How Transistors Work – A Simple Explanation

The transistor is a simple component that you can use to build a lot of fun projects. In this hands-on guide, you’ll learn how transistors work so that you can use them in your next circuit.

And it’s actually pretty easy, once you learn the basics. I’ll focus on the two most common transistors here; the BJT and the MOSFET.

NPN transistor on a breadboard

The transistor works like an electronic switch. It can turn a current ON and OFF. A simple way to think about it is to look at the transistor as a relay without any moving parts. A transistor is similar to a relay in the sense that you can use it to turn something ON and OFF.

But a transistor can also be turned partly on, which is useful for building amplifiers.

How Transistors Work (BJT)

Let’s start with the classic NPN transistor. It’s a Bipolar Junction Transistor (BJT) and has three legs:

  • Base (b)
  • Collector (c)
  • Emitter (e)
Schematic symbol of an NPN transistor with pin names

If you turn it ON, current can flow through it from the collector to the emitter. When it’s OFF, no current can flow.

In the example circuit below, the transistor is OFF. That means no current can flow through it, so the Light-Emitting Diode (LED) is also off.

A transistor turned off so that no current can flow through it.

To turn the transistor ON, you need a voltage of about 0.7V between the base and the emitter.

If you had a 0.7V battery, you could have connected it between the base and emitter, and the transistor would have turned ON.

Since most of us don’t have a 0.7V battery, how do we turn on the transistor?

Easy! The base-to-emitter part of a transistor works like a diode. A diode has a forward voltage that it “grabs” from the available voltage. If you add a resistor in series, the rest of the voltage drops across the resistor.

So you’ll automatically get around 0.7V by adding a resistor.

This is the same principle you use to limit the current through an LED to make sure it doesn’t blow up.

If you also add a pushbutton, you can control the transistor, and thereby the LED, ON and OFF with a button:

How transistors work (NPN type)

Choosing Component Values

To choose the component values, there’s one more thing you need to know about how transistors work:

When a current flows from the base to the emitter, the transistor turns on so that a larger current can flow from the collector to the emitter.

How transistors work (NPN)

There is a connection between the sizes of the two currents. This is called the gain of the transistor.

For a general-purpose transistor, such as the BC547 or 2N3904, this could be around 100.

That means that if you have 0.1 mA flowing from the base to the emitter, you can have 10 mA (100 times more) flowing from collector to emitter.

What resistor value do you need for R1 to get 0.1mA flowing?

If the battery is 9V, and the base-to-emitter of the transistor grabs 0.7V, then there’s 8.3V left across the resistor.

You can use Ohm’s law to find the resistor value:

Ohm's law for resistance
The Ohm’s Law triangle

R = \frac{V}{I} = \frac{8.3V}{0.0001A} = 83000 \Omega

So you need a resistor of 83 kΩ. That’s not a standard value, but 82 kΩ is, and it’s close enough.

R2 is there to limit the current to the LED. You can choose the value you would have chosen if you were to connect the LED and resistor directly to the 9V battery, without the transistor. For example, 1 kΩ should work fine.

Check out the video explanation I made on the transistor a few years back (forgive the old-school quality):

How To Choose a Transistor

The NPN transistor is the most common of the Bipolar Junction Transistors (BJT). But there is another one called a PNP transistor that works the same way, just that all the currents are in the opposite direction.

When choosing a transistor, the most important thing to keep in mind is how much current the transistor can support. This is called the Collector Current (IC).

How a MOSFET Transistor Works

The MOSFET transistor is another very common type of transistor. It also has three pins:

  • Gate (g)
  • Source (s)
  • Drain (d)
MOSFET symbol (n-channel)
MOSFET symbol (N-channel)

A MOSFET works similar to the BJT transistor, but with one important difference:

In the BJT transistor, the current from base to emitter decides how much current can flow from collector to emitter.

