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How do I find the right transistor?

Discussion in 'General Electronics Discussion' started by Mongrel Shark, Jun 6, 2012.

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  1. Mongrel Shark

    Mongrel Shark

    Jun 6, 2012
    I have been playing around with Joule Thief's for the last few weeks. I am very interested in getting the supply voltage as low as possible. I can get light (not much) from around .2v using an ac141k germanium transistor. It's rated pretty high though. Has a pretty narrow range I can adjust with the pot (50k) I use to control the base. Same thing happens when I use high value silicon transistors in most joule thief circuits I have made.

    So I need a few low value germanium transistors. A germanium equivalent of a bc107 or 2n3904 would be a great start.

    But is there some way I can search for this without viewing hundreds of data sheets. That often don't have the low thresholds (ie most of the information I want is not on data sheets) on them....

    Surely someone much smarter than me has been frustrated by this and made a chart or search tool or something?

    The catalog for my local tech store has a good chart, that shows all the transistors in a logical order. But they don't stock Germanium or low value silicon (had to mail order bc107).

    I've been staring at data sheets for three days. My brain is melting! Meeellllllttttinnnnngggg!

    Can anyone offer some advice?
  2. davenn

    davenn Moderator

    Sep 5, 2009
    hi there
    welcome to the forums :)

    Germaniun transistors are pretty much a thing of the past
    You may be extremely lucky to find the rare "new Old Stock" supplier that may have some tucked away.

  3. Mongrel Shark

    Mongrel Shark

    Jun 6, 2012
    I can get tons of them on ebay.... I just need to work out which ones I want. Some can be a bit pricy.. Like I said the Data sheets aren't that helpful. Tells me what will cook 'em. But not what they need to run. Some show losses, which is a little helpful. I can geustimate from what the overload is, and get in the ball park.

    Problem is there is no organization. If I was getting transistors from my local store. I can just look up or down the chart as I want higher or lower values. Check the ratings, and make an educated guess. normally gets me pretty close.

    Anyone got a link to an old germanium chart or something? I don't need to know exactly the right bits. playing around and working that out is the fun bit. I just need to get in the ball park...

    Like I said. looking up endless data sheets is making my brain melt...
  4. Mongrel Shark

    Mongrel Shark

    Jun 6, 2012
    Well after 3 days of trying not to ask stupid questions on forums.

    I finally caved and asked my question.

    Only to find this a few min later.


    Anyway problem solved. That was quick.

    I think I may hang around for a bit anyhow. been looking for a decent group of more experienced techies to answer the odd question.. I may even be able to help someone else...
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    Yeah, from you post #3 above, I would have directed you to the sticky at the top of th datasheets section which gives a lot of information about searching for this information.

    But it looks like you're already there. :)
  6. john monks

    john monks

    Mar 9, 2012
    I face the same problem at work. I finely gave in. I downloaded tons of data on thousands of transistors and op amps into a database, and wrote my own program to search for parts meeting my own criteria. This proved to be a big time saver for me. The problem you face is that new transistors are coming on line all the time and you can't keep up. And you don't want a transistor that just works, you want the best transistor.
  7. Mongrel Shark

    Mongrel Shark

    Jun 6, 2012
    Anyone out there interested in getting the manufacturers, some of the datasheet archives, and maybe some of the bigger retailers to all co-operate on an open source project like this?

    So we can go to a website and enter in the criteria we need, and get it narrowed down to a few hundred options rather than hundreds of thousands of data sheets, with only one or two lines that actually have important info...

    I've lost track of the times this week I have downloaded a data sheet. only to scroll down to page 3 and find I just wasted 5 min. Seriously probably wasted 20 hours this week. No wonder it cost a fortune to get a circuit designed. your paying an engeneer top rate to google stuff for hours on end....

    Sounds like it would make things easier for everyone once it's done. As well as improving sales.

    It's a bit like rule 35. If I can't find it I must create it.

    Only I don't have more than a few hours a week to donate to the project...

    Anyone else interested in throwing a few hours a week at this?

    And you want it in less that 4 hours searching! And you certainly don't want to have to buy and try 50 different parts. Although this is probably why the manufacturers haven't done this already... But How much does it cost the industry in total labor hours?

    This is not the only forum I have asked this question on. three days now and still not a decent, acceptable answer... I thought you electronics people where smart? Why do you all put up with this?

    No wonder it's so hard for nOOb's to get started....
  8. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    You just pick one that will do the job.

    There's no point looking for perfection, it never exists.

    For a non-critical task (and I suspect this is) you just choose something almost at random.

    If you're relying on the limits of the specification of some device then you're probably not doing it right.
  9. Mongrel Shark

    Mongrel Shark

    Jun 6, 2012
    For my current project. I want something that will run on the lowest possible volts. while giving me a bit of frequency adjustment from 30hz up to around a khz, maybe a little higher.... I don't mind swapping transistors to get different hertz ranges.

