SMD DC Power filtering caps - Page 1
Forum section: electronics / beginners

author: paulca
author_rank: Super Contributor
timestamp: February 20, 2018, 09:56:30 am
content: I am trying to design a board using surface mount only. It will be powering a small number of WS2811 RGB LEDs from a 2A 12V DC wall wart.I figure I need a 16-20V 100uF cap or greater across the input power rails.I'm finding the little canned aluminum caps seem quite expensive, I'm wondering if I should be looking at a different type?Also for ceramic disc replacements in 0805 surface mount do you normally go straight to multi-layer ceramic?

author: fourtytwo42
author_rank: Super Contributor
timestamp: February 20, 2018, 02:43:38 pm
content: Quote from: paulca on February 20, 2018, 09:56:30 amWhy ?

author: paulca
author_rank: Super Contributor
timestamp: February 20, 2018, 02:53:59 pm
content: Because every circuit I have seen has a power decoupling cap on the power rails.There is also a 5 volt regulator powering an MCU in the circuit, the power rail decoupling cap will smooth out main power burps and bumps?

author: fourtytwo42
author_rank: Super Contributor
timestamp: February 20, 2018, 03:42:13 pm
content: So your not into calculating the required capacitance then ?Seems to me your making a hard job out of it by selecting what on the face of it seems an excessivly large component, I expected a reply where you would justify that, not just because I have seen it elsewhereNormally a regulated wall wart would have it's own output capacitor so you only need to satisfy the input requirements of your 5V regulator and that is what type ?Does your board use the 12V for anything other than the 5V regulator ?

author: paulca
author_rank: Super Contributor
timestamp: February 20, 2018, 03:47:21 pm
content: The board powers a strip of blinking RGB LEDs.The LM78L05 requires 330nF on input and 10nF on output.You are right though, I'm used to just lifting a capacitor out of a box. I don't have SMDs though and at the price they seem to cost I'm not likely to buy a stock pile. Unless I buy bulk cheapies off ebay.How would one calculate the capacitance required? I don't have a data sheet for the AC/DC wart and in fact how can I know which wall wart someone chooses to use? I don't know the noise ripple caused by the LEDs except that they cause havoc with my audio amp when connected on the same 12V rail.

author: fourtytwo42
author_rank: Super Contributor
timestamp: February 20, 2018, 04:01:21 pm
content: Many potential answers there, so there is something else on the 12V rail namely an unspecified amplifier. Are the leds also on the 12V rail or on 5V ?You can quantify the wall wart by subjecting it to some ripple current and measuring the resulting ripple voltage, do you have a scope ?It is important to try and find out how the LED switching noise is getting into the amplifier, is it only as you say when they share the same 12V supply or is it also caused by shared grounds and/or proximity. You may need some additional rail filtering.

author: paulca
author_rank: Super Contributor
timestamp: February 20, 2018, 06:21:37 pm
content: The amplifer is a red heron. I run things on my desk of a battery powered DC rail, including strips of LEDs. I have found the LEDs can dump a load of noise onto the rail, not always, but I just took that to mean they will "dirty the power"I am making a board to run a strip of LEDs which I intend to put into a lamp and give to someone as a present. So, while I will supply a generic, jelly bean wall wart I can't say they will always power it with that wall wart. So it has to accept any-old 12V wall wart that is powerful enough. Like every other product that runs on a 12V DC supply wall wart.The LEDs will only consume 500mA, maybe 1A peak. The wall wart I have is rated for 2A.If I need to test the ripple on it, that's a bit silly if in 2 years time the wall wart gets lost and another one takes it's place.I was thinking large capacitance as the load can go from 10mA to 1A very rapidly and repeatedly.To be honest my first prototype board version of such an LED controller didn't have any caps and it worked fine.So maybe I just provide what is specified for the 5V regulator and forget decoupling the power rails.

author: paulca
author_rank: Super Contributor
timestamp: February 20, 2018, 06:24:40 pm
content: Quote from: fourtytwo42 on February 20, 2018, 04:01:21 pmSorry, when I re-read this I recall when I got hideous noise on the amplifier. It was when I connected an Arduino UNO to run the LEDs, I connected the PC ground via the USB to the system. My PC is a noise factory. I'm surprised the USB DAC manages to filter almost all of it off it's ground.Also, in the device in question the 12V will power the LEDs. The 5V regulator will power the ATTiny85 chip.