In the MOSFET transistor, the voltage between gate and source decides how much current can flow from drain to source.

Example: How To Turn ON a MOSFET

Below is an example circuit for turning on a MOSFET.

How MOSFET transistors work

To turn a MOSFET transistor on, you need a voltage between gate and source that is higher than the threshold voltage of your transistor. For example, the BS170 has a gate-source threshold voltage of 2.1V. (You’ll find this info in the datasheet).

The threshold voltage of a MOSFET is actually the voltage where it turns off. So to turn the transistor properly on, you need a voltage a bit higher than that.

How much higher depends on how much current you’d like to have flowing (and you’ll find that info in the datasheet). If you go a couple of volts above the threshold, that’s usually more than enough for low-current things like turning on an LED.

Note that even if you use a high enough voltage to have 1A current flowing, it doesn’t mean you’ll get 1A. It just means that you could have 1A flowing if you wanted to. But it’s whatever you connect to it that decides the actual current.

So you can go as high as you want, as long as you make sure you don’t go above the maximum gate-source voltage limit (which is 20V for the BS170).

In the example above, the gate is connected to 9V when you push the button. This turns the transistor on.

Choosing Component Values

The value of R1 isn’t crucial, but around 10 kΩ should work fine. Its purpose is to turn off the MOSFET (more about that below).

R2 sets the brightness of the LED. 1 kΩ should work fine for most LEDs.

Q1 could be almost any n-channel MOSFET, for example, BS170.

How To Turn OFF a MOSFET?

One important thing to learn about the MOSFET is that it also acts a bit like a capacitor. That is, the gate-source part. When you apply a voltage between gate and source, this voltage stays there until it’s discharged.

Without the resistor (R1) in the example above, the transistor wouldn’t turn off. With the resistor, there is a path for the gate-source capacitor to discharge so that the transistor turns off again.

How To Choose a MOSFET Transistor

The above example uses an N-channel MOSFET. P-channel MOSFETs work the same way, just that the current flows in the opposite direction, and the gate to source voltage must be negative to turn it on.

There are thousands of different MOSFETs to choose from. But if you want to build the example circuit above and want a specific recommendation, BS170 and IRF510 are two commons ones.

Two things to keep in mind when choosing a MOSFET is:

  • The gate-to-source threshold voltage. You need a voltage higher than this to turn the transistor on.
  • The Continuous Drain Current. This is the maximum amount of current that can flow through your transistor.

There are other important parameters to keep in mind, depending on what you’re making. But that’s out of the scope of this article. Keep the two parameters above in mind and you’ll have a good starting point.

MOSFET Gate Current

If you want to control a MOSFET from for example an Arduino or Raspberry Pi, there is another thing you need to keep in mind; the current that flows into the gate when you turn the transistor on.

As briefly mentioned above, the gate-to-source of a MOSFET acts as a capacitor.

That means once it’s charged, no more current flows through it. So when a MOSFET is on, there is no current flowing through the gate.

But when a MOSFET is being turned on, there is a current, just like when you charge a capacitor. For a small fraction of a second, there can be a lot of current flowing.

To protect your Arduino (or whatever you’re using) from too much current, you need to add a MOSFET gate resistor:

Often 1000 Ω is a good enough value for this. Use Ohm’s law to check for your specific case.

Why Do You Need A Transistor?

A common question I get is why do we need the transistor? Why not connect the LED and resistor directly to the battery?

The advantage of a transistor is that you can use a small current or voltage to control a much larger current and voltage.

That’s super useful if you want to control things like motors, high-power LEDs, speakers, relays, and more from a Raspberry Pi/Arduino/microcontroller. The output pins from these boards can usually only provide a few milliamperes at 5V. So if you want to control your 110V outdoor patio lights, you can’t do it directly from the pin.