    So listing the burnout specs is pretty useless....

    Like (*steve*) says, if your relying on the limits of some specification, your probably doing something wrong.

    I'ts not critical now. During R&D stage. But I want to enable the third world (and everyone else) to be able to have lighting that is virtually free after a small set up cost. Making it simple enough that people not familiar with electronics can play around with it too.

    Right now I am at the start of what I hope to be an open source project that will make people all over the world less dependent on centralized power networks.

    My big problem finding Semis, is the specs I want are just not listed. Guessing is just amateurish, fun, But amateurish and time consuming. Why can't I just type what I want into a search and get most of the junk eliminated?

    How much money and time would this save big engineering firms?

    If the manufacturers and retailers want me to buy their kit. I need to be able to find it!

    Datasheet archive and the likes are not very good unless you already know what part you want specs for...

    All I want to do is search by specs, not part no... Doesn't seem like a huge ask. I bet the manufacturers all have this info on a database somewhere. By not sharing the info they cost everyone else in the industry, an uncountable figure.

    It's like no one want's to sell transistors....

    All I'm doing is asking for help. On public forums that claim to exist to help people. Yet people keep trying to shut me down? Why is this?

    Are john and I the only ones that want to be able to find a part to do a job in less than a few hours?

    Seriously. 3 days searching and I still don't even know if I'm in the ball park. Germanium Transistors are too expensive to buy tons of them....

    I shouldn't have too!

    What happened to customer service?

    I go to a website looking to spend some cash. and come away downloading a LOIC and studying how to use it... Is that the sort of reaction you would want from people that could be your bread and butter?
  10. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    Well, how might it be done? I'm amazed that you have discovered what engineers have failed to realise for so many years!

    Let's say I'm after an N channel power mosfet with some particular characteristics. I quite like International Rectifier as a supplier, so I go to their site.

    I note with some excitement that they have a link for N Channel power mosfets.

    I click on that link and find that I can search for an N channel power mosfet by characteristics.

    Absolutely amazing. Why do they hide this information so well? Why don't people know about it? You've opened my eyes!!!
  11. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    But seriously, I'd rarely use this sort of thing.

    My zeroth option is to just select a device I'm used to working with.

    My first option is to look at what I've done before and to use the same part.

    Next I might look at parts I've used for similar tasks and check to see if they're sufficient.

    Next I might go to somewhere like Digikey to do a parametric search on parts from many manufacturers and see what the options are.

    Going to particular manufacturers sites is pretty much a last resort because I may have to go to several (they don't want to point you to competitor's products).

    In the days of olde there were books listing thousands of devices with cross-reference information. These days there are easier ways.

    Lastly you come to practical aspects. You might want a device that's a couple of cents cheaper because it will lower unit costs.
  12. CocaCola


    Apr 7, 2012
    In addition to what Steve (edit initially stated 2 post up) stated in many/most cases a generic transistor (you will notice some become real popular like say the 2n2222 ? ) will serve the purpose and it's simply counterproductive to waste the time finding that 'perfect' one unless absolutely necessary for the design...

    *** Edit damn Steve you tossed up the same idea 'use what you know works' as mine, just little sooner...
    Last edited: Jun 7, 2012
  13. Mongrel Shark

    Mongrel Shark

    Jun 6, 2012
    So Since I asked for help selecting a germanium Transistor. with low base voltage. (under .2v is ideal, lower the better) with a slow frequency 50-1000herzt range.
    If there is something non germanium that will work i'd be keen to try that too.

    Do you think you might actually be able to help me with my first search?

    I don't care If they don't sell it. I can find a part once I know what it is.

    I don't know how perfect I need until I have some idea of whats available...

    What do I type into what page? I looked at the links provided. They don't seem to help.... I asked for help with transistors, not mofsets,

    I'm sure once I've done it once I'll be fine. I just don't get why it takes so many posts on 3 forums. and still no actual help....

    I must say this forum has been the most helpful. But there is more of a don't ask questions attitude than actually offering help.

    I have been googling for well over 20 hours. more like 40. Clearly I am missing something.

    How about clearing that up rather than being rude?

    Sounds like there are a few people reading this that can help. yet 12 posts and a few links later... Nothing actually useful or productive.

    And this is the forum with nice people....
  14. Mongrel Shark

    Mongrel Shark

    Jun 6, 2012
    Obviously I'm new to semis. and trying to work with stuff older than me. so I don't have anything I've used before....
  15. Mongrel Shark

    Mongrel Shark

    Jun 6, 2012
  16. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    It is quite likely that there are no germanium transistors listed.

    You could start looking for mosfets with a very low Vgs(th) though. But you'd have to redesign your circuit to use them.