author: fourtytwo42
author_rank: Super Contributor
timestamp: February 20, 2018, 07:45:45 pm
content: Quote from: paulca on February 20, 2018, 06:24:40 pmNo worries I was trying to understand your system and hence the need for this large capacitorSo what current do the leds draw ?Ground loops with PC-audio are a pain hence I feed the PC-audio into balanced inputs, no ground connection! Is it a desktop PC or a laptop BTW that makes a differance ?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 20, 2018, 08:45:56 pm
content: You calculate it thus:Realize the capacitor is there to control the supply impedance.You have three constraints on impedance:1. Stability: for the regulator2. Dynamics: if you have a peaky load, will it under/overshoot an excessive amount?3. Inrush: the input should be well damped and/or clamped, so that if someone hot-plugs the DC adapter, nothing blows up.- Stability is simple enough, but hard to quantify, because they don't ever draw stability diagrams for regulator inputs. They rarely draw them for outputs, as it is (usually in terms of ESR vs. C, for LDOs that are susceptible to instability, but absence of such a diagram is not evidence for absence of instability -- most are just lazy!). Let's say you should have an impedance that is below, say, 10 ohms, and dominant resistive or capacitive, with no crazy peaks. This is most likely to behave, and be representative of the regulator's intended environment.- Dynamics are easier, because we can put numbers here. A 78L05 delivers maximum 100mA, so let's say the maximum step load change is 100mA. This step must not cause dropout (rising current step) or overvoltage (falling current step). A typical case might be 1V of allowable ripple, in which case you get a 10 ohm equivalent. Any lower impedance will do.- Inrush works similarly, but instead of a current step, we assume a voltage step, at the adapter's open circuit voltage. Consider the typical adapter: a transformer, rectifier and electrolytic capacitors, then ~2m of cable (usually twin lead, Zo ~ 100 ohms). OCV is typically 1.5 times nameplate (they have pretty awful regulation). The input voltage step causes a voltage and current hump, with the cable having equivalent inductance, and the two capacitors (one inside the adapter, one on your device) acting in series. This is a simple RLC series resonant circuit.In any case, the main offender is the cable: it has equivalent inductance, so dominates the impedance above a few kHz. How much inductance? If Zo ~ 100 ohms, that's about a third the impedance of free space (377 ohms, but I'm rounding down to account for dielectric constant), so the inductance is about a third of mu_0, or (1.257) / 3 ~= 0.4 uH/m. So a two meter cable is 0.8uH. A simple transmission line argument, reduced to the simplest of ratios.The adapter will probably be on the order of 1000uF and 0.1 ohm ESR. Unless you need a large capacitor (>100uF), we can assume this is a Thevenin voltage source with OCV and ESR as given.Inrush is probably the most stringent of the three conditions. But it can be assisted with a TVS (say a P6KE18A or SMAJ18A, which clamps the overshoot at 20-25V, well within the 78L05's capability), so we don't need to design to it, necessarily.If we do: we need C large enough that sqrt(L/C) is smaller than total ESR.Suppose we use 1uF ceramic, with ESR ~ 0.1 ohm. Z = sqrt((0.8 ) / (1)) is about 1 ohm, which is 10 times ESR, so will ring like a motherfucker. What's worse, it won't merely ring up to twice the input (~36V), but it will overshoot considerably more, due to ceramic capacitor saturation (maybe 60V or more!). We could clamp this with a TVS, but we still have the problem that the impedance peak is ~10 ohms at the resonant frequency (which, really, may not be a problem, given the assumptions mentioned earlier).We might then add 4.7uF in parallel with this, with a 1 ohm series resistor, to dampen the network. (Or an electrolytic with the same parameters, but you can't find an electrolytic with stable ESR, so it's not the best.) Overshoot still sucks (need TVS), but all the dynamics are satisfied: the impedance is capacitive above ~180kHz, and the peak impedance is about 1 ohm.If we go with electrolytic alone, we'll expect to find a part with ESR > 0.2 ohms in smaller values. Plus the adapter's ESR, we need to find C such that: 0.3 ohm <= sqrt((0.8uH) / C). Chug chug chug, and we see 10uF or more will do.When C is relatively large, the supply voltage comes up relatively quickly (most of the charging current is dropped across the ESR), and doesn't overshoot, but just kind of whumps up to its normal value. I might go with 47uF just because it's a little bigger... safer, maybe?Note that peak currents are tremendous here! An 18V supply into 10uF 0.3 ohm is a peak current around 60A, if only for a few microseconds! You don't want to route this through any semiconductors, if you can help it.You can still use a TVS, though it won't save you much at this point -- if someone plugs in a reverse polarity or excessively high voltage adapter, the TVS is toast, and the device is simply dead. A series diode addresses reverse polarity; overvoltage might be better treated with a crowbar (a SIDAC, perhaps?) and polyfuse. This is really only needed for higher level foolproofing where product reliability is paramount.Tim

