Help deciding about optocoupler - Page 1
Forum section: electronics / beginners

author: nour
author_rank: Frequent Contributor
timestamp: February 15, 2016, 05:13:31 pm
content: I have a couple of incidents that I have blown up a microcontroller because of the input/out misusing (by applying high voltage to IO pins)So I want to add optocoupler isolator to my input/out pins which I am using in my prototyping and learningthe basic Idea is to add optocoupler that can handle switching mosfet/bjt or read input from outside in reasonable speed(not sure how much would that be!)this is basicly will be for just my development/learning boards, I want them to be idiot-proofif there is any better ideas regarding protecting the IO pins please don't hesitate to add yours, may be mine is just too bad or not applicable (I am not an expert after all)of course I am looking for a cheap optocoupler that I can buy in quantities like 50 or 100 pieces (max)preferable from tme.eu (because they are the cheapest supplier for me right now and the fastest)

author: masoudium
author_rank: Contributor
timestamp: February 15, 2016, 05:57:43 pm
content: That's what I use for my projects:http://www.tme.eu/en/details/tlp181bl/smd-optocouplers-transistor-output/toshiba/

author: nour
author_rank: Frequent Contributor
timestamp: February 15, 2016, 06:17:25 pm
content: why this one, do you have a specific reasons also it is expensive to buy 100 piece almost 32 $(that's too much for some optocouplers)

author: masoudium
author_rank: Contributor
timestamp: February 15, 2016, 08:14:20 pm
content: Oh, maybe that's not good for you. I use them because I don't have to buy them as I desolder them from the unusable boards that I have.

author: uncle_bob
author_rank: Unknown
timestamp: February 15, 2016, 08:30:29 pm
content: Quote from: nour on February 15, 2016, 06:17:25 pmHiThat's a pretty cheap opto coupler.What is your price goal?Bob

author: michaeliv
author_rank: Frequent Contributor
timestamp: February 15, 2016, 09:19:32 pm
content: You're unlikely to find very cheap outside of delivery from China.Cheapest would be aroud 3c eachhttp://www.aliexpress.com/item/50pcs-Pack-PC817-EL817-817-DIP-Optocoupler-Electronic-Accessories-Free-Shipping-A022/32587756016.htmlDepending on your application you could also use resistor + Zenner to ground or resistor + Diode to VCC

author: nour
author_rank: Frequent Contributor
timestamp: February 15, 2016, 09:29:13 pm
content: http://www.tme.eu/en/katalog/optocouplers_112291/#id_category=112291&s_field=niski_prog&s_order=ASC&page=1I just opened the optocoupler page on tme and arrange them asccan someone tell me about the first 20 if I can choose any of themalso can someone please tell me why he chosen whatever he have chosenI mean what is the criteria?No ali Family please

author: MT
author_rank: Super Contributor
timestamp: February 15, 2016, 09:29:49 pm
content: Quote from: nour on February 15, 2016, 05:13:31 pmThere are loads of cheap optos, CNY17 kind of cheap allrounder... Motorola once had a some type of buss isolator chip cant remeber the number.Why not simply use loads of NPN as I/O. Or buss transivers with open drain.http://www.tme.eu/se/details/cny17-1-l/optokopplare-transistorutgang-tht/liteon/cny17-1/Since you dont know your speed use below, else figure out your on/off switch speeds most cheap ones hovers around the usec range anyway.http://www.tme.eu/en/details/el817/tht-optocouplers-transistor-output/everlight/

author: uncle_bob
author_rank: Unknown
timestamp: February 15, 2016, 09:51:40 pm
content: Quote from: nour on February 15, 2016, 09:29:13 pmHiThe prices on that list are roughly 1/3 of what they are at places I would trust.Be careful....Bob

author: nour
author_rank: Frequent Contributor
timestamp: February 15, 2016, 10:01:13 pm
content: Quote from: uncle_bob on February 15, 2016, 09:51:40 pmwhat do you mean??TME suppose to be one of the trusted places to buy components right?

author: Muxr
author_rank: Super Contributor
timestamp: February 15, 2016, 10:12:41 pm
content: I think TME is good, lots of people have bought stuff on TME and I haven't heard of an issues yet. Although being from States I have no reason to shop there so I don't have first hand experience with them.I think if you're dealing with multiple channels you might want to look into multichannel optos to save some money that way.Quick search at the place I buy components shows this guy:http://www.mouser.com/ProductDetail/Lite-On/LTV-846/?qs=sGAEpiMZZMteimceiIVCBx3Y662pemTNzbYJ80qgGrs%3d4 channel for 70c a pop. 11.5c per channel compared to the one listed above. There is also an SMD version which costs 1c more. And you get a pretty nice break if you order in quantity.

