Beginner Concepts: Designing transistor control circuits

Need to switch something on or off using a microcontroller? Using a transistor is one of the best ways to do this, but how exactly do you design properly for transistor switching? [Ben Krasnow] put together a tutorial in which he does an excellent job of explaining the ins and outs of designing transistor control circuits.

We’ve embedded his twenty-minute video after the break. In it he talks about the use of transistors, the difference between NPN and PNP transistors, and the design specifics you need to know when working with them. We think that beginners will find [Ben’s] demonstration of how to calculates Hfe, which is the base current necessary to fully switch the transistor. If this is gibberish to you, have no fear. [Ben’s] instruction is clear and easily understandable.

The one thing we missed in the video is clarification about base current protection for PNP transistors. [Ben] mentions that there’s no easy circuitry that can be used on the base of a PNP  to regulate flow from the emitter to the base, but he doesn’t elaborate. Otherwise, it’s everything we could have wanted on the topic.

18 thoughts on “Beginner Concepts: Designing transistor control circuits

  1. Ok, so even after doing transistor theory at uni, this guy explains transistor switching SO much clearer than was taught to me. Great video. Much of the DC stuff makes so much more sense. He’s right about the AC signal analysis, that’s just annoying… Two thumbs up!

  2. “demonstration of how to calculates Hfe, which is the base current necessary to fully switch the transistor”


    Hfe is the current gain of the transistor. I.e. the ratio of input current to output current.

    Based on this, I think I will skip the video thanks. I suggest anyone wanting to learn how to use a transistor checks out a reputable website such as:

    Alternatively (shock horror), check out a book from your local library.

  3. >>Excellent! Now I can just link people this video when they ask :D

    These kind of comments as well as the “i’ll tackle this project if I have electricity in my house and spare money one day”

    Are the new “ME TOO”…

    And sort of… make you miss the old “me too”

  4. It looks like the Hfe error is a HaD typo, and not represented in the video.

    A very nice video. As much as there were some details I wantd to see, it’s likely better added with additional videos to avoid overwhelming a beginner.

  5. @Mike Szczys

    In regards to “The one thing we missed in the video is clarification about base current protection for PNP transistors”, Ben’s statement was probably not the most accurate (even though not wrong) – the “issue” with driving PNP in this configuration is more of the “voltage level shifting” then “current protection”.

    If you are driving the transistor’s base with 0V to 5V while emitter is on 12V (Vbe is then -7V to -12V), you can’t fully turn the transistor off as there is some base current flowing at all times.

    You need to drive base in the way that Vbe is under the threshold voltage (let’s say less then -0.5V), meaning that your driving logic needs to be able to go up almost to the supply voltage (12V in this case)). (NOTE: for the sake of accuracy, Vbe should have been written as VBE, but limited font size use renders VBE useless (DC vs. AC signal notations)).

    One way to overcome this problem is to put a voltage shifter (a plain 7.5V zener diode or 13 regular diodes in series with the base’s resistor would do it in this case) so 5V on your input turns the transistor off, while 0V creates sufficient voltage drop to turn it on.

  6. Video is very instructive, although has some potentially “damaging” design directions.

    In the case described, those design “misconceptions” are not dangerous, but give that the concept can be used in other implementations, I suppose some corrections are due.

    1) Always look for the worst case scenario. 2N2222A transistor has declared hFE as “not lower then 75″, so let’s use 75 (not 100 as in video)

    2) To minimize power dissipated on the transistor, always drive it as deep into saturation as possible (meaning, base current needs to be (quite) larger then Ic/hFE). While respecting the limit of the max allowed Ib (200mA here), it’s always good to drive Ib 2 to 5 times over the requested value. That will assure as low collector voltage (Vce in this case) as possible.

    3) Stated input voltage from the driving logic circuit (said to be 5V), is probably overly optimistic. As an example, an old fashioned TTL circuits (like 7400), can’t drive output higher then 3.5V

    Given Rb=4k7, Vin=3.5V, Vbe=0.7V, hFEmin=75 we get
    Ic = 75*(3.5-0.7)/Rb = 45mA, apparently not sufficient for this application.

    To be sure it correctly works even in the worst case scenario, the calculation should rather be

    Ib = 3*Ic/hFEmin = 2.2mA

    Rb = (Vin-Vbesat)/Ib = (3.5-1)/0.0022 = 1.2k

    (another safeguard here, Vbe in active state is 0.7V, but in saturation it can go as high as 2V, consult the data sheet)

  7. He should have used the old “mnemonic” to help with distinguishing PNP vs NPN schematic symbols. (P)ointing i(N) [yeah, there’s no second ‘P’ for this one that I remember] and (N)ot (P)ointing i(N).

  8. @ehrichweiss

    Thanks, I keep checking every time.

    But I don’t see the problem when working with digital circuits, just saturate the transistor and you’ve got the potential, maybe a voltage divider if you need less than Vcc. Then again I’m still just a student.

  9. Just nitpicking here, but a bipolar transistor is not “just a plain transistor”. A bipolar transistor is a bipolar transistor, just like a field effect transistor is a field effect transistor.

    Also worth pointing out that the reason NPN’s are more common is that for the same amount of money you get a faster better device than a PNP, because the electron mobility is higher than hole mobility in silicon. Same reason why NMOS FET’s are better than PMOS.

  10. Very nice video!

    I like how he adds the reason for requiring the resistor at the end, made the same mistake a couple times when I was starting out as well but never truly understood.

    Again, awesome!

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