author: paulca
author_rank: Super Contributor
timestamp: February 20, 2018, 09:56:27 pm
content: Wow! ThanksInformation overload!So if I tried and put some numbers on things.The strip considers of 20 sets of 3 in series leds, giving a total of 60 LEDs. Each of the 60 LEDs is made from 3 LEDs, Red, Green, Blue. Each LED can pull 18mA. So that's a total of 180 LEDs and atheoreticaltotal current of 18*180 = 3.2 Amps.https://cdn-shop.adafruit.com/datasheets/WS2811.pdfSo "in theory" I could program the LEDs to strobe on/off PWM style from 0A to 3.2Amps. The ATTiny at 16Mhz could probably manage to strobe them pretty fast. The data write time would allow 100uS duty cycle. Though I'm not sure the ATTiny is fast enough to do that.The 3.2Amp figure is from the linked datasheet, but I have tested this strip at full white and it didn't pull more than 2.5 amps.However I don't intend to program any strobe flashes, as the lamp I'm making is meant to be mellow. Full brightness white would require a much better PSU to get 2.5-3.2 amps.Running the LED animation programs I intend to use in the lamp it usually sits around 500mA. It is cycling rainbows around at about 30% brightness with occasional white "confetti" effect sparking around it. with only a handful of LEDs going full white. If I take the brightness up high I can get it to peak at 1 Amp.I will add a selection button that will cycle through: Rainbows, Basic White, Warm white, Off. Of those "Basic White" is highest current, I will test on the bench and set the brightness to not exceed about 1 Amp. This gives me tones of headroom on a basic 12V 2A wall wart and avoid heating it up. Although it will probably cook a 500mA wall wart.I suppose, avoiding flashes, the only current spikes I need to worry about are the LEDs coming on in "Basic white" mode. Even then the "on" time will be limited. The board will power the ATTiny85 will boot and it will start writing data to the LEDs. It takes approximately 2.5uS per set of 3 series LEDs. So it will set the full set of 20 to white in 50uS.I'm not particularly worried about dropping a few volts to the LEDs. I would be more worried about the sudden current draw upsetting the voltage regulator and browning out and crashing the MCU. The MCU is good down to 3V at least, so that is also unlikely.The worst case is for the MCU to crash, or the LED driver chips to crash and go full brightness white constant. That 'could' pull as much as 2.5 or 3.2 Amps if you believe the datasheet.The question would be, in this unlikely scenario, will it overload the wall wart or the copper strip the LEDs are mounted to? A poly fuse would aid things here, it would cut out before anything got too hot to melt which would reset things back to rebooting the ATTiny and normallity would restore.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 21, 2018, 12:36:15 am
content: Nah, a polyfuse won't open under that condition, it'll just brown out and start forgetting bits, and self limit.Actually, a polyfuse might not even open under a fault condition, because wall warts are usually "impedance limited" (meaning they can cook in their own juices, but probably not start a fire, when shorted).What's a WS2811 do under brownout conditions, anyway -- anyone know? They're popular enough, someone must've discovered this before...If nothing else, the 5V rail won't pull much below 3V because of LED Vf + controller dropout.Definitely a good opportunity for a cheap little TPS54200 or the like. That's >5W wasted in a linear regulator!Tim

author: paulca
author_rank: Super Contributor
timestamp: February 21, 2018, 07:19:43 am
content: Quote from: T3sl4co1l on February 21, 2018, 12:36:15 amEh? The 5V reg is only powering the ATTiny, so it should only be passing around 10mA, so 12-5 * 0.01A 70mW.The 12V goes straight to the LED strip. I deliberately choose 12V WS2811s due to low amperage. The downside of the LEDs being grouped in 3s and not "individually" addressable is fine as the lamp sits inside a diffused lamp shade, so you can't see the LEDs anyway, just the soft glow.

