How to look for parts - Page 1
Forum section: electronics / beginners

author: cuccureddo
author_rank: Newbie
timestamp: April 26, 2010, 08:55:17 pm
content: Hey all.I'm developing my very first circuit, nothing complex, just a first attempt.Anyways the thing i'm struggling the most with is looking for parts. How am i supposed to do that? Let's say i need like an opamp or a binary counter how do i look for it? I tried going on digikey or mouser but i got literally overwhelmed. I've been asked for informations that, at this stage, i dont know/havent tought yet like temp range, package format and all sort of things.And even if i refine the search parameters enough to get a list then i have to read through something like 20 datasheetsI mean it cant be right can it? Is it just like that or am i doing something wrong? Is there a "catalog" i can use which is divided into sections like for example digital circuits, amplifiers, semiconductors etcIt took me quite a long time just to find a simple 7490I know this is a very noobish question, but i feel... lostthank you

author: allanw
author_rank: Frequent Contributor
timestamp: April 26, 2010, 10:17:04 pm
content: Try sorting by price or by quantity available. If you don't need a very specific op-amp for example, just picking a popular one that has the performance you need is probably good enough. Otherwise it's basically impossible to pick from among the thousands of IC's that perform the same task.

author: EEVblog
author_rank: Unknown
timestamp: April 26, 2010, 10:49:29 pm
content: Unfortunately that's pretty much what design engineers spend most of their time doing, trying to find the most suitable part for the job.You'll generally have enough basic requirements like package type, speed or power consumption etc in order to get the list down to a manageable first level. But for common parts this can often still be hundreds or thousands of parts as you've discovered, so you need to whittle it down some more. You can do this by selecting "in stock" parts only, and then you'd sort by price and maybe some extra fields like power consumption. Even with hundreds of parts still left, you might pick purely by price.And you'd also look for it being a generic part number too that is available from a bunch of different manufacturers.Then it's common to search through say 5 different datasheets before finding a suitable part.Your example 7490 is usually pretty simple, as generally they are "jelly bean" parts that perform the same across all manufactures, so it won't matter which one, just pick the first on the list. So you'd just search by package type, in stock, and price. You might then use octopart.com or findchips.com to make sure it's available from different suppliers.Dave.

author: DJPhil
author_rank: Frequent Contributor
timestamp: April 26, 2010, 11:10:10 pm
content: As I understand it, even for the professionals this is a pain, and often constitutes the largest portion of design time. Don't feel bad, this is one of the more difficult things to master. It's further complicated by the speed that ICs are constantly being revised, discontinued, or released for the first time, and that makes it hard to stay current.Thus the concept of the 'jellybean' is born. When you want to breadboard a circuit to test an idea or slap something together quickly to solve a problem it's a good idea to have some inexpensive standbys handy. These get used when you simply need 'Opamp, Mk. 1, standard grade' and are usually very cheap. The goal is to always have a few handy, so you keep a stock and replenish it like you would a resistor of a commonly used value. This doesn't always work well if what you're doing requires high performance, but if you're hobbying your way along it can be a good way to start.Engineers (or hobbyists, labs, companies) that have different areas of focus or specialization will have different ideas of what makes a good jellybean opamp, for example, because different component trade-offs are valuable to them. Someone who's doing a bit of everything might have a dozen or more different jellybean opamps to cover their bases.Being a hobbyist, I personally like this way of doing things as it seems like a fair system for working in the free sample services of the manufacturers. I've got no qualms about asking for a sample of a dual voltage, low dropout, low quiescent current regulator with independent standby and soft power on, but I can't keep a straight face when thinking about asking for samples of an LM317 for 'evaluation' that's been in production for years and years.(P.S. I just made up that regulator, so nobody ask me where to get one)Discrete logic is harder to do this with due to the overwhelming variety of tasks you'd have to cover. Others can likely give good advice on this, I've got no real experience in digital logic or mixed signal parts.You can also look for circuits online that perform similar tasks to the one you're designing for and see what parts they use. Failing all else, you can always ask for advice here! If you describe what you're trying to do I'm sure there's someone who has the experience to recommend a few part numbers.Some of my jellybeans:Opamps for audio = assorted TL07X and TL08X (where x equals 1,2,4 for single, dual, quad), LM833Opamps general = LM324 quadOpamps precision = OP07Comparator general = LM339 quad3 pin Vreg = LM317T positive and LM339T negative (TO-220), LM317L (TO-92), and a handful of fixed 7805 and 7812Oscillator = any 555 clone, any 556 clone (dual 555), I'm using NE556To everyone: what do you guys keep handy at all times?Hope that helps

