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Substituting a transistor

Discussion in 'Electronic Basics' started by [email protected], Apr 15, 2006.

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  1. Guest

    I am going to trade a BC547 transistor for a TSxxxx transistor (Taiwan

    In the datasheet of the BC547 I have read the following:


    I took a look on the website of Taiwan Semiconductors. Here's a nice
    overview of their transistors and their V's and Ic:

    So I explored the list and tried to find a matching substitutor. But I
    notices that sometimes the V's are the same, but then the Ic is too
    high. And then when I seem to find a transistor with the correct Ic,
    it's the V's are way too high again. In short: there's no perfectly
    matching transistor.

    Does this mean non-of these transistors can replace my BC547? I read on
    this newsgroup that very often any transistor will do. (which doesn't
    really make sense imho, since often there are several types of
    transistors used in the same circuit). Still I will have to make a
    decision. So I guess not all factors are as important and that there
    will be a transistor that will be good enough?

    My main question is: what is the most important factor of a transistor?
    Is it the Ic, Vcbo, Vces or the Vebo. And do my V's and Ic have to be
    at least as high of the values of my previous transistor, or should
    they only be maximum the same value and not cross it? Or do I just need
    to find values as close as possible?

    Thank you in advance,
    I hope somebody can clear this up for me

    (In case anybody wonders... It's really more as an experiment I am
    substituting the transistor.)
  2. I am not seeing your specs on the data sheet:

    But to get back to your question, many substitutes might be possible,
    but to decide what specs must be well matched and which may simple be
    exceeded, you need to understand what the transistor is limited by in
    the circuit in question. If you want a substitute that will work in
    any application, all the specs must be equaled (at least). Besides
    collector voltage and current ratings, you might need to consider DC
    gain at low current, the current that has the peak gain and the gain
    at the current where the gain falls to something like half of the peak
    gain. You might have to be concerned with saturation voltage (if
    being used in a switching application), and collector to base
    capacitance and fT, if speed is a concern. And you might compare
    noise figure if that is important (and the two data sheets are at all
    similar in their definitions and test conditions).
  3. Chris

    Chris Guest

    Mr. Popelish has given good advice. But I'd like to know why you
    didn't provide the exact part number of the transistor you're

    I'd assume you would probably be able to replace a TSxxxx with a
    BCxxxx. ;-)

    If you want an almost certainly good substitute, you need to know what
    you're substituting. Make sure you equal or exceed all the relevant
    specs. And there are many other specs besides working voltage and
    maximum current.

    But if there are no exact matches, then you have to get a circuit
    schematic and work out exactly what the circuit is doing. Figure out
    what's important and what's not. For instance, if you're using a
    transistor as a slow speed switching relay driver, f(t) probably isn't
    going to be too important. Nor are various capacitances. You'd be
    more concerned with h(fe), Ic(max), and V(ceo).

    Many times, especially with power transistors and high frequency
    circuits, there is no good substitute.

    Good luck
  4. Phil Allison

    Phil Allison Guest

    ** Nothing close to a BC547 here - even the pin outs are different.

    ** Transistor type number substitution is not a simple exercise.

    The very first thing is to get the polarity, package and pin outs the same.

    Vce, Ic and Hfe (beta) need to be the same or somewhat higher.

    Then check the Ft (gain-bandwidth product) is similar.

    Then the power dissipation figure, which can vary greatly even with the same

    Some devices are specialised for radio frequency work or switching, so look
    at the maker's description to.

    ......... Phil
  5. Phil Allison

    Phil Allison Guest


    ** Huh ???????

    The OP mentioned it was a BC547 no less than three times.

    ......... Phil
  6. Chris

    Chris Guest

    Thanks, Mr. Allison. I read "I am going to trade a BC547 transistor
    for a TSxxxx transistor (Taiwan Semiconductors)." and somehow
    interpreted that to mean he was replacing a Taiwan Semiconductors
    something with a BC547, instead of the other way around.

    My mistake. Thanks again for the spot.

  7. Guest

    This is very interesting for me. I have a datasheet from "Vishay
    It shows:

    while your specifications from "Fairchild" show

    and I also found the specifications of "Philips"

    I guess I was wrong when I thought I could just download circuits from
    the internet, buy the parts and assemble them.
    (When even a partnumber can have different meanings) But I'll go with
    the 45,50,6,500 as you showed me.
  8. Guest

    Thank you for your reply Chris.
    Vceo = 400V
    Vcbo = 700V
    Vebo = 9V
    Ic = 800mA

    I guess this could then replace the

    Vceo = 45V
    Vcbo = 50V
    Vebo = 6V
    Ic = 500mA

    Would this work? (By the way it will probably be used in applications
    like a PIC-microcontroller programmer and a USBtoSerialport Converter)