author: fourtytwo42
author_rank: Super Contributor
timestamp: February 21, 2018, 09:43:48 am
content: Quote from: paulca on February 20, 2018, 09:56:27 pmIf they are made up of 3 in series then 180 leds consumes 180/3*0.018 or 1Amp right ?Is the wall wart regulated, have you measured the ofload voltage ?

author: paulca
author_rank: Super Contributor
timestamp: February 21, 2018, 10:26:55 am
content: Quote from: fourtytwo42 on February 21, 2018, 09:43:48 amYou might be write, I have a few sets of these LEDs, so I might be remembering the wrong test. I'll fire them up full white on the bench supply and check. But it makes sense. The chip datasheet says 3 LEDs at 18.5mA constant current. So that would be 18.5mA across all three, which at 12V results in nearly the same wattage than 18.5mA through 1 LED at 5V.I measured it's open circuit voltage and it seemed find, around 12.2V. I will measure it's on load voltage tonight.

author: paulca
author_rank: Super Contributor
timestamp: February 21, 2018, 07:26:31 pm
content: So here is the schematic to clear up some confusions.The 330nF cap on the 12V rail is the one I'm stumbling with. The 100nF's are for the Tiny RESET pin and power pins.The rest is fairly simple.Grr... forgot a pull up on the reset pin. Although come to think, I've used an ATTiny before without one. Must check the minimal circuit in the datasheet again.

author: soubitos
author_rank: Frequent Contributor
timestamp: February 21, 2018, 07:36:15 pm
content: Unless you want to fit your assembled pcb into a tiny space or you got to production and need to lower assembly cost etc why on earth dont you use large ready available caps you already have i am sure?

author: fourtytwo42
author_rank: Super Contributor
timestamp: February 21, 2018, 07:42:12 pm
content: You forgot to measure the wallwart onload voltageAnyway your 5V regulator will prevent most rubbish on the 12V from reaching the micro so I really would not worry to much. Providing your wallwart has reasonable regulation your leds should not flicker. If you really want to belt & braces your 5V regulator you could add a resistor 10-100R between it with it's input capacitor and 12V so that it's input capacitor serves the regulator as intended and not the leds

author: paulca
author_rank: Super Contributor
timestamp: February 21, 2018, 09:02:32 pm
content: Quote from: soubitos on February 21, 2018, 07:36:15 pmI'm hoping to fit it into one of these:http://www.hammondmfg.com/pdf/1551G.pdfThe SMD stuff leaves plenty of room on the board and I probably could put a 100uF THT cap down on it's side somewhere. There is probably enough room.Quite annoyingly the regulator comes in either TO-92 or SOIC-8. I'm not convinced the SOIC-8 saves much space to be honest.

author: paulca
author_rank: Super Contributor
timestamp: February 21, 2018, 09:13:10 pm
content: Quote from: fourtytwo42 on February 21, 2018, 07:42:12 pmA little "how you doing" for a test but.Open Circuit: 12.32V steadyMeasured at the end of the LED circuit while they are running in the prototype: 12.23 - 12.28V depending on the load.The prototype is running a little dimmer than the final version will, but I can't be bothered to reprogram the board tonight.Indicatively the wall wart seems fine under load. I expect that even doubling the load will not pull it under 12V.

author: paulca
author_rank: Super Contributor
timestamp: February 21, 2018, 09:14:20 pm
content: Out of interest, this is what it looks like. It's just running the demo real from fast LEDs, but the pattern I am writing for it is based on the smooth rainbow cycle (similar to 0:19 in the video) and maybe some sparkles.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 21, 2018, 09:18:12 pm
content: Quote from: paulca on February 21, 2018, 07:19:43 amOh, I had it in my mind that they were the 5V kind.Do check abs max then -- adapters have high OCV.Tim

author: paulca
author_rank: Super Contributor
timestamp: February 21, 2018, 09:25:10 pm
content: Quote from: T3sl4co1l on February 21, 2018, 09:18:12 pmYea, I have a monster strip of 5Vs they are a different kettle of fish. It's only 1 meter, but it has 144 leds on it and my 5A PSU frowns at me suspiciously when I connect them to it. I haven't tested them at max bright yet except to do a strobe light pulsing them, which was a beer induced risky move anyway. Insanely bright and disorientating. The 5A PSU huffed but hung in there.Max current for that strip would be 7.5 Amps and they would need powered at both ends.

