What's a guy to do? Hi, Colin:- Okay, the most important thing is to make sure that with the minimum line voltage, maximum load and minimum capacitor and worst-case voltage- regulator drop that the regulator never "drops out". The first part is a function of the line voltage range you want to cover (-20%? -25% from nominal), the actual transformer output voltage under fairly heavy loading (more on that later) and the diode drop(s). The regulator dropout you get from the data sheet. It is 2V at 1A 25'C typical, and 2.5V guaranteed over temperature. Which one should you use? Right, 2.5V. Then you have to look at the ripple. You can make the assumption that the capacitor is only charged in brief pulses the peaks of the AC waveform. Thus, we have a peak-to-peak ripple of Vr = (tx * I)/C (time for one half cycle with full-wave rectification or one cycle with half-wave, I is the regulator input current, C is the capacitance). So, with a 4400uF cap, 60Hz, full-wave, you'll have Vr = 1.9V (p-p). That means your average input voltage measured on a DVM had better be higher than about 8.5V at minimum line input and maximum load, because the "troughs" of the input waveform will be down at 7.6V, just about the 7.5V minimum. Note that half-wave rectification means the caps have to be doubled for the same ripple. 1000uF is not enough for any reasonable 1A supply, but it's often fine if you only need 100mA or 200mA. The actual transformer voltage will be a bit lower than you might expect because the 1A is supplied in brief pulses of much higher current, depending on the transformer winding resistance and other factors. The other requirement is that the ripple current be less than the maximum spec on the capacitor, and the more below that the better, because the capacitor will last longer. Using two capacitors means that you can double the ripple current spec unless your layout is really silly. >Also, data sheets indicate 0.33 "or larger" on the input if an appreciable >distance from the filter caps, but published plans seem to use 0.1 or 0.22. >Anyone know why? Maybe they are not far from the filter cap? I don't use any unless the regulator is off-board, but there is a bit of a conflicting requirement- the capacitor should be kept cool (away from regulator) but it has heavy traces to, and should be close to the regulator. 1/2" or 1" is no problem. If it's a through-hole electrolytic, 10uF/50V is about the smallest that makes sense unless you're into the really small (7mm or less) heights. >And, on the output, 0.1 seems to be standard. Motorola says values less than >0.1 could cause instability. Wouldn't this tempt you to use more than 0.1? 7805's don't require an output capacitor for stability. The output cap reduces the output impedance. What Motorola means is that you should not put a 0.01uF capacitor on there, if you do use one (and you usually should) then make it or them >= 0.1uF total. Usually there's more distributed capacitance around the board on chip bypasses anyway. LDOs and negative voltage regulators do require an output cap and you should follow the data sheet recommendations. Hope this helps, Spehro Pefhany --"it's the network..." "The Journey is the reward" speff@interlog.com Info for manufacturers: http://www.trexon.com Embedded software/hardware/analog Info for designers: http://www.speff.com 9/11 United we Stand -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.