Instead, you could do it through a relay. But even the relay usually needs more current than the pin can provide. So you’d need a transistor to control the relay:

Connect left side of the resistor to an output pin (ex from Arduino) to control the relay

But transistors are also useful for simpler sensor circuits, like this light sensor circuit, the touch sensor circuit, or the H-Bridge circuit.

We use transistors in almost all circuits. It’s really the most important component in electronics.

The Transistor as an Amplifier

The transistor is also what makes amplifiers work. Instead of having just two states (ON/OFF) it can also be anywhere in between “fully on” and “fully off”.

That means a small signal with almost no energy can control a transistor to create a much stronger copy of that signal in the collector-emitter (or drain-source) part of the transistor. Thereby, the transistor can amplify small signals.

Below is a simple amplifier to drive a speaker. The higher the input voltage, the higher the current from base to emitter, and the higher the current through the speaker.

A varying input voltage makes the current in the speaker vary, which creates sound.

Common emitter amplifier

Normally, you’d add a couple of more resistors to bias the transistor. Otherwise you’ll get a lot of distortion. But that’s for another article.

If you want to learn more about using the transistor as an amplifier, electronics-lab.com has some nice tutorials that go through the three basic BJT amplifier setups.

Questions?

Do you understand how transistors work now? Or are you still confused? Let me know in the comments below.

More Transistors Tutorials

233 thoughts on “How Transistors Work – A Simple Explanation”

        • I thought that electrons were the emf that pushed current through a circuit. Is that where i am confused?

          Reply
          • Voltage is the emf that pushes current.

            Electrons moving from negative to positive often makes up the current But not always. Sometimes, for example in semiconductors, you have charge carriers moving in the opposite direction.

            Anyway, whatever way the particles flow, it leads to a flow of charge in one direction. And that’s what current it – flow of charge.

            If negative charges (like electrons) flow from negative to positive, it behaves exactly like positive charges flowing from positive to negative.

            But the important thing to remember is that it does not matter which way you decide to think about current:

            You can choose to follow the electron flow and think of current as something that flows from negative to positive if you want, and you’ll get the exact same result as someone who thinks about current from positive to negative – as long as you stick to the same direction all the time.

          • the term you are looking for is “conventional current” and it (theoretically only) flows from the positive to negative – as per the direction of a diode or the emitter on a transistor. But as above “electron current” flows negative to positive. I think it is something to do with the time line about when they discovered that electrons flow in the opposite direction.

  1. my question : is the only work of a transistor, to act as a switch or as an amplifier and why is it necessary to use it in a circuit?

    Reply
    • The transistor can be used as a switch or as an amplifier. I’ll write about the amplifier case in some article later. But basically the transistor doesn’t have to be only ON or OFF, it can also be anywhere in between :)

      Cheers!
      Oyvind

      Reply
  2. I just cant thankyou enough for this. I have my physics test on Electronic Devices this friday and transistors was giving me a tough time.
    Thankyou:-)

    Reply
  3. I’ve got a fancy book here that shows a bunch of different transistor types.
    What about a PNP transistor? Does it do the same thing but in the other direction?

    Reply
    • In the PNP case, the transistor is on with no voltage on base, and off when you apply voltage. So the opposite of NPN. Here is a circuit in two different versions, one with npn and one with pnp: build-electronic-circuits.com/ldr-circuit-diagram

      Cheers!
      Oyvind

      Reply
  4. I just want to know y a transistor is used in a circuit as it needs two power source and then it is a waste of power also.is it necessary to use it in a device.and also i feel very complecate to understand your video

    Reply
    • Say you had a microcontroller with a light sensor. The microntroller usually operates on 3.3 or 5 volts. Say you want to turn on a DC motor when the sensor senses light. The DC motor may require 12 volts to operate, which the microcontroller can’t supply. So you can hook the microcontroller signal line to the base of a transistor whose collector and emitter are connected to a 12 volt circuit with the motor. Now when the light sensor is activated, the microcontroller sends a small voltage to the transistor which causes the 12 volt motor circuit to activate.