author: soubitos
author_rank: Frequent Contributor
timestamp: February 21, 2018, 11:08:58 pm
content: Quote from: paulca on February 21, 2018, 09:02:32 pmWhen you reach a capacity of x00 uF's the capacitors are "larger" anyway, depending on characteristics like voltage ESR, temp etc.. you can use a 470uF/25V TH capacitor using the footprint of say a 5845 inductor. you simply bend the caps leads 90 degrees then solder the cap then bend it carefully another 90% so the soldered leads are under the capacitor... this way its low profile, on the pcb and no holes needed....

author: David Hess
author_rank: Super Contributor
timestamp: February 22, 2018, 08:44:38 pm
content: Quote from: paulca on February 20, 2018, 03:47:21 pmThe LM78L05 requires a high frequency decoupling capacitor on its input if it is far from the bulk input filtering capacitor. It requires no high frequency decoupling on its output and a small bulk filtering output capacitor is preferred. High frequency decoupling goes at the load and not the source.

author: paulca
author_rank: Super Contributor
timestamp: February 22, 2018, 10:37:33 pm
content: So if I don't put a bulk input filter on the board it will be about 3 feet away from the one in the wall wart.Can I then get away with just a 330nF on the reg input side?On the output side, can I use just one 100nF across the power to the ATTiny, in place of the reg output cap and the ATTiny power filter cap? They are like 10mm apart at most.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 23, 2018, 01:40:19 am
content: Recommend > 10uF electrolytic at input, that's OK at output or use another electrolytic.Tim

author: David Hess
author_rank: Super Contributor
timestamp: February 23, 2018, 04:46:25 am
content: Quote from: T3sl4co1l on February 23, 2018, 01:40:19 amOr maybe a little more at the input but for a subtle reason which has nothing to do with regulator stability.Some transformer wall warts have an alarming amount of leakage inductance so if the load on the output of the regulator is abruptly removed, the input voltage can rise high enough to burn out the regulator unless there is some bulk input capacitance to absorb it. Bob Pease wrote about this once.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 23, 2018, 08:41:21 am
content: Quote from: David Hess on February 23, 2018, 04:46:25 amI elaborated on inrush and such earlier, for which 10uF would be borderline, and more would be just fine.Was that in this thread, or the other? I don't even remember anymore, these questions all blur togetherTim

author: paulca
author_rank: Super Contributor
timestamp: February 23, 2018, 10:27:50 am
content: Either way I'm back to my first post.The only reason I asked is not that the electrolytic caps are too expensive, it's just that when I see things being suspiciously expensive I wonder if it's because I'm picking the wrong (rare, low demand) part.Further looking capacitors seem to vary in price dramatically. There are 100nF multi-layers that cost £0.012 in a strip of 100, but there are ones that don't look any different from spec which are £0.19 each!I would buy a cheap chinese multi-pack, but I doubt very much I will use most of the values. Ideally I'd like a kit which has 100 each of the 10 most used values.1, 10, 100, 330, 1000 uF10, 100nF22pFIs about all I have ever had to use so far.However the pF, nF packs are at least cheap.How risky would a pack of surface mount aluminium caps from china be?

author: soubitos
author_rank: Frequent Contributor
timestamp: February 23, 2018, 08:47:21 pm
content: Quote from: paulca on February 23, 2018, 10:27:50 amDepends on what you buy from whom you buy it and what you expect to get...I am sure brands like CapXon ChengX might not be top notch but for normal use, in non critical applications they are more than fine. Then you got Lelon, Rubycon brands that are not really Chinese but you can source them from China at good prices.. the problem is, if you can trust the average aliexpress seller to send you genuine parts ... for the most part, if the price is too good to be true, and the seller sells odd bits of products... avoid it...

author: paulca
author_rank: Super Contributor
timestamp: February 23, 2018, 09:43:33 pm
content: So I made about my 5th attempt to layout the board."Piper" by the way is my daughter. This is her comfort night light. She is 7 months old.