Which type of 0.1uF cap should I use with an LM317? - Page 1
Forum section: electronics / projects

author: dentaku
author_rank: Frequent Contributor
timestamp: June 07, 2014, 02:44:43 am
content: After stopping for quite a while I've decided to finally build a +/- 20V power supply with a 317 and 337 and it got me wondering about types of capacitors.I need a 0.1uF cap between the Input of the LM317 and Ground so I'm curious about what type of capacitor is appropriate here?I'm attaching a photo of five (maybe 6) different types of caps I've found in my bins.Anything I should never use in this situation?What are those big X2 250V things anyway?Are they designed for AC/line voltage?I breadboarded the positive side of the power supply today and used a "CAPACITOR POLY FILM 0.1UF, 100V, 5%, RADIAL; Product Range:CORNELL DUBILIER - 160 Series; Capacitance:0.1ÂµF; Capacitance Tolerance:Â± 5%; Capacitor Dielectric" because they're easy to use but I'm not sure if another type would be better.

author: Afrotechmods
author_rank: Unknown
timestamp: June 07, 2014, 03:02:12 am
content: The ceramic (orange or blue) ones should be fine assuming they are rated for whatever input voltage range you will be using. Depending on what you are building you may not even need one.The X2 caps are indeed intended for mains filtering but can be used for other things too.

author: SeanB
author_rank: Super Contributor
timestamp: June 07, 2014, 05:07:42 am
content: Use any of the film ones and not the ceramics, as those you show there have a breakdown voltage of 25V, and at that are likely not going to be 100n any more but likely under 30n. The green mylar or the yellow one next to it are perfect in this application.

author: dentaku
author_rank: Frequent Contributor
timestamp: June 07, 2014, 02:37:33 pm
content: Quote from: SeanB on June 07, 2014, 05:07:42 amThat's good to know.So for the green one, according to an AVX datasheet I just found, 2A 104J would be2 = 200VA = C0G (NP0)104 = 100000 pF or 0.1uFof courseJ = ±5%That DOES seem like it's just the right capacitor for the job. I'm going to swap the white film cap with the green one and see if it's good. The white film cap is brand new and the green one is just recycled stuff but my multimeter's CAP setting says it's 108.3nF so it's probably fine.The funny thing is that I posted this last night and this morning Dave added a new video about variability in ceramic caps at different voltages.http://www.eevblog.com/2014/06/07/eevblog-626-ceramic-capacitor-voltage-dependency/By the way, the other side of my power supply uses an LM337T and the datasheet says...*Cin is required if regulator is located more than 4 inches from power supply filter.* A 1.0 mF solid tantalum or 10 mF aluminum electrolytic is recommended.**CO is necessary for stability. A 1.0 mF solid tantalum or 10 mF aluminum electrolytic** is recommended.In the past I just used 10uF electrolytics but since then I've found a bunch of tantalums.Is it worth using tantalums or should I just stick with my little 10uF Nichicon electros?

author: mariush
author_rank: Super Contributor
timestamp: June 07, 2014, 02:59:24 pm
content: Tantalums have low esr, which can cause problems to some linear regulators (for example LM2941 or MIC2941 need a 22uF on out, but its esr needs to be above 0.1 ohm for loop stability)... see page 8 here at application hints:http://www.ti.com.cn/cn/lit/ds/symlink/lm2941.pdfLM317/LM337 don't have this problem, as far as I know, so you could use tantalum capacitors but it has no benefit. Also, tantalum capacitors can fail shorted and burn (with flames) and they don't like overvoltage (explode violently or burn in flames unlike electrolytic capacitors which just swell up or pop) so if your regulator fails in some way allowing more than the rated voltage of the tantalum, you could burn your power supply. Since there's no benefit to using them in your circuit, I would avoid them.10uF is the minimum recommended output capacitance, you can use more than that, for example 100uF or 220uF, it just doesn't improve output much so they don't recommend something that may be a bit more expensive. You often see capacitors larger than minimum or recommended value because there's no real savings for the manufacturer to go with smaller ones, or the capacitor size makes them handle heat better, especially if they're near heatsinks (larger can could be 4000h@105c compared to smaller 2000h@105c)As an extra tip... it's not the case here, but generally if you have a circuit powered by batteries and you use a LM317 or some other linear regulator, it will help to have a large input capacitor before the regulator, something like 470-1000uF. Like I said, you're probably using a transformer so it doesn't apply to you (and you already have the large capacitors after the transformer anyway)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 07, 2014, 03:29:35 pm
content: 2A is the voltage code -- the number-letter code is typical of small (SMT) electrolytics and films. 2A I think is around 100V.Yeah,http://www.niccomp.com/help/voltage-codes.aspfor example.Regarding ESR, tantalum is lower than aluminum electrolytic, but still pretty sizable. The real value of tantalum is actually two parts: the stability of capacitance and ESR with respect to manufacturing, temperature and voltage, and also the ESR range which is almost perfectly suited to most supply filtering applications -- not too low (encourages resonances, lots of LDOs and switchers unstable with low ESRs), not too high (doesn't dampen resonances, too much ripple and dissipation). Whereas ceramics and aluminum polymers are often too low for bulk applications, and regulators that don't claim "any capacitor" bypassrequirea series resistor to maintain stability (good luck finding how much though!).For linear (non-LDO) regulators, I don't think it matters. Electrolytics have been used forever, so they can't be too sensitive to high ESR, and low ESR doesn't seem to be a problem either.Tim

