Jelly Beans - Page 1
Forum section: electronics / beginners

author: NivagSwerdna
author_rank: Super Contributor
timestamp: July 09, 2016, 04:02:43 pm
content: I'm new to this SMD stuff but I have a schematic and working on the layout...Now for the layout I need to pick components that are cheap and available from Farnell (or similar)...I've come to the conclusion that 0603 is the best price point for R and C so I just layout on that basis...but for other components... especially inductors...How do you decide on parts from somewhere like Farnell that has 10000s of the things?My current dilemma is a 4.7uH inductor for a MCP1640

author: NivagSwerdna
author_rank: Super Contributor
timestamp: July 09, 2016, 04:25:13 pm
content: Even if I restrict it to say at least 1A rated current, ESR < 0.2,.... I still get hundreds of the things!

author: Howardlong
author_rank: Super Contributor
timestamp: July 09, 2016, 04:38:12 pm
content: Inductors are especially hard. I would just accept you're rarely going to have the right one, especially in power electronics.For RF, I keep E12 stock of both multilayer and wirewound, with wirewound sizes optimised to maximise Q.

author: zapta
author_rank: Super Contributor
timestamp: July 09, 2016, 05:03:21 pm
content: Quote from: NivagSwerdna on July 09, 2016, 04:25:13 pmIf there are many products that fit your requirements, what is the problem then?

author: NivagSwerdna
author_rank: Super Contributor
timestamp: July 09, 2016, 07:58:54 pm
content: Quote from: zapta on July 09, 2016, 05:03:21 pmIt makes ordering very slow.. IMHO the parametric filtering on the farnell site is slow to use.I added a few extra criteria... cheap, available, well-stocked and for my application at least partially shielded...ended up with a SRN4012-4R7M which was quite cheap... then went to Mouser and found it listed as obsolete!Wurth 744031004http://uk.farnell.com/wurth-elektronik/744031004/choke-smd-4-7uh-1-2a/dp/1635834Almost 3x the price

author: Howardlong
author_rank: Super Contributor
timestamp: July 09, 2016, 09:50:01 pm
content: There are one or two manufacturers who turn up on distributor sites with wildly high prices, Wurth is one, Coilcraft is another. For that reason, apart from for very small runs, they don't make the BOM, but they do make nice lab kits, of which I have several, but when you move to production they've priced themselves out. If you have no visibility of indicative pricing at the prototyping stage, why would you use them in production when you could choose something from Panasonic at 1/5th the price?Unlike Rs & Cs, wound components tend to be relatively expensive parts.Granted, I am sure there are a few mega corps who have insider deals with their own sales reps and negotiating muscle, but for everyone else, no thanks.

author: basinstreetdesign
author_rank: Frequent Contributor
timestamp: July 10, 2016, 01:37:30 am
content: For most of my stuff I use through-hole parts because I have lots of them but for SMT R's and C's I use 0805 size because they are the smallest size part I want to attempt to solder with an ordinary iron and that works fairly well. Just have a good bent needle-nose tweezer to pick them up with and hold them down with when applying solder. Cost is generally a lesser factor because of the low quantities.I get practically all of my parts from DigiKey because their parametric search capabilities are the best, you know exactly what they have at any moment and delivery is fairly quick (last time, the standard, cheapest delivery charge of CDN$8 got my stuff to me in my small out-of-the-way town the next day) Every time I do a buy for a project I get another dozen values, about 50 pcs ea on cut tape, included which I don't really need yet to fill out my collection. But for L's or whatever other passives just get what you need when you need it plus 3-5 other values you might have to try to get your circuit working. As for discrete Q's, D's get 50 or so of the most common and general purpose ones, such as XXX3904, XXX3906 BJTs, etc.After a while you will have a fairly solid collect that will serve for almost anything you want to do.

author: tggzzz
author_rank: Super Contributor
timestamp: July 10, 2016, 08:17:03 am
content: Inductors are a much more fickle class of component than inductors or resistors. Frequently you will have to decide on which properties are most important (e.g. current or resonant frequency), and that will largely fix the size.For RF filters you can get a wide range of 0603 inductors, but they aren't cheap. In the absence of a specific requirement, consider getting a "kit" on ebay, but re-evaluate their suitability when you are planning to use them for a specific purpose.For interference suppression, sometimes SMD ferrites are useful, sometimes not, sometimes wirewound components are useful, sometimes not. Get some and hope, or design a circuit and buy the specific component.For PSUs, work out what the design requires, and get those.