      Reply
  5. Finally! Thank you for the straight forward explanation. Whenever I need a refresher, I don’t want to know about doped this and that. I just want to know how it turns on and off. Brilliant!

    Reply
  6. till now I did not understand how transistor works as a switch but just know I understood by our explanation , thanks for ur valuable time n explanation ….please explain us some more imp. concepts :)

    Reply
  7. You are seriously an awesome teacher .
    I am doing the same, as you told but my circuit is not working :p
    so can plz tell me, what could be the default .

    Reply
  8. Hi, Øyvind. That was awesome explanation. Can you explain me the importance of the resistor at the base of the transistor.? because my led didn’t turn on without that resistor.! And I also observed that when the resistor wasn’t connected(i.e, led still off), the transistor got heated up faster than when the resistor was connected(i.e, led on). Can you explain me what actually happened inside that transistor..?
    Thanks Øyvind.

    Reply
  9. You may check with the help of DMM in diode mode and by keeping positive terminal in P-block and negative terminal in N-block it shows some value with beep sound it is working. If it is open not working.

    Reply
  10. I love this article so much, I’ve been really confused about transistors and this helped clear things up. My only question is that there’s so many types of transistors and I was wondering if there’s really that much of a difference between them.

    Reply
    • Hey Emmet,

      Great=)

      The difference between the different types are mostly details like how much current they can handle, how much voltage that is needed to turn them on, how fast they can switch on and off +++

      But they work the same way.

      And you have the “inverted” type of transistor that instead of being off when there is no voltage on the base and on when there is, it’s the opposite.

      Cheers!
      Oyvind

      Reply
  11. Awesome demo of teaching circuits.

    You showed physical elements of circuit with the circuit diagram (given in books). Wow!

    Keep teaching

    Reply
    • When there’s no current flowing from base to emitter, yes the npn works as an insulator, and no current can flow from collector to emitter.

      Best,
      Oyvind

      Reply
  12. I’ve been having so much trouble understanding transistors that it was completely stopping my progression. ive read countless tutorials on alot of great sites. I finally get it! this might be the best birthday present ever!

    Reply
  13. Øyvind , I had been struggling with this concept for two days now and you just made it clear like a transistor, I could say- gave my brain a little nudge and a bolt of electricity of understanding passed through my head!
    Hmm.
    I should probably take a lesson on simplifying things from you. XD
    Thank you, really!

    Reply
  14. Really very useful site for beginner like myself. I just purchased an arduno starter kit. Now i can use it. Lot of thanks

    Reply
    • Great to hear!

      There’s no list of articles. You can use the search function at the top right of the page.

      Best,
      Oyvind

      Reply
  15. good job.great website.but how do i electrically adjust the size of voltage applied to the base to either turn it ON or OFF, bcos right now it seems the transistor will remain on so long the battery and that resistor is connected.

    Reply
  16. U make my day happiest. I had a big problem on transistors bt after reading ua article everything is going smoothly, plz explain on how to reduce this voltage into 0.7v, and how to arrange the resistors to reduce voltage, thx,……l salute u sir.

    Reply
  17. we are doing project on piezoelectric plates to charge mobile .they produce enough voltage but very low current in micro ampere ..can we used transistor here to amplify current ??

    Reply
    • You need to supply the current for a transistor, it won’t magically give you more current from nothing. But it’s possible to charge a capacitor with the voltage from the plates, the use the capacitor to supply the current to whatever you are charging.

      Oyvind

      Reply
  18. nice sir well explained .didn’t got any explaination like this . briefly and simply explained….I appreciate that……thanks

    Reply
  19. Mr admin
    I didn’t understand the “amplifying” portion.Could you please explain in a simple manner for me.I’m new to electronics.

    Reply
    • The current in an NPN transistor flows from base to emitter and from collector to emitter. Not from collector to base.

