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Building a comparator

Discussion in 'General Electronics Discussion' started by LectricCircuit, May 22, 2017.

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  1. LectricCircuit

    LectricCircuit

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    Apr 7, 2017
    wondering if anyone could suggest a schematic I can use to build a comparator circuit that will turn on a LED at 8.4v when two lithium ion cells are done charging in series. Thought of using a couple Zeners in series but that way produces a gradual glow as the voltage rises and isn't as precise. A comparator would be off until the on-state voltage is reached and then will turn on without the messy gradual glowing effect. Problem is I'm fairly new to electronics and I'm able to follow schematics and such but I don't know how to create my own circuit off the top of my head with all the equations and such. I have already built the charger and it charges the battery great and stops at 8.42v. Just looking to add the charge complete LED to my existing circuit if possible.
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

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    Have you tried your favorite search engine? Just enter e.g. 'comparator schematic' ynd your monitor will be flooded with hits.
    Tip: Zener diodes make miserable references for use in a decent comparator circuit, The're just not precise enough in most cases.
     
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  3. Audioguru

    Audioguru

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    Sep 24, 2016
    You need to learn about charging a Lithium battery. 8.4V is two fully charged lithium cells.

    The charger is supposed to be a balanced charger so that one cell is never over-charged. A balanced charger monitors the voltage or each cell and bypasses current around the cell that is detected to be fully charged first.
    The charger MUST limit the current and limit the voltage to 4.20V per cell.
    When a cell voltage reaches 4.20V then it is not fully charged, it is about 75%. The charging current drops to a low amount when a cell is fully charged, then the charger should detect it and turn off, lighting an LED.

    You should use a Lithium battery charger IC that does all that stuff and more. It detects if the battery has been discharged too low which might cause it to explode or catch on fire if it is normally charged so it tries charging with a low current and if the voltage does not rise then it quits and gives an alarm.
     
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  4. BobK

    BobK

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    It amazes me that people with little knowledge keep trying to build LiIon battery chargers when the likes of Apple, Samsung and Boeing can't get it right.

    Bob
     
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  5. LectricCircuit

    LectricCircuit

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    Apr 7, 2017
    Can't learn if you don't try. Can't think of any successful person that would say "oh I shouldn't try this because some big company can't even get it right". I see nothing wrong with trying to learn and experiment to gain further knowledge. Knowledge is power and if you have the mentality of giving up or not trying something new to gain knowledge I feel sorry for you. That's what these kind of sites are for is to ask people for advice and help. Not to degrade someone for trying something outside of their normal capabilities.
     
  6. LectricCircuit

    LectricCircuit

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    Apr 7, 2017
    I actually just started looking up datasheets for different lithium ion charger IC's. This is just becoming a hobby if mine so I'm learning more and more each day. They have some pretty nice IC's that have some very useful functions for charging. I'm trying to learn all I can so I can stay safe and not blow up or set my house on fire so thanks for the advice and I am going to go with an IC. I do have a current limiting resistor in my circuit allowing 800ma charge rate but the part I do not have in my circuit is any kind if balancing function so I will not be using it until I either figure out how to add a balance function to it or get the suitable IC and start from scratch. Thanks a lot for the advice.
     
  7. bushtech

    bushtech

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    73's de Edd likes this.
  8. Audioguru

    Audioguru

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    Sep 24, 2016
    The Digikey article talks about charging a single Li-ion cell, not a balanced charger for charging more than one cell.
     
  9. BobK

    BobK

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    There are plenty of things you can learn about electronics that are less likely to set your house on fire. Charging Lithium Ion batteries is one of the most dangerous things you can do that does not involve high voltage, which is another thing I would not recommend for beginners.

    If you were just learning rock climbing would you try a solo free ascent of El Capitan? There is a difference between fearfulness and prudence.

    Bob
     
  10. bushtech

    bushtech

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    Sorry Audioguru, my bad. But it is a good starting point I think.
     