author: tautech
author_rank: Super Contributor
timestamp: July 10, 2016, 08:36:28 am
content: Quote from: NivagSwerdna on July 09, 2016, 04:02:43 pmWhen I did a one off project with a 1640 I just got a free sample direct from Coilcraft as was specified in the 1640 datasheet. Worked fine and if I was to use a 1640 again I'd source the same Coilcraft inductor from any of the major suppliers that stock it.Those little SOT23/6 boost regulators are honking along @ 500 KHz so jelly bean inductor it is not.Use what's not recommended in the datasheet only at your peril.Pay attention to the recommended layout too and despite that I did I had to use much higher than the suggested output capacitance to get ripple to acceptable levels. So much so that MLCC wouldn't cut it and tantalum was required.

author: NivagSwerdna
author_rank: Super Contributor
timestamp: July 10, 2016, 08:37:22 am
content: Quote from: basinstreetdesign on July 10, 2016, 01:37:30 amFor my first SMD design I went for 0805 but I'm now very happy with 0603 and in fact I find 0805 far too big and clunky now!Also 0603 seems to be much cheaper.

author: NivagSwerdna
author_rank: Super Contributor
timestamp: July 10, 2016, 08:43:37 am
content: Quote from: tautech on July 10, 2016, 08:36:28 amThanks for that. I was intending tantalum on the output and the point about the spec of the inductor is well taken. The coilcraft on the datasheet is very expensive but I see your point.I think my major problem is the slowness of the parameteric filtering on the supplier web sites, or maybe I need to be more patient.

author: KL27x
author_rank: Super Contributor
timestamp: July 10, 2016, 08:14:12 pm
content: QuoteYou made me do a double take.I bought a few of these inductors from Mouser (US) within the last week. Along with some SRN4026-4R7M. To evaluate and replace other obsolete inductor in one of my circuits.So I check today, and SRN4012 is gone from Mouser, just like that...The SRN4012 is very very short/flat, like shorter than an SOIC chip. So I suppose it would save some space in certain situations. The 4026 actually has a very slightly smaller footprint and is still shorter than my output cap and diode. And it is pretty robust, physically.I actually never got around to testing the SRN4012, and now I guess I'll just bin it. The 4026 works great for me and is still showing in production.QuoteHey, let me know how this works for you. What kind of max sustainable output you can get. I have tried finding a cheaper alternative for LT1308. So far, 1308 is still king of the hill for output current. Maybe I'm not so good at pcb layout, but I am able to get somewhere around a quarter to half the rated max current for any of these boost IC's. So it looks like a lot of cheaper alternatives are out there, but I can't get the output.With the 1308, here's my results with a 20 ohm load at 5V out (250mA): it will work down to 3V, 750mA input current or so, if it has 3.6+ish volts float voltage available for startup. About 475mA input at 3.65V to sustain ~250mA @ 5V out. Now I'm curious how bad is my circuit. I can probably reliably get around 300mA output on a fresh battery, but 250mA is about the practical max for it to be able to start up when the battery is low.I tried MIP2288, and I almost bought MCP1640... but if my track record vs boost IC continues, I will be lucky to get 100mA output at 5V.

author: batteksystem
author_rank: Regular Contributor
timestamp: July 15, 2016, 06:00:57 am
content: Quote from: Howardlong on July 09, 2016, 09:50:01 pmI used to get free samples from the local coilcraft offices in hong kong as an university student.

author: charlespax
author_rank: Regular Contributor
timestamp: July 15, 2016, 08:32:00 am
content: Quote from: NivagSwerdna on July 09, 2016, 04:02:43 pmMy first step is usually to see what is being used on breakout boards from SparkFun, Adafruit, Seeed, etc.. If they have a breakout board that basically does what you're trying to accomplish, see what inductor they're using.You can also check prices on taobao.com for you part you want to use. If you ever want to graduate to making assembled boards in China, you'll wan to use something common there. A parting being cheap and plentiful on Digikey can turn into being rare and expensive in China.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: July 15, 2016, 09:19:19 am
content: Quote from: NivagSwerdna on July 09, 2016, 04:02:43 pmFirst off, you go to Digikey and use their parametric search, which is way more useable than Farnell ( even if you're going to buy from Farnell)Filter by value, in stock, select a sensible range of saturation current ratings, and cut tape assuming you're using SMDs in less than reels.Enter your build qty and do sort by price.That should get you to a sensible-sized list to look through.