author: masoudium
author_rank: Contributor
timestamp: February 15, 2016, 10:20:53 pm
content: Quote from: nour on February 15, 2016, 09:29:13 pmWhat do you mean by these sentences. speak clear please.

author: nour
author_rank: Frequent Contributor
timestamp: February 15, 2016, 10:35:12 pm
content: Quote from: masoudium on February 15, 2016, 10:20:53 pmI thought it was clear enoughanyway if someone choose a particular optocoupler I would like him to declare the reasons, what criteria he decided to use to choose this particular part(isolation level, transfer ratio, voltage, extra)

author: uncle_bob
author_rank: Unknown
timestamp: February 15, 2016, 10:41:48 pm
content: Quote from: nour on February 15, 2016, 10:01:13 pmHiOk here is what I mean90% of the time when you see prices that are 1/3 of the correct prices you are dealing with fake parts or something similar.Since I have no absolute proof that they are indeed selling fakes, I try to be careful about exactly what I say.Bob

author: xtoffer
author_rank: Contributor
timestamp: February 15, 2016, 10:49:12 pm
content: I believe that unless you have specific requirements on isolation characteristics, speed, current draw etc.. it doesn't matter so much which you choose. A combined row of them sounds convenient though, and if you only need either output or input it shouldn't be too messy.As an alternative (depending on isolation requirement of course) I would suggest to have a look at if the micro has clamping diodes on the I/O pins. If so you might get away with a resistor in series as decent protection.

author: masoudium
author_rank: Contributor
timestamp: February 15, 2016, 11:13:18 pm
content: This is what you asked:Quote from: nour on February 15, 2016, 05:13:31 pmYou haven't mentioned any quantitative specification for your need.I introduced you the solution that I use for the exact situation (Switching transistors with uC). I didn't know the price as I'm using used parts, and It's not important as I use them for my prototype projects. First make things clear with yourself.PS. Don't ask your questions in a third person way when I'm present here.Quote from: nour on February 15, 2016, 09:29:13 pmand I'm still not sure what you meant by "ali family"?!

author: uncle_bob
author_rank: Unknown
timestamp: February 16, 2016, 12:22:15 am
content: HiOk let's back off to some very basic stuff. Opto isolator 101:These beasts started out as discrete parts. You have an LED that drives a photo transistor. You put a couple of resistors on it and it (sort of) works. It may not be as fast as you need (how fast do you need?). It may not be as low current as you need (?). It may not hit the logic levels you need (?). It's a DC input device. That may be ok (?). It has various breakdown and current limits.Next up were darlington based parts. They have much better gain. They do not saturate as well. They generally are slower parts.After that people started making logic output parts. In goes a LED and out comes a CMOS level. Generally more money. Often less fuss. Could be a bit faster than some of the others.After that the odd stuff starts to show up. Devices that are set up to have well defined transfer ratios for analog applications. Stuff that will drive a TRIAC. Parts with unusual logic outputs (rare).Throughout all this, you will pay for speed. 150 megabits will be more expensive than 15 megabits. 1.5 megabit costs still less. That keeps on going as your bit rate drops. With some parts the "advertised" bit rate is quite hard to actually get. With a logic output part it might be pretty easy. When there is a single letter between a $15 part and a $1.50 part ... watch out for what you get.That's a quick walk through. They all have some common features:Both sides of the coupler need power from somewhere. It may be current in an LED, but it is still power. They all have a voltage isolation spec. It often is dependent on board layout as well. If you *need* 5KV, check the layout requirements.There is no such thing as a "one size fits all" isolator. One in a really small package will never ever hit a high isolation voltage. A high gain part is not going to be as fast as a good low gain part. Your application will dictate what you need.Bob

author: wraper
author_rank: Unknown
timestamp: February 16, 2016, 01:17:10 am
content: Unless you need something high speed, EL817 or any other kind of PC817 would be just finehttp://www.tme.eu/lv/details/el817/transoptori-tranzistoru-izeja-tht/everlight/Most of other optocouplers will perform about the same. You can safely use them up to about 10-20 kHz frequency.If you need something with higher speed, price will increase by order of magnitude, H11L1 - H11L3 are among the cheapesthttp://www.tme.eu/lv/details/h11l1/transoptori-logiska-izeja-tht/fairchild-semiconductor/h11l1m/

author: wraper
author_rank: Unknown
timestamp: February 16, 2016, 01:29:12 am
content: Quote from: uncle_bob on February 15, 2016, 10:41:48 pmThey are not fakes, TME is a valid distributor. Those are not "correct prices" but the result of the greed of many other distributors. If the part is actually expensive, it will be expensive at TME too. But if the part is actually cheap, you can get it with much less margin from TME.