    LectricCircuit likes this.
  11. LectricCircuit

    LectricCircuit

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    Apr 7, 2017
    I am being safe about it. It's not like I'm just throwing some batteries on there and leaving my house letting them go unmonitored. I have a store bought charger that I use regularly I just wanted to build a charger just to do it. Also this isnt my first project. I've built many things prior to this. This is just the next level I'm stepping up to. This is also why I'm on here asking questions and looking up datasheets and reading articles and gaining all the knowledge I can before i decide I have a complete and final charger I can use with no problem. I charged the cells one time with the charger I built and monitored them the entire time and when they were done I used my multimeter and they're both charged to 4.2v. But even with that being said I have not continued to use the charger because I want more safety functions such as the balancing function that audioguru mentioned. I just wanted to make sure the circuit I had already made will in fact charge the cells. With fire extinguisher on hand. When I say I'm new to electronics I don't mean I'm just dipping my feet in for the first time. I'm just not a professional circuitry engineer so I don't know how to just come up with stuff off the top of my head as far as what value resistors and capacitors. I am assuming that with your first comment being directed towards people pretty much being dumb for trying to build lithium chargers when big companies can't even get it right, that you are knowledgeable about this sort of thing. Or at least I'd hope so if you're saying that, that you have the knowledge to back it up. But instead of belittleing someone for maybe not knowing as much as you, why not give some suggestions. Why not help them out. Tell them some ways to add safety functions to their project or suggest a schematic that you know works good. Explain to someone how it works. The whole purpose of life is to gain knowledge and information and then pass it on to the next person. Help someone out instead of telling them they're an idiot for trying. That's all I was trying to get at. I'm still young and I have a lot of years ahead of me and I love learning new things and I think it's a shame that people waist knowledge that's free for the taking. I like to achieve not give up. And if I climbed some mountains for a while and wanted to push myself to the next level you bet your ass I'd go for Everest. Especially if someone told me I couldn't do it. You will never feel victorious or accomplished if you don't push yourself to try new things. Nobody cares if you climb the Appalachian mountains a hundred times but you climb everest one time and people look up to you for your courage a bravery. But even though I think climbing mountains is a little different than building electronics lol I think I got my basic idea out there. I just want to keep learning as much as I can. To give someone knowledge is a lifelong gift that can be passed from generation to generation. So not to sound like an asshole but if you could help me out with some suggestions I would really appreciate it and you could feel good about yourself for passing on the information you learned from someone else but if you don't want to help me out than please don't waist your time or my time by belittleing me and pretty much laughing at me because "it amazes you that people try". There was a time that you didn't know about this stuff either until you gave it a try. Just asking for you to not look down on me because I haven't learned it yet.

    PS: sorry for ranting I just get upset when I see someone trying something new and someone that knows about the stuff won't help them but instead tells them they're stupid for trying. Especially when it's me that is the one trying something new.
     
  12. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    ... but you don't have to repeat the mistakes others have already made - and learned from these mistakes.

    One entry for learning about different battery chemistries and their properties is battery university. They also have a section on charging Li-Ion batteries.

    Then don't do it.
    Bob's advice is sound. Houses have burnt down from exploding Li-Ion batteries. Instead of going into 'rant mode'. take some time to consider your request and the answers you've been given. Try to take the viewpoint of an outsider and looka at both your post and the answers in an impartial way.
    I'll attribute this rant to your youth. But please keep in mind that advice from someone with much more experience than you currently have should always be taken into consideration. You're free to take your own route, even free to make or repeat mistakes, but you should respect the other forum members.

    You could have made your intention (going to learn something) clear in the original post instead of just asking for a specific circuit. This is a point we quite often come across: Instead of presenting their problem and asking for help, people often ask for a specific circuit which they think may solve their problem, but which in reality is not even the second best way of doing things.

    Thanks for reading,
    Harald
     
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  13. LectricCircuit

    LectricCircuit

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    Apr 7, 2017
    Well considering I was going off of a schematic that was used by many others I assumed it would be correct. And I am not interested in repeating mistakes others have made nor is that what I asked for. I asked for help on designing a comparator because from reading the material I read, that is what was presented as a solution hence why I asked for a comparator circuit design. If this theory is wrong all someone had to do was say hey you should go about it this way instead. But no I'm not going to listen to someone telling me to give up and not try. If you have knowledge about this sort of thing why not pass it on in a polite way to encourage a young man to go about it in a safe way instead of laugh at the person for not knowing as much as you. If it would be better for me to ask in a general way instead of a specific circuit than just say that from the get go and suggest something you have in mind. I just don't see the sense in discouraging a young man and telling him not to learn. Even if you were to say I think you should read the article on this website.... Or here's a link to a lithium ion safety article you should check out before building something like this. That would be much better than turning them away is all I'm saying. You have years and years of knowledge. Share it with the younger generation so we can pass it on to our kids and so forth. Idk maybe we just have different views. Maybe it's my young age maybe it's your older ways who knows. I don't know why it's so hard now in days for people to lend a helping hand. Like I said any help suggesting any ideas is appreciated. That's what this was all about in the first place was me asking for help. If not it's fine I'll just keep doing my own research and keep trying it on my own.
     
  14. Harald Kapp

    Harald Kapp Moderator Moderator

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    Nov 17, 2011
    That's the problem with unreflected use of unchecked "information" from the internet.
    "Eat sh*t, millions of flies can't be wrong" ;)

    Nobody is laughing at you - nor at any one else here. I'm sorry if you got this impression. The friendly people on this forum all happily share their experience. All they expect - as you do, too - is respect. Not rants.

    Unfortunately this is a behvior common to many people: asking for a specific circuit they have in mind instead of presenting the challenge they're facing and asking for ideas how to solve the issues at hand. In your case you could have asked how to charge a Li-Ion battery. We could then have come up with some ideas, maybe after requesting some more details about the battery.
    We face a dilemma here: In similar cases we got nasty replies from a poster who complained that we didn't answer his original question but presented an alternative solution.

    Sorry if you feel being turned away. I don't think that was the intention of the other repsonders in this thread. Li-Ion can be safe or dangerous, depending how you handle ist. I think that's what the other answers were about.

    I gave you these links.
    Besides, you could have gone into more detail about your project/studies in your post #5 and you could have asked for that kind of information. This would have shown us that you are considerate and willing to learn and the thread could have taken a completely different route.

    Going into "rant mode" is not going to get you along. Neither on this forum nor on any other (imho). Stay cool - you're going to get help here :cool:
     
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