    Thank you in advance
  9. Phil Allison

    Phil Allison Guest

    ** A TO220 pack device - you fucking moron.

    ** NO - you half witted baboon.

    .......... Phil
  10. Chris

    Chris Guest

    Hi, BramGo. I'm not sure why you're focused on Taiwan Semiconductor
    (unless you work there). There are a couple of problems with your
    possible substitution. Both transistors are NPN. However, the
    replacement you're suggesting is a much larger TO-220 package. While
    it exceeds the BC547 on voltage specs by an order of magnitude, it's a
    relatively slow transistor (f(t) = 4MHz). That's going to be an issue
    if you're trying to switch voltages at microsecond speeds. It won't
    switch as quickly. Also, the pinouts are wrong. Like a lot of power
    transistors, the TS13007 has a BCE pinout. That isn't close to the
    BC547 -- if you wanted to force it into an existing board, you'd have
    to twist around some leads. Also, if you've got an existing board, the
    physical board holes for a TO-92 transistor are too small for a TO-220
    lead -- it may not fit in the holes.

    NPN transistors of the BC547 type are called gumball parts -- they're
    commonly manufactured by many sources, and are relatively inexpensive.

    The BC547 is a gumball-type part. If you look at the data sheet:

    a couple of things come to mind. First, the NPN transistor part pinout
    is a little odd -- the leads on the TO-92 package are backwards from
    the standard EBC configuration (looking at the flat part on the front
    of the package, with the leads down, reading from left to right). The
    BC547 is CBE. If you have a circuit board you're working with, you can
    solve this problem by replacing with a more standard transistor, and
    putting it in backwards (like the NTE123A mentioned above). If you're
    doing a prototype, though, you don't have to worry about it.

    The second thing that comes to mind looking at the datasheet is that if
    you're using this as a voltage switch at microsecond speeds, you might
    be able to do better. This Fairchild data sheet says maximum current
    is 100mA, with maximum power dissipation 500mW. Many TO-92 transistors
    can switch more current.

    On the plus side, the BC547 is optimized for higher voltage gain and
    speed at that current.

    If I were looking at this, I'd see if your application is using
    voltages over 40V (from your description, probably not). I'd then grab
    an NPN transistor called the 2N3904 (Vceo = 40V, Ic(max) = 200mA, all
    other specs similar) and just put the transistor in the board backwards
    if this is a replacement application.

    Look at the similarities on the datasheets, and see for yourself. By
    the way, if you're going to post again, please mention if you're
    repairing an existing board, or building something new. Also please
    describe more about what your transistor is doing.

    Good luck
  11. JeffM

    JeffM Guest

    I am going to trade a BC547 transistor for a TSxxxx transistor
    Foley questioned your apparantly arbitrary exclusion
    of all but a single source. I do as well.$$Search?OpenForm

    Greg Szekeres apparantly has a cross-reference page, but,
    having run it thru the WorldWide Web Consortium's HTML checker,
    I have to wonder what horrible Horrible HORRIBLE tool
    he used to construct it:

    Here's an old dead-tree variant:
    ISBN 0-672-21335-4

    The word "gumball" (aka "jellybean") was also used.
    That could be apt.
    The more details you provide,
    the better the advice you are likely to get.
  12. Guest

    Hi again,

    Thank you for all your comments.
    And thank you for those hyperlinks.

    The sequence of pins aren't really that important. I am using a
    prototyping board so it's easy to switch some wires.

    Electronics is totally new to me to be honest. In september I started
    studying Industrial Engineering in the field of Electronics. Normally
    it would take 4 year to graduate in it. But I received an adapted
    scheme due to my previous studies in the field of Computer Science. So
    I am doing the first 3 years in just 1 year. The problem is: most of my
    teachers assume I allready had over 2 year experience with the basics
    of analog electronics. But I'm actually a total beginner. Next year
    I'll have to write a big paper and do a training period in order to
    graduate. So in preperation of that I am trying to make up the
    experience I don't have.

    Choosing transistors is pretty complicated for me. Maybe it would be
    easier if I would better understand the circuits. I don't get the exact
    meaning of it in certain situations. (I do have a basic knowledge of
    what a transistor is: I know it can be used as a kind of amplifier
    depending on the current on the base.) But there are certain situations
    where I do not understand the importance of it. For example in the
    diagram on the following webpage:

    It would be nice if somebody could give a little bit of background
  13. Rich Grise

    Rich Grise Guest

    It would probably "work", depending on your application, but you don't
    need anywhere that high of a voltage:

    Try an MPSA06:
    Vcbo = 90V
    Vce0 = 80V
    Vebo = 4V <- You'll have to watch your reverse base voltage, but if that's
    under control, it should work fine.
    Ic = 500 mA

    Good Luck!
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