Post by thread:[PICLIST] {EE}: Filtering Caps -- a complete myste

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? 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? 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? As I say, it's a mystery to me. Any enlightenment on the Zen of capacitor selection would be appreciated. Thanks, Colin -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

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" spam_OUTspeffTakeThisOuTinterlog.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.

You can almost certainly find some design papers on the net. Briefly - > 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? Depends largely on what peak ripple YOU wan't to tolerate and why. Larger caps generally never hurt performance. Smaller caps might. Also depends on "headroom" you ae providing the regulator. As this drops you can tolerate less ripple before regulator dropout and will ned larger caps in the limiting case. Note - quite incidentally to this discussion, that a SMALL series R feeding the main filter cap will often IMPROVE noise performance and will guive caps a MUCH more gentle time as cap charging is "spread" across more of the AC cycle. Many don't use this method. A little arcane (despite its apparent simplicty) and you need to read more about it if doing it. Leave this until you are happy with basic filtering. . > 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? This is NOT a ripple spec per se but an impedance one related to stability of the *AMPLIFIER* that also happens to be a regulator. (If you don't think it's an amplifier, explain how it oscillates when you don't talk nicely to it :-) ). The 0.33 is probably design worst case and the 0.2 is someone knowing it will work OK with a bit less and the 0.1 is people being lazy or assuming it is NOT an appreciable distance from the filter caps. > 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? Yes. 0.1 uF is the mother duck value. They are commkn and cheap and also usually about OK. Note that 0.1 uF was about the right value for general bypass capacitors in older systems or new slower systems. It "just so happens" that for a typical 0.1 uF monolithic ceramic cap the series resonant combination of cap plus lead inductance for a cap soldered with minimum lead length was good for processors with clocks in the 1 to 10 MHz range which was good for most systems. If you arenusing a 50 MHz clock (or 100 or 133 or 200 or ....) then you want smaller bypass caps. The world has moved on buy many designs haven't. Much more on this subject but this should start things off. As regards caps around actual regulator, until you are comfortable doing otherwise, do what the data book says unless it's really hard to do so - and then, do it anyway at first. Note that some regulators have min and max cap size specs or spec in terms of ESR rather than capacitance. > As I say, it's a mystery to me. Any enlightenment on the Zen of capacitor {Quote hidden}> selection would be appreciated. > > Thanks, > Colin > > -- > 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.

Colin Constant 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? > T=RC T=1/F F= ?120Hz. (or 100 Hz , or 60 Hz or 50 Hz...) (Full wave rectified transformer in the USA is 120 Hz.) R = E/I E = Raw unregulated voltage. (8 volts MIN. for your 7805) I = 1 Amp. > >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? > I use a 1 Microfarad on the input. a .1 is too small, and I have had oscillations. {Quote hidden}> > >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? > > As I say, it's a mystery to me. Any enlightenment on the Zen of capacitor >selection would be appreciated. > >Thanks, >Colin > >-- >http://www.piclist.com hint: The PICList is archived three different >ways. See http://www.piclist.com/#archives for details. > > > -- * | __O Thomas C. Sefranek .....tcsKILLspam@spam@cmcorp.com |_-\<,_ Amateur Radio Operator: WA1RHP (*)/ (*) Bicycle mobile on 145.41, 448.625 MHz ARRL Instructor, Technical Specialist, VE Contact. hamradio.cmcorp.com/inventory/Inventory.html http://www.harvardrepeater.org -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

True confessions of a circuit board layout man........... There is no real magic in selecting filter or bypass capacitors for a given circuit. I have been in the business of laying out circuitboards since 1972. I have laid out over 1000 circuitboards and designed many products with power supplies. I've had engineers hand me a schematic (TTL, CMOS, analog) and have been told to "scatter" bypass caps where ever I have the room. True, there are many theoretical articles and references as to what the proper capacitor values should be but this is not the time and place to to it. There are two types of power supply capacitors. Filter and bypass. In specifying the exact value of a filter cap, you would need to know the parameters of the DC source, half or full wave rectifier, no load peak voltage, minimum and maximum load current, temperature, and probably a few more things I can't remember. A lot also depends on whether this is a one time project or a product that will go into production for consumer, industrial/automotive applications, mil-spec, etc. Obviously, the most important factor is the amount of ripple that can be withstood under full load by your project. The more filtering capacitor, the better....to an extent. Too much can be costly and bulky. Not enough, and your project will become unstable under load. Bypass capacitors in your circuit should be placed liberally. Usually as close to the +5 and ground pins of the most sensitive devices you use. Not every chip has to have them although a processor, PLL, counters, should be bypassed. The value for most digital circuits is 0.1uf. From 1 to ~50 mhz, use 0.01uf. Above 50mhz use a 0.001uf. If you want to get into the theory and design of a power supply, a good reference would be the ARRL handbook available from http://www.arrl.org This is a good all around reference for most common electronic parts. If you just want your project to work, like your post, (7805 1A) make sure your DC input is no less than 3 volts over your regulated output, 470 to 1000uf is adequate for filtering and NEVER omit a 0.1uf cap across the input of the 78xx and ground pin, as close to the device as possible to keep it from oscillating. Don't put too much capacitance on the output of the 78xx. A no load condition and a drop in voltage on the input of the78xx will blow it. I have seen some circuits with a diode placed across the 78xx, anode to the output, cathode to the input to protect it. Rick Colin Constant wrote: {Quote hidden}> 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? > > 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? > > 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? > > As I say, it's a mystery to me. Any enlightenment on the Zen of capacitor > selection would be appreciated. > > Thanks, > Colin > > -- > 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.

