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Discussion in 'General Electronics Discussion' started by SparkyCal, Apr 29, 2020.

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  1. SparkyCal


    Mar 11, 2020
    I have studied how transistors work as a switch or amplifier. Although I have not mastered it, I am getting here.

    I have a transistor related question.

    I bought two transistor assortment kits. One kit has 10 different types of transistors. They are:

    BC327 BC337 BC517. BC547. BC548. BC549. BC550. BC556 BC557. BC558

    The other box has:

    S8050. S8550 S9012 S9013 S9014 2N3904 2N3906. C1815 A1015. 13001

    I know there is a difference between NPN and PNPs and I know it is about whether they are on or off when applying a current.

    I don't understand however, how to know when to use what transistor? For example, I ran out of BC547. Is it possible to use BC548s? How do you know how to substitute one for another or what certain models are commonly used for?

    Any hints?

  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Quick answer:
    The reason for the many different types of transistors is the many different parameters they have. Plus companies throwing their own product onto the market with parameters almost equal to transistors but unique names - just to be different :(
    If you need a replacement for a transistor you ideally find one with (almost) identical parameters. Look up "transistor cross reference table" to find lists of transistors (or other components) that usually can be used as replacement for one another.
    Many circuits are not very sensitive to parameters (e.g. when a transistor is used as switch, high frequency gain may be irrelevant), In former times you could find transistors labeled in a schematic as TUN (Transistor Universal NPN) or TUP (Transistor Universal PNP) and you could use whatever you found in your collection.

    With a bit more detail:
    To find a suitable replacement for a transistor in a specific circuit you need to know the relevant design parameters that led the developer of the circuit to chose this transistor. These may be e.g. gain, max. collector current and collector-emitter breakdown voltage. You then look up transistors with similar or better specs. For example if the replacement has a higher c-e breakdown voltage, that is o.k
    But: Not in every case a "better" parameter is good for the circuit, For example higher (better) gain would usually be considered acceptable, but there are circuits where too much of gain can lead to instability.
    Other parameters may be less relevant and may differ without affecting circuit performance.
    The problem with this approach is that mostly you are not given the information to make an educated decision based on details of parameter selection. So mostly you'll have to revert to the first strategy above and select a transistor with parameters as similar as possible.
    Or simply grab any one and give it a try. But be prepared to exchange this transistor a few times until you find a suitable replacement.
  3. Nanren888


    Nov 8, 2015
    Data sheets. A bit easier for similar transistors as they tend to be on the same sheet.
    Your question BC547, can I use a BC548?
    Depends on what's important. Let's look at the differences.
    The table shows that the only differences are the maximum voltages.
    We can see that the BC547 has higher voltages, but the BC548 is listed as 30V, so if the circuit needs 30 volts or more, a BC548 is not a good substitute.
    The max current seems to be the same.
    The maximum powers match.
    The highest frequency seems to be the same,
    HFE, the large signal gain minimum and maximum also match.
    Hope this helps.

    Attached Files:

    davenn and Harald Kapp like this.
  4. Nanren888


    Nov 8, 2015
    table including other parameters.
  5. ramussons


    Jun 10, 2014
    Why do they give lower rated transistors in the first place? All the 10 transistors could have been BC546.

    Or is it exploiting the human psychology that believes a "Variety" pack is better than a "Mono" pack? :D:D:D
  6. bertus

    bertus Moderator

    Nov 8, 2019
  7. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    Veeeery likely.
  8. Audioguru


    Sep 24, 2016
    Be careful about the pinouts.
    The BCxxx transistors have the European CBE pinout but most of the other ones have the American EBC pinout. The 13001 transistor has both the previous pinouts plus the oriental BCE and ECB pinouts.
  9. SparkyCal


    Mar 11, 2020
    Thanks for all the useful tips and links. This helped me a lot. For the most part the circuits I will be building are pretty simple. But it is good to know how to check whether I can get away with subbing out transistors and how to determine that.

    Thanks again for taking the time to help me out. Slowly but surely, i am learning. I mentioned on another thread that I finallky got a Astable- Multivibrator circuit to work on a regular soldered board. It's a big accomplishment for me ;-)
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