      Reply
  20. What happens when i use a battery of 1.5v to power the transistor, and again how do i combine resistors to get the 0.7v required

    Reply
    • Actually the base-emitter connection is a diode. A diode will get it’s diode voltage over itself as long as you have enough voltage. Put a resistor in series with the base, and the rest of the voltage will drop over the resistor.

      Reply
  21. Great explanation !! And now that I understand the transistor, the presence of R1 is a mystery. Why is it in the circuit ?

    Reply
  22. hey Mr. Oyvind your blog was very interesting and helpful to me than my Electronics lecture classes in university.you gave me the main concepts easily and I will proceed practicing to be better.my question is why does a transistor burns off ?

    Reply
    • Yes. But you have to replace it with same type (NPN with NPN, PNP with PNP). And if it’s not just a general purpose transistor, you need to make sure your replacement transistor can handle whatever circuit requirements that might be (ex current or speed).

      Best,
      Oyvind

      Reply
  23. i think i am getting somewhere with this…please can i have one more practical example of why we need the transistor in our circuits please i would appreciate it

    Reply
  24. So ive been looking into transisters i get the basics of it but i dont understand why you need it in a circut as a switch, why not just use a switch then. I also understand the uses for amplification, but to me the transister seems redundant as a switch. Can you please clairify?

    Reply
  25. I really love this website, it’s good for engineers……..but how would I connect this transistor with components to amplify current. Coz it can’t amplify on its own….thanks

    Reply
    • Hi,

      A small current from base to emitter gives a large current from collector to emitter. So the small current gets amplified to a large current.

      Best,
      Oyvind

      Reply
  26. Awesome Oyvind!!!, I now understand clearly what/how transistors work.But!!!!!!!!!- i have a question.Why do you place the transistor rather after the LED and the Resistor.

    enoh

    Reply
    • Great to hear!
      If you have the LED and resistor before, you will have 0V at the emitter of the transistor. This means that 0.7V on base will turn the transistor on. If you have the LED and resistor after, you need to have the 0.7V + the voltage for the resistor and LED on the base. It’s not really a problem, but everything is much easier if they are before.

      Reply
  27. can a transistor have just two terminals? I am trying to identify a particular components that looks like a transistor but it has just two legs. How do i send the pix?

    Reply
  28. hi Øyvind Nydal Dahl
    you are so clever
    you should have been my teacher when I took electronics course
    about 15 years ago
    wow so helpful
    by
    Abe

    Reply
      • thank u sir
        u means that the positions of + and -ve changes and line current conduct the small voltage battery(0.7V)
        thank you for the explanation
        which component i have to use in AC Supply iam trying to do automatic water level controller

        Reply
  29. Hi how can a common base transistor work, if the BE junction is a diode? I see the input coming in on the emitter on common base configs. If the input is on the emitter, how can current flow to the base, which is grounded, if the diode is reverse biased as in the case of an NPN?

    Reply
  30. Will you please tell me about the internal working of transistor, how electrons and holes flows through transistor and how do they affect working of transistor?

    .

    Reply
  31. What is the difference between NPN and PNP in application? Is it the direction of current? Is it the same 0.7V need to apply from base to emitter to turn the transistor ON? Thank you.

    Reply
    • Yes, the direction of current is opposite. It’s the same 0.7V, but because the current is opposite, you need 0.7V less on the base than on the emitter to turn the transistor on.

      Reply
    • That depends on the transistor.

      About 100 mA is normal for general purpose transistors. But, you have transistors that can handle much more.

      Reply
  32. Thank you for your beautiful explanation.keeping feeding us with easy to digest explanations. My question is ; can we place the transistor in another place within the circuit like for example between the 9v and led? And get the same result?

    Reply
    • Yes, it’s possible.

      But you need 0.7V between base and emitter, and with that new setup you no longer knows what voltage you have on the emitter. Thereby you make it much harder to figure out the voltage for the base.