At 09:02 AM 3/6/02 +1100, you wrote: >I've never seen this approach with 78xx regs - interesting. I've only >seen it with 317 types. It's valid- look at the internal schematic for the 7805, here's a pointer to one, and analyze what happens for Vout > Vin. http://www.onsemi.com/pub/Collateral/MC7800-D.PDF It's only a concern under the following conditions: Substantial capacitance on the output (but if you need it, you need it) AND either A) A very heavy load on the unregulated supply such that the current from the resulting dv/dt at switch-off could cause excessive current through the regulator. This is rather an unlikely scenario. Or B) There is some possibility that the input could be short- circuited and there is no series diode. This is more likely where you'd use it. If there is a bridge or polarity protection diode, it's not a concern unless there is a possibility that it could be shorted on the *other* side of the diode. Not all data sheets cover this issue, so it's a good point. Like an input capacitor, it's wasteful to put it in there if it is unnecessary. (BTW, on the previous post, 470uF is about 10 times too small for 50/60Hz mains filter for typical 1A supply, also the ripple current rating will not likely be high enough to be reliable). Best regards, Spehro Pefhany --"it's the network..." "The Journey is the reward" .....speffKILLspam.....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.

Yeah, 470 might be a little wimpy. I guess I usually keep my loads below 0.5a @ 5v with the 7805 unless I want a combination power supply/space heater. ;-) Another "condition" that can occur is a mutiple regulator power supply from one source. +5, +12, +18 volt from the same rectifier (the +12 or +18 not short circuit proof). Here, with a large cap on the output of the 5v reg, a heavy load or short on another regulator could drop Vin under Vout. Rick Spehro Pefhany wrote: {Quote hidden}> At 09:02 AM 3/6/02 +1100, you wrote: > > >I've never seen this approach with 78xx regs - interesting. I've only > >seen it with 317 types. > > It's valid- look at the internal schematic for the 7805, here's > a pointer to one, and analyze what happens for Vout > Vin. > > http://www.onsemi.com/pub/Collateral/MC7800-D.PDF > > It's only a concern under the following conditions: > > Substantial capacitance on the output (but if you need it, you > need it) > > AND either > > A) A very heavy load on the unregulated supply such that the > current from the resulting dv/dt at switch-off could > cause excessive current through the regulator. This is > rather an unlikely scenario. > Or > > B) There is some possibility that the input could be short- > circuited and there is no series diode. This is more > likely where you'd use it. If there is a bridge or > polarity protection diode, it's not a concern unless > there is a possibility that it could be shorted on the > *other* side of the diode. > > Not all data sheets cover this issue, so it's a good point. Like > an input capacitor, it's wasteful to put it in there if it is > unnecessary. > > (BTW, on the previous post, 470uF is about 10 times too small > for 50/60Hz mains filter for typical 1A supply, also the ripple > current rating will not likely be high enough to be reliable). > > Best regards, > > Spehro Pefhany --"it's the network..." "The Journey is the reward" > EraseMEspeffspam_OUTTakeThisOuTinterlog.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. -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

At 09:12 AM 3/6/02 +1300, Russell wrote: ><snip> > > 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? > >Depends largely on what peak ripple YOU wan't to tolerate and why. >Larger caps generally never hurt performance. Smaller caps might. ><snip> Too large a filter capacitor will result in too long a rise time on the final regulated voltage. PIC microprocessors have a maximum "come-up" time specification for start-up reliability. ================================================================ Robert A. LaBudde, PhD, PAS, Dpl. ACAFS e-mail: ralspam_OUTlcfltd.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 causas scire" ================================================================ -- http://www.piclist.com#nomail Going offline? Don't AutoReply us! email @spam@listservKILLspammitvma.mit.edu with SET PICList DIGEST in the body