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Playing with Transistors: NPN 2N3904 Transistor Experiment

Playing with Transistors: NPN 2N3904 Transistor Experiment



I've been playing with electronics for 40 years, but there's still stuff to learn - even basic stuff about how transistors work. Here I measure the Collector/Emitter junction voltage when the transistor is turned on.

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Tags: transistors

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33 Comments

  1. Date: November 21, 2015 at 19:12
    Author: USS Liberty

    You should start your own "electronics fundamentals" video series.

  2. Date: November 21, 2015 at 19:20
    Author: FairCogent

    Great video. You failed putting those links though, at least as far as I can tell.

  3. Date: November 21, 2015 at 19:20

    You left us hanging on the strange circuit with the 220K emitter resistor. Did you figure it out?

  4. Date: November 21, 2015 at 19:21
    Author: Jonas Hansen

    I have just spent 2 weeks (not full time), figuring this out myself for pnp and npn transistors. Through your inspirational videos, i got inspired to create night lights for my kids with rechargeable batteries and a bare bones atmega328 that will power up them selves during the day and hopefully will run for months/years. As i wanted to create a dark sensor using a solar cell to charge batteries during the day and switch the circuit off while charging and turning it back on when night falls.

    Awesome stuff Julian, love it !

  5. Date: November 21, 2015 at 20:31
    Author: Nathan Campos

    I highly recommend you read the chapter about transistors in The Art of Electronics, the explanation is superb, easy to understand and very in-depth.

  6. Date: November 21, 2015 at 21:27
    Author: deslomeslager

    I never understood why we use an NPN transistor with the emitter on the negative rail. This means we are pushing 1 mA through the base and loose it. I would always think it is better to put the NPN on the positive rail. This way you can send the 1 mA current from base to emitter as well through the load (the LED). am I right about this? That would be like DUH, why has this been done wrong for 50 years?

  7. Date: November 21, 2015 at 23:13
    Author: Espen Lodden

    +Julian Ilett Thanks for another good video. Just what I needed tonight, as I had my 2N2222 transistor connected wrong. You mentioned that you would link two videos, but the links did not come up on my screen. Could you perhaps link them in the video description? Cheerio!

  8. Date: November 22, 2015 at 04:02
    Author: Digger D

    Hi Julian, watching your vids often.  You might find W2AEW's site helpful – he has a video  on Transistor Bias Circuits and more.  I'm still trying to figure out transistors myself.  I understand the basics but there is still a lot to know.

  9. Date: November 22, 2015 at 08:25

    When a transistor gets enough base current, about .7-.8V base-emitter, it enters saturation, where the collector-base voltage actually becomes negative, that is, the collector voltage with respect to your emitter ground falls below the voltage of the base with respect to ground. Saturation is the state used for switching devices on and off, and for digital logic. Before saturation, with slightly lower base-emitter voltage it operates in what is known as the linear region, where the hFE is a gain in collector current that is reasonably linear with base current. The linear region is used for analog amplification. Below the .65 V or so base-emitter voltage the transistor is in cut-off, that is virtually no collector current flows except a very small leakage. To study and become acquainted with these regions connect a pot to the base to supply and vary the base current and watch the various voltages.

  10. Date: November 22, 2015 at 14:32
    Author: robec75

    I confirm something someone else already wrote, Vcesat is roughly 0.2V…. school days stuff 🙂

  11. Date: November 22, 2015 at 22:34
    Author: magicjc2012

    Nice vidéo, very clear and didactic, can you the reference of the transistor tester you are using. many thanks

  12. Date: November 22, 2015 at 22:42
    Author: Eclipse

    It's actually much more simple and intuitive than all of the educational mathematics models. You may note that when the transistor is 'off', it's 'dropping' all of the voltage it sees. A BJT may be viewed as a current-controlled resistor in actuality. The more current flowing from base to emitter (in an NPN), the less 'impedance' it will provide to collector-emitter current.

    For a circuit of nine volts, all of that voltage must be 'dropped' by some form of impedance, thus performing electrical 'work'. It's analogous to calculating solar cell MPPT. So you may note that when a battery is said to be 'disconnected', it's not actually fully so, it's just experiencing the impedance of the atmosphere.

    The voltage then across the transistor will be proportional to how much is dropped by other parts of the circuit relative to its own impedance which is set by base current. An 'ideal' transistor would drop zero voltage at currents below its set 'allowance'. A real transistor however usually has a minimum voltage drop, which in this case was around 200mv.