      Reply
  33. Thanks for the great explanation. Really helped!!! My question: Since LED is before the resistor in the current flow, wouldn’t it burn out because of high voltage. Shouldn’t we place the resistor before LED

    Reply
    • Hey Arjav,

      A resistor reduces the flow of current in the whole circuit. Just like when you squeeze a garden hose, there’s less water flowing in the whole hose, not just after the point where you squeeze.

      Best,
      Oyvind

      Reply
  34. This is really help full. Can you also explain about transistorized series voltage regulator and voltage regulator with current limiting technique?

    Reply
  35. Dear Oyvind:

    Please delete my previous post as this post is an updated proofread version.

    I do not yet understand how a transistor can function as an amplifier. I am also trying to learn how to interpret schematics. This is what I see from your example:

    I see two subcircuits. The first on contains the the 0.7 V DC battery with the base and emitter of transistor, the functional equivalent of a diode. In the 2nd subcircuit, we have the 9 volt battery, the LED, the resistor, and the entire transistor. The first subcircuit is the on/off switch of the other. I see on/off as basically a pulse of infinite length terminated only when the battery loses enough charge to fall below the 0.7 V threshold or when the battery is disconnected. When the switch is turned on, so is the 2nd subcircuit. This 2nd subcircuit lets either 9 volts DC go through or nothing at all. (A questions on this will be deferred to the end.). We have a Math Problem in that we need enough resistance in R1 to prevent L1 and possibly Q1 from frying. Hence, we need to know what material the LED is made of ( Is there a General Table out there for all of the different semiconductors?). How do you calculate R1 given the 9 Volts, the LED Material, and the Transistor (Is there a table for Transistors?).

    As to the Amplifier question, i.e. the question previously deferred, what if we replace the 0.7 Volts DC with something AC? Do we get Square Wave? I really don’t see the amplification of a 0.7 Volts AC sine wave into a 9 Volt AC sine wave from this circuit.

    Regards

    Reply

    Reply
    • Hi Clayton,

      It seems like you have a good understanding of this circuit.

      You’re right that we need enough resistance in R1 to prevent L1 from frying. Q1 can handle much more current than the LED, so as long as we keep the LED safe, the transistor will be safe.

      In this circuit, it’s okay to simplify and say that there’s zero resistance in the transistor. That means you can look at the LED and resistor as if they were connected directly to a 9V battery and calculate the correct resistance:
      https://www.build-electronic-circuits.com/current-limiting-resistor/

      If you replace the 0.7V battery with an AC source you will not get a square wave. The transistor can operate in different regions. So if you set up the transistor in such a way that the voltage varies around a specific point, you’ll get an amplifier. For this to happen, you want to set the transistor into its “active region”. That’s another article for the future, but you can start reading here:
      https://learn.sparkfun.com/tutorials/transistors/operation-modes

      Best,
      Oyvind

      Reply
  36. As soon as you start start something on Amplifiers, I will have more questions which I cannot reasonably ask in this particular thread, though my AC question was a relevant attempt. I do have applications that will need them.

    Reply
  37. Hello! myself Ram Tripathi, i am a class 8th student from India, i am an electronics enthusiast and a regular vistor of your website for tutorials, I need your financial help to build my electronics project which i have planned to do this summer, please contact through email if you are interested to help me.

    Reply
    • You can find it in the datasheet of the transistor. If for example you have a bc547, just google bc547 datasheet and you’ll find the datasheet. Somewhere in there you’ll find an image that shows which is which.

      Best,
      Oyvind

      Reply
  38. hi on a common base config NPN transistor, how can current flow into the emitter if the diode from b to e is forward biased? Seems that the end of emitter would be the cathode end of a diode and nothing could enter
    on an PNP its easy to see but not NPN
    thanks and regards

    Reply
  39. Thanks a lot, I understand what you say, but I have a question: What about if my transistor is BC557 which mean that the arrow is from the EMITTER to the BASE, then how it works ?

