Buying components without getting specs. Problem? - Page 1
Forum section: electronics / beginners

author: Decoman
author_rank: Regular Contributor
timestamp: December 18, 2017, 08:24:39 am
content: So I have for the last few weeks ordered small stuff off eBay, like wiring, LEDs and mechanical buttons for use with an electrical circuit, but although I have taken care to take a screenshot of the spec's for such components when that info is shown, there seem to be stuff I have ordered that don't have any specs associated with them in the sales listing.I am now wondering, disregarding the iffy predicament of buying electrical components off some random seller on eBay, if it will be near impossible or perhaps too difficult, to work with such un-spec'ed components.I think I've learned that you can do a measurement (diode test) on a diode with a multimeter to get an idea of how much resistance the diode offer in terms of sucking up voltage, but what about contact buttons?Presumably, I would want to know, how they work (easy I guess), but also any ampere rating and voltage rating.Presumably, the amp rating for such parts would then have to be guessed, in cases where I do not have specs for the components I bought.Note, I do not yet know, if there are perchance specs included already in the packaging when buying some minor amount of components off eBay, which, if so, could solve my issue here in knowing what specs these components have.As a bonus question, I am also wondering how to work with Surface Mounted Device (SMDs) if not intending to use the SMDs with a board, but instead, trying to install an SMD button for an installation and not some board. Any hints as to how one can work with SMDs is appreciated, though I suppose I can try youtube for that particular issue of mine.

author: nicalitz
author_rank: Regular Contributor
timestamp: December 18, 2017, 06:33:45 pm
content: when it comes to ebay components, you get what you pay for really. the ratings and characteristics of components are determined through rigorous testing and simulation, performed at the manufacturer's expense. many ebay components come from less reputable manufacturers that simply 'copy the recipe'. they also cut corners to reduce costs, and often forgo testing altogether.if you buy a button, expect something that does what a button does. make electrical contact between two terminals when pressed. no more, no less. great for prototyping and playing around, not that great for professional work.as a rough guideline: if the voltage and current ratings of a part become significant, you are better of paying a bit more for high quality parts from a reputable manufacturer

author: danadak
author_rank: Super Contributor
timestamp: December 18, 2017, 11:36:00 pm
content: On he matter of working with SMD, lots of videos on youtube.Quite a few threads on eevblog on this topic, simply do some searches.Regards, Dana.

author: suicidaleggroll
author_rank: Super Contributor
timestamp: December 19, 2017, 12:01:03 am
content: Quote from: Decoman on December 18, 2017, 08:24:39 amThat's cute. Just FYI - it doesn't really matter if a "spec sheet" is provided with some knockoff component from a shady seller on eBay, which 99% of what you're going to find is. They copy the real part, cutting every single corner they possibly can, then cut and paste the real part's spec sheet and claim theirs is the same. Other than pinout it's basically useless. Unless you're specifically buying NOS components from reputable sellers with real supply lines, you should consider any component you buy on eBay to be a cheap knockoff with no real specifications or testing.

author: paulca
author_rank: Super Contributor
timestamp: December 19, 2017, 12:35:05 pm
content: The flip side of this story of course is that genuine components start out costing $1 in a Chinese factory. They get shipped to a wholesaler who sells them to retail for $5. The retailer then supplies them to you for $20.The cost of doing business in the west is huge compared to China. This is exactly why western companies do their manufacturing and often design in China. Then you have all the regulatory stuff, the marketing, the advertising, the colourful box, the fancy websites, the support call centres....When you order them from ebay you are often just cutting out a load of that mark up and buying direct from the original manufacturer or a local wholesaler who also supplies retailers in your locale. (You buy them at $5). Even buying for a local importer of the same and paying a little more can be economical, especially given that if a batch start returning to that seller as DOA or not fit for purpose they will stop selling from that supply. So they act a little bit like a tat filter.The downside, as has been mentioned. You have virtually no way of knowing if you are buying a bargain cheap sourced genuine part or a cheap nasty knock off.So it comes down to, "How much do you care?". If you are playing, hobbying and doing amateur stuff where the cheap knock-off will probably work for you, then who cares.At the other end of the spectrum, if you are building a medical device for intensive care, or military weapons control system, completely different story.In the middle you get into the fights with the bean counters. If they spot a part currently sourced at $0.25 that can be got for $0.10 of questionable quality, they will take that fight to the engineers to try and get that component swapped in. Depending on the products market, the mean-time-to-failure hit they take might be worth it. It's a disposable society. If your kamagotchie or other gimmick device fails 6 months after Christmas, who cares?Also, lets not forget that a lot of the products we buy in the west today are actually designed, built and sourced in China. They are sold in ebay for $5 from China or $50 from a known retailer who put it in a lovely colourful box and do a bit of fancy marketing around it. It's still the same cheap Chinese tat. I got caught with this before after the product failed repeatedly, was extremely fragile and became too expensive to maintain I found out they are sold direct from China for less than a tenth what I paid for.

author: bd139
author_rank: Super Contributor
timestamp: December 19, 2017, 01:14:20 pm
content: I don't find ebay or China viable for most things. Recently I put together a prototype for an analogue front end on a project I'm working on. I priced up China and due to the "prepacked" nature of things (random lots of resistors or a million of one value), I couldn't get what I needed for less than RS locally.Total BOM from China was about £22. Total BOM from RS was £14.RS won because that's a rock solid supply chain, all the stock arrived next day and it was all branded and it was cheaper!I've had some total shit from ebay over the years as well. If RS sell me shit, they will fix it next day no questions asked. If you can test it properly then yes go for it, but how do you test a hooky Analog Devices log amp and is that worth your time?