  13. Date: November 23, 2015 at 15:19
    Author: douro20

    We tend to take these things for granted when MOSFETs are a lot easier to work with.

  14. Date: November 23, 2015 at 16:10
    Author: Greg Cook

    Julian,Very good video.  Thank you.  Where did you get the transistor tester?

  15. Date: November 24, 2015 at 00:57

    hi julian , thanks for the video. please tell me , how do you connect the probes to the aligator clips?
    i have the same transistor tester , do you think it can be wrong when it says that a transistor pr mosfet is ok? it would be very usefull to show us how to test an optocoupler and other devices like this.
    thank you!

  16. Date: November 24, 2015 at 06:47
    Author: xanataph

    Yes, it's incredible how much there is to know about electronics, even what are considered "basic" components. The saturation C-E voltage can be quite low, but it depends on how much current is flowing through. Last week I actually used a couple of 2N3055 power transistors as blocking diodes in one of my solar systems. When the base is connected to the collector they basically behave as diodes. But with a Schottky like (or perhaps even better than) performance in respect to the voltage drop. The only thing to watch out for is that unlike proper diodes they have a very low PIV indeed. I was able to get away with it due to there being a 12 volt (nominal) system voltage, it wouldn't have worked with 24 volts as then reverse breakdown would have occurred.

  17. Date: November 25, 2015 at 13:30
    Author: Billy Sugger

    The standard silicon PN junction forward voltage is around 650mA at room temperature, and changes with precise doping but also changes a lot with temperature. In fact, you can make a crude temperature sensor using one. Your initial tester Vbe of 714mV was reasonable, the second result of 780mV suggests the junction temperature has risen during the test, and your measured 800mV in circuit reflects the heating due to the collector current driving the LED. This is why in circuits where matching Vbe is important, like current mirror circuits, it's vitally important to ensure good thermal coupling between the two transistors, which should also be selected from the same manufacturing batch to ensure similar doping.

    The configuration with a resistor in the emitter but not the base turns the transistor into a switchable current sink, the collector current being approximately equal (given large hFE) to emitter voltage divided by emitter resistance. The IC switches that current on and off.

  18. Date: November 25, 2015 at 15:56
    Author: Ronen Shemesh

    whenever the transistor goes to saturation (beyond the maximum beta) Vce drops to the minimum value that can be taken from the datasheet, generally Vce(sat) ~= 0.2v

  19. Date: November 26, 2015 at 05:47
    Author: bluetorch13

    I have the same multimeter, love it. But blew both fuses and are more expensive than the multimeter… sad…

  20. Date: December 9, 2015 at 15:04
    Author: tehaxor69

    Hi Julian, have you heard of a one transistor oscillator via reverse biasing a BJT NPN transistor?

    Here is a good video demonstrating it.
    https://www.youtube.com/watch?v=rpGOKGrcpAk

    I was wondering if you would do your own video giving the one transistor oscillator a go.

  21. Date: February 7, 2016 at 20:50
    Author: Jeff Morgan

    How cute where did you get those plastic letter "B, E, C" pods? Nice job

  22. Date: April 10, 2016 at 07:26
    Author: Gordy Barn

    You should of added the LED voltage drop for the proper voltage from Collector to Emitter.

  23. Date: April 10, 2016 at 07:49
    Author: Gordy Barn

    I have been going crazy trying to grasp other peoples explanations just how a transistor works and am confused by the many variations they use to figure the proper biasing. There has to be a much simpler way and maybe one day I will find a way that I understand and works for me. So far , I have found one article on the web however, there are typo's that need to be corrected.
    I guess the only way to understand them is to build a circuit like Julian has and experiment.

  24. Date: April 10, 2016 at 07:53
    Author: Gordy Barn

    I worked as an Electronic Technician for the US Government for 28 years and in all that time, I never had the need to understand how transistors work much less any other discrete component. I learned how the circuits were supposed to function and if they did not work properly, I knew what circuit was the problem. After a while, you just learn common sense trouble shooting.

  25. Date: November 1, 2016 at 14:07
    Author: Saksham Gupta

    how much base resistor value should be if i am using using bc547 at 5v 75ma?

  26. Date: February 12, 2017 at 10:33
    Author: Dan Slade

    Dave and Derek mentioned in the same video. good stuff 😉

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