author: cuccureddo
author_rank: Newbie
timestamp: April 26, 2010, 11:29:52 pm
content: Thanks guys, that was helpful.I'm still kinda lost and overwhelmed, but at least i know i'm not aloneI'll try to follow your tips and see if i can shorten the list a bit. I mean if i have to dig like 5-7 datasheets that's quite doable, but 20 for each IC no wayYes asking around is another solution, but i would like to keep that as a last resource because it's a bit like cheating. By that i dont mean that is a shame to do so, just that you "sweat" less hence you learn less. But ye sometimes it's the only wayThanks again

author: Unknown
author_rank: Unknown
timestamp: April 27, 2010, 09:39:19 am
content: I agree that if you don't have any exotic requirements (which you can use with parametric search), you should stick to jellybean parts, like the op-amps that DJPhil lists, the LM324 is also known as the LM358 (dual). For transistors, the jellybean ones are 2N3904/2N3906. For diodes, 1N400x (50/60Hz rectifier), 1N4148 (fast, low-current). Just look at what appears frequently in schematics, these components are usually cheap, easily available and made by multiple manufacturers. Sometimes a designer will use an exotic part that they happened to have in their drawer, watch out for that.I find it sometimes helpful to sort the Digikey search results by number in stock (I don't believe Mouser lets you do that), this will often separate the popular parts from the exotic parts. Up front, exclude everything made by Maxim except the MAX232.

author: ElektroQuark
author_rank: Unknown
timestamp: April 27, 2010, 10:08:48 am
content: Quote from: alm on April 27, 2010, 09:39:19 amWhy?

author: mkissin
author_rank: Regular Contributor
timestamp: April 28, 2010, 05:05:15 am
content: Quote from: EdoNork on April 27, 2010, 10:08:48 amI assume he's talking about price. Their MAX232is listed on Digikey as US$3.31 each, while the TI version is listed at US$1.16 each (part numbers MAX232CPE+-ND and 296-1402-5-ND, respectively). However, Maxim also makes some extremely specific and high-spec components, which aren't as easy to get cheaper alternatives for. Similar things can be said for Analog Devices, Linear Technology, Burr-Brown (owned by TI now), and so forth.Most basic components can be second-sourced, meaning that more than one manufacturer makes pin-compatible parts (sometimes with slightly different part numbers). For example, a whole bunch of manfacturers make 1N4141 or 1N914 diodes, all of which are roughly equal. However, some of these are not quite perfectly equal and will differ in some of the ranges of the specs, thus, if you are using a part at the extreme of its range, it pays to be very careful and make sure that you refer to the datasheet of the manufacturer that actually produced the version of the part you are using.For a specific example, take the 1N4148 diode. Digikey lists current stock from 6 different manufacturers, ranging from 26c each down to 12c in single quantities. However, they are not precisely the same. The Fairchild Semi version lists a maximum reverse current at Vr=20V and T=150 degrees of 50uA. The Diodes Inc version guarentees a maxumum of 30uA under the same conditions.

author: ElektroQuark
author_rank: Unknown
timestamp: April 28, 2010, 07:49:31 am
content: Yeap.I had a problem with MAX232 and TI ones, they require different value capacitors. Reading the datasheet solved the problem but...

author: Unknown
author_rank: Unknown
timestamp: April 28, 2010, 09:30:12 am
content: Quote from: mkissin on April 28, 2010, 05:05:15 amI was talking about availability outside of samples. Many of their components are single-sourced, and are hard to get from distributors. Plenty of stories of designers that designed in a Maxim component, found out they couldn't get them in production quantities, and had to change the design to use some other component. I believe Dave has ranted about this in some past video blog. It often seems like the only Maxim component that's widely distributed (and second-sourced) is the MAX232. This is less of an issue for hobbyists, so it was somewhat tongue-in-the-cheek.Quote from: mkissin on April 28, 2010, 05:05:15 amAbsolutely true. It's usually good practice to design for the worst-specced version. Speccing 1N4148/1N914 usually just means 'a fast, low-power silicium diode', most designs don't depend on specific properties.

author: mkissin
author_rank: Regular Contributor
timestamp: April 28, 2010, 08:57:36 pm
content: Quote from: alm on April 28, 2010, 09:30:12 amAh, right. I definately agree with you on that. You need to be really careful when using parts that cannot be second-sourced. I got burned myself on that (along with what seemed like half the internet), when I was designing a realy simple function generator around the MAX038 chip. It was a really nice chip, so much more awesome than the XR2206, until Maxim decided overnight to discontinue it, along with what seemed like a third of their portfolio. I still have a couple of MAX038 chips sitting on my shelf, but there's little point in using them now.Quote from: alm on April 28, 2010, 09:30:12 amFixed that for youUnless you can afford to test every part that comes through your door for performance, assume each part will perform at its worst. Most of the time, you'll be pleasantly surprised, and the 50MHz scope you designed will run perfectly at 100MHz, but sometimes it will simply meet the originally spec'd 50MHz.

author: Simon
author_rank: Unknown
timestamp: May 02, 2010, 07:59:28 am
content: why not just tell us your requitrements and we can suggest a part (as I do when I'm after something new)