    What you're doing is looking for the perfect buggy-whip, unaware that whilst such things once existed, they are no longer needed by very many people and unless you're very very special, probably not needed by you either.

    edit: if you search digikey for "germanium" you'll find a duck egg.
  17. KrisBlueNZ

    KrisBlueNZ Sadly passed away in 2015

    Nov 28, 2011
    Mongrel Shark, I don't mean to sound nasty, but you really have got hold of the wrong end of the stick. The wrong end of two sticks, actually.

    First, there is very little demand for a complete list of all devices of any particular category from all manufacturers, and as a former professional design engineer I completely agree with Steve's comments. It is not usual for a designer to have an obsession with a specific parameter and no idea of which manufacturers are likely to make a part that fits the bill; in any case, companies like Digikey and Element 14 have a good range and good parametric selection guides for components that are still in production.
    The main factor in this case is that germanium transistors are no longer used in current designs (except perhaps in some extremely specialised fields that I've not heard of) and will not be available in quantity at low cost for your invention to use.
    Also, the low base threshold voltage of germanium transistors is a by-product of the characteristics of the semiconductor material, and was never a particularly important specification, so (a) you're unlikely to find a germanium transistor from the past that was designed specifically for low base voltage, and (b) the base voltage was never a very well-controlled parameter so there is likely to be significant variation from one device to the next, even from the same manufacturer.

    My second point is that if you REALLY want some useful feedback, post a schematic and description of your invention, whatever it is, rather than just vague hints about how amazing and important it will be.
    It may be that your perceived need for a low base voltage threshold can be worked around, or may become irrelevant because of other limiting factors in the design.
    It is unusual, though not impossible, for an inexperienced designer to come up with an original, workable and useful design, especially without guidance and on the first attempt, and we may be able to save you a lot of time by focussing your efforts on critical problems that you may not have considered.
  18. Mongrel Shark

    Mongrel Shark

    Jun 6, 2012
    Just Frustrated.

    I'm sorry if it seems like I'm taking my frustrations out on people here.

    It's just that I have been looking into the whole circuit for weeks now, and the transistor is the hold up. It doesn't need to be perfect. closeish will be fine for early prototyping. I'll have a much better idea what I want after that.

    You guys are the nicest electronics forum regulars I have come across. Other forums seem to be full of intellectual elitists.

    Like I said. this is the third one and the first to post a link to a site as useful as digikey.

    If I do post my full idea. I think it will be here and youtube at the same time. With a website and a few other places as well.

    I'd really like to get a few of the different sections of the end circuit working better than what I have, before I do that though. If it works I want lots of people replicating it quickly. So common parts (ie not germanium) would be great. just not sure if it's plausible.
    That it's so hard to find the info I want is encouraging in a way. Means not many have tried the same thing....

    I'd love to post a diagram or schematic. But I'm still playing around a lot. it's changing daily I'm also worried someone will patent my idea before I can open source it properly.
    Although I guess if I talk about it here that may help protect it as public property. I'm really not looking to get rich off this. But others might.

    The basic part I am working on at the moment, and feel I need low voltage transistors for. Is a self regulating oscillator, or joule thief. I want to take trace amounts of power and pulse it into something that can light an LED or similar highly efficient light. I'm getting there...
    This is just stage one. There is more later. I might start a thread soon to help maintain my idea. Its not a new one. I've wanted to do this for 20 years. When I first came up with it, almost everyone, especially my teachers at school, told me I would have to break the laws of thermodynamics. I don't believe that is the case. I had it from a very reliable source that it was plausible. As I always knew. The more I have researched and observed, the more sure I get it will work. I think it's even been done before.....

    But the first stage of the circuit needs to be able to collect low voltages at small micro amps, Then pulse it out in a more useable level.

    I have my first germanium version on you-tube here
    While it works well. the transistor is all wrong. I ordered that one for something else, It's just all I had. Still better than the BC347 or 2n3904 or the other common BC3-- NPN's I have tried. I have some BC107's in the post. Not expecting much from them though. I want to go for lower volts and less amps. The transistor seems to be the problem...

    I'm pretty certain the base voltage requirement and the switching speed of the first transistor, or Q1 if you like, are going to be main factors. Just about everything else can be worked around....

    As you will see if you watch some of my videos. I have tons of reading to do on brain melting topics like Harmonics and induction coils, and will still need help. you will also see I have a decent ground to build knowledge on...
    It's not about money, it's not about fame. I just want this to be for everyone. to make the world that tiny bit better.

    The digikey link answered my original question. SO perhaps it is time for a new thread. I know it's time to read my girl a bedtime story...

    I'll be back a little later with more info, and probably a new thread. I'm still very new around here, so if the projects section is the wrong place to host an open source project. Let me know.

    Thank's to everyone for their tolerance to my frustration and vagueness. All will become clear soon.
  19. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
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