author: nour
author_rank: Frequent Contributor
timestamp: February 16, 2016, 06:12:33 am
content: Quote from: masoudium on February 15, 2016, 11:13:18 pmit is not like I am in a court of law and I faked some facts, it is an open discussion and I have asked about the specs in the second reply, it is not like I came after a 100 entries and out of the blue changed the topic to how car engine worksplease be more flexible, and it is not like I am questioning you, you have added your valuable reply, so thanks for the information, if you want to enrich me with more knowledge, I will be happy to listen from youQuote from: masoudium on February 15, 2016, 11:13:18 pmcalm down man I feel heat in the talk, you think you are the only person in the topic, I have wrote this talking for every person who might write reply later, I didn't mean by (he) you, I meant the person that will add entry in general not youabout ali family I will let you discover this yourselfuse google and you will someday know what I am talking about (or read the member's replies in order and you will understand)Hope I have cleared any misunderstanding

author: nour
author_rank: Frequent Contributor
timestamp: February 16, 2016, 07:12:23 am
content: Quote from: uncle_bob on February 16, 2016, 12:22:15 amthanks a lot for your reply, you have cleared some misunderstanding for me about those small devices, I think I will reconsider choosing with another standardsI think I will probably end up buying 2 or 3 categoriesHigh speed with-in megabits --> will be expensive which means I will buy small quantitieslow speed for slow applications that require switching with the range of 10 khz or lessQuote from: uncle_bob on February 15, 2016, 10:41:48 pmyes I agree with you, but that apply to place like ebay, amazon and of course aliexpress, but not for will known distributor like TMEI have bought from them several times, they are very good, may be not enough to be 100% sure but I can for sure say they are good(hopefully)I actually do this all the time when buying from ebay, if for example saw some item being sold on say digikey with 49 $ and on ebay you can get it with less than a 1$at this moment I will just close my eyes and pretend I have never seen this item beforehere is an example with LM338from digikeyhttp://www.digikey.com/product-detail/en/LM338K%20STEEL/LM338K%20STEEL-ND/6224from Chinese seller from ShenZhen, chinahttp://www.ebay.com/itm/311418393584?_trksid=p2057872.m2749.l2649&ssPageName=STRK%3AMEBIDX%3AITand you have all the freedom in the world to guess what they have added inside the IC package

author: Muxr
author_rank: Super Contributor
timestamp: February 16, 2016, 07:38:25 am
content: Quote from: nour on February 16, 2016, 07:12:23 amSome are original parts but salvaged from old gear.

author: nour
author_rank: Frequent Contributor
timestamp: February 16, 2016, 07:44:33 am
content: Quote from: Muxr on February 16, 2016, 07:38:25 ammay be but the list is declared as NEWhave you tried this one before ?

author: Muxr
author_rank: Super Contributor
timestamp: February 16, 2016, 07:56:28 am
content: Quote from: nour on February 16, 2016, 07:44:33 amHaven't tried this particular part. But that's been my experience with some components from China. If it's significantly lower in price it's either fake, salvaged, or just a surplus that made it into the gray market. But more often than not the first two.I am familiar with Alice11... seller on Ebay. He's quite famous. Sells a lot of goods.

author: nour
author_rank: Frequent Contributor
timestamp: February 16, 2016, 08:59:23 am
content: this particular item from this particular seller is fake down to the bottom of the ocean of fakenessI have once bought this LM338 from him, I bought like 5 or more, I don't really remember, was long time agoguess what, when building the circuit it will work just great but the moment I put any noticeable load on the output it stop working instantaneously and foreverI remember the printing on the package was easily removed by just scraping it by your fingernailof course I just throw all of them away in the garbage never bother test all of them actually and from that moment I have learned my lessons and do my homework before buying from any seller, at least from chinait was a real waste of time and money