author: dentaku
author_rank: Frequent Contributor
timestamp: June 07, 2014, 03:42:26 pm
content: Quote from: mariush on June 07, 2014, 02:59:24 pmYup, I've already got an 1800uF and a 3300uF electrolytic between my bridge rectifier and the input of the regulator anyway.Thanks for the good info.

author: dentaku
author_rank: Frequent Contributor
timestamp: June 07, 2014, 03:53:13 pm
content: Quote from: T3sl4co1l on June 07, 2014, 03:29:35 pmOK, those codes aren't for old through hole caps. Oh well, 100V is still perfectly fine.I've bookmarked that page and saved the .PDF. It looks quite useful.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 07, 2014, 04:07:16 pm
content: Quote from: dentaku on June 07, 2014, 02:44:43 amOh, and yes: X/Y ratings tell about the safety of connecting them across the line.An ordinary metallized film might burn and catch fire if subjected to overvoltage. Not a great idea. So, they came up with standards, which must be applied to capacitors for across-the-line (X1/Y2) and line-to-ground (Y1) purposes.An X or Y type capacitorneed not be good at being a capacitor. They do need to fail gracefully, perhaps at worst letting out some smoke, but not catching fire (or self-extinguishing if they do). They are intended for EMI filtering applications, so they still have to do some filtering, but the ESR can be relatively high -- helpful in a filter to dampen resonances, but dangerous around high ripple currents, which can cause heating (like an active PFC converter, which might draw ~50% current ripple at the operating frequency).Note that any self-healing process necessarily reduces the capacitance, sometimes by a huge amount (if half the electrode area gets cut off by enough breakdown action, well, yeah..). So you should probably check that the salvaged capacitor is still within spec.The Y1 type caps are almost always blue or yellow ceramic disk types, unusually thick. The nature of line transients is, line-to-ground can be large (2.5kV or so), due to say lightning strikes, while the line-to-line can be high but isn't usually as bad under the same conditions (maybe 400-1000V?). Mainly, loads on the line keep line-to-line voltage less crazy than line-to-ground, which can be kind of flapping-in-the-wind. (If you want to know these figures precisely, by all means look up the standards -- I haven't read them in detail myself, so don't take any of this seriously!)For general applications, as long as they are within spec, EMI / X2 caps are pretty good, not usually good for the peak current that you'd want a snubber or resonant capacitor rated for, but certainly enough for, say, extra DC bus filtering (let alone anything that doesn't stress them, like timing, or small-signal oscillators).Y1 caps are usually class II dielectric (see Dave's #626 today!), but given the peak voltage rating and "geez that thing is freaking THICK" of it all, you won't notice much change at normal line levels (say ~250VAC). They aren't high-Q, I wouldn't recommend them for resonant applications (indeed, I've subjected a 150pF 275VAC cap to 500kHz at an amp or a few, and boy did it heat up quickly, not to mention the distorted waveform!), but other than that, they are also good general purpose caps.Tim