author: nicalitz
author_rank: Regular Contributor
timestamp: December 19, 2017, 01:54:38 pm
content: this makes it all the more important to be 'savvy' when you're buying parts. if i go to my local hobby electronics store, i expect to get parts sourced from questionable chinese manufacturers. the retailer might have done some effort to get information on these parts, some technical specs, but i would have very little confidence that the information is accurate and reliable. i know that i can get the same parts shipped from china for much less, assuming im willing to wait a bit longer.on the other hand, you have components from reputable manufacturers, which you can buy from RS, Mouser, Digikey, etc. some of these parts are still manufactured in china, but the level of quality control imposed on these manufacturing facilities are much higher. also, sample parts are often tested to verify compliance to the technical specifications of the product. yes you have your military/space rated parts which cater for a niche market and are priced accordingly, but your typical consumer parts, which are used in everyday electronics, are often manufactured in astronomical quantities. this dwarfs the cost of imposing quality control, and drives down costs in a competitive market. even for electronic hobbiests, these parts dont break the bank, and are SIGNIFICANTLY better characterized and more reliable than your average ebay part.i always have a few ebay parts to play around with, but i rarely use them for serious projects. given the option of using a $0.01 capacitor off ebay or a $0.1 murata cap from mouser, id go with the murata 99% of the time. but to each their own i guess

author: paulca
author_rank: Super Contributor
timestamp: December 19, 2017, 03:08:14 pm
content: You can also bet, when a run of 1 million 10nF caps fails a few spot checks and get put in the reject bin, that someone in China will have them on ebay in bags of 1000 within 24 hours.

author: nicalitz
author_rank: Regular Contributor
timestamp: December 19, 2017, 03:17:30 pm
content: without a doubt. in such case, i would actually much prefer if ebay sellers were honest and disclosed exactly where the parts came from and what checks were failed. if i can make an informed decision, i would actually consider buying those parts.buy im sure the manufacturers explicitly prohibit the seller from disclosing this information seeing as it can harm their reputation

author: bd139
author_rank: Super Contributor
timestamp: December 19, 2017, 03:20:00 pm
content: It’s not even that. Sometimes they don’t know. They can change hands a few times as they go around the Chinese markets.

author: nicalitz
author_rank: Regular Contributor
timestamp: December 19, 2017, 03:26:00 pm
content: and im sure each 'hand' adds a markup. makes you think how little those rejects were originally sold for

author: bd139
author_rank: Super Contributor
timestamp: December 19, 2017, 03:38:05 pm
content: The manufacturer may have even paid someone to take them away.If it’s not powdered then it’ll end up back on the market.

author: bson
author_rank: Unknown
timestamp: December 19, 2017, 04:20:38 pm
content: And it turns out those SMD 10ohm resistors you bought are 1000ohm, and were discarded because they were mislabeled 100 instead of 102. So you paid $3.50 for 250 instead of $5 from Mouser, then waited 3 weeks for the China post package to arrive, and then as stuff doesn't work with them just give up and pay $5 for the real deal - plus $20 next day shipping. Plus you get to desolder and replace the mislabeled ones.

author: wraper
author_rank: Unknown
timestamp: December 19, 2017, 04:38:25 pm
content: Quote from: bson on December 19, 2017, 04:20:38 pmMouser is expensive for passives. At TME for $2.75 you can get a full reel of 5000 Royal OHM 5% resistors, for $4.5 reel of Vishay 1% resistors. Overseas express shipping with DHL or UPS $9.90. Add VAT to EU orders.

author: phil from seattle
author_rank: Super Contributor
timestamp: December 19, 2017, 04:40:48 pm
content: ebay/ali express parts are often rejects, sometimes counterfeit, sometimes genuine. You just never know. I will buy commodity stuff - switches, connectors, pin headers, ... off ebay. But if you want datasheets and some confidence in getting what you order, stick to Mouser/Digikey/... Often the prices are pretty close.For assemblies, ebay/ali/china can't be beat. For example, 320x240 2.2" color TFT displays for $5. They work surprisingly well.

author: nicalitz
author_rank: Regular Contributor
timestamp: December 19, 2017, 04:52:45 pm
content: Quote from: wraper on December 19, 2017, 04:38:25 pmi've never heard om TME, don't think many people down here use them. they seem like a very well established distributor. in short, how would you compare them to Mouser, Digikey? in terms of variety, stock, reliability, support, etc?

author: wraper
author_rank: Unknown
timestamp: December 19, 2017, 05:03:02 pm
content: Quote from: nicalitz on December 19, 2017, 04:52:45 pmThey have lower variety but prices are much cheaper. Especially for passives and other discreet components. They sometimes used ESD dissipative bags when ESD shielding bags should've been used. But this does not matter for passives.If compared with RS, their procedures seem to be better. For example, they used moisture barrier bags, humidity indicator card and desiccant for moisture sensitive components unlike what I've got from RS.

author: bd139
author_rank: Super Contributor
timestamp: December 19, 2017, 05:15:45 pm
content: TME are pretty good. I can order from them at 4pm today and have stuff delivered to me tomorrow at 10AM. That’s Poland to U.K. If I order RS here at same time then it gets to me about 5pm. That’s same country.