    Reply
  40. How is this automatic ?

    We have to do some “event”, for giving .7 volt to emitter, then only LED will turn on automatically.

    But still we have to manually do some action ( like turning on switch ) to supply 0.7 volt to base to emitter, how can we make it completely automatic though ?

    Reply
  41. I’d like to clear up something else here. You used two independent power sources with your demonstration circuit. The 0.7 volt power source is the signal which is inputted through the base. Your 9 volt power source is just extra power that is inputted through the collector. The output through the emitter is the amplified signal, fact that which combines the 0.7 volt signal with 9 volts of power. My emphasis here is on the fact that the power sources are independent. Am I correct so far?

    Let’s take it to the next level. If I want to boost a signal from a 555 chip with a 386 chip, would the power sources will either have to be independent or the result of a voltage divider?

    Reply
  42. Your articles helped a lot when I began electronics in 2016 thanks 4 this but now I want to use transistors as power amps but the biasing a transistor is complicated and how the input is amplified also is confussing I don’t know if u can make it easy thanks.

    Reply
  43. SIMPLY THE BEST OUT THERE. ALL MY BEST THOUGHTS AND FEELINGS HAVE BEEN EXPRESSED IN THE PREVIOUS REPLIES. THE BITS HAVE FINALLY BEGAN FORMING PATTERNS IN MY BRAIN. THANKS TO MY GREATEST TEACHER I MUST CONFESS!

    Reply
  44. I don’t understand why it looks like the resistor comes after the led. What is the direction of current flow when the transistor turns on and how does the resistor protect the led ?Thanks.

    Reply
  45. thanks a lot for such a simple explanation.
    I’m from mechanical background, working with industrial robotics and it helped me in understanding the electronics circuits.
    WR/pardeep malik

    Reply
  46. Very nice simple explanation! I’m helping my son with his electrician study and your article and video is helping clear the confusion for us!

    Reply
  47. Hello.

    I liked reading this article. It was very interesting to learn about it in a simpler perspective.

    I’m currently attempting to make my own homemade wireless charger, and multiple sources have told me to use a transistor. I have questions as to why it is used, is it used as an amplifier, and if so, do you add the numbers shown on the package (e.g., 60v) to the already existing current (e.g., if I soldered a 5V 1A phone charger to the circuit).

    Here’s a link to the transistor I’ve been told to use: https://m.ebay.com/itm/10-x-PN2222A-PN2222-Transistor-NPN-40-Volts-600-mA/250833387082?epid=0&hash=item3a66d5be4a%3Ag%3AxlIAAOxyY3ZRzEIE&_trkparms=pageci%253A134aa294-be32-11e7-b118-74dbd180ec71%257Cparentrq%253A7247439c15f0a990c36c14b6fffaacaa%257Ciid%253A3

    My goal is that I get at least 5-15 watts being received on the receiving end (USB-C male plugged into Moto Z Play) to charge my phone.

    If you have any tips or information on this subject, please email me.

    Reply
  48. Thanks Man. I bought my son an electronics kit for his birthday , which he loves, but whilst I was able to explain most of what was happening, I struggled to explain how transistors worked. Your article has really helped. (3 1/2 years after you wrote it!!). Cheers dude.

    Reply
  49. This is THE best explaination I have come across.

    Simple accurate and easy for a newbie like me to understand

    P states and emitters meant nothing

    Your explanation and diagram was perfect

    I has saved your article and put a copy of it in my electronic bits box so I can refer back to it

    Reply
  50. Well Explained, Thanks. But i think after explaining what is it you should also explain why it is used and where it is used so that we learners get a very clear understanding on it. Anyway good job and you explained what is it in a very understanding manner, Thanks for your efforts and congratulations!!! keep up the good work brother.

    Reply
  51. Excellent. In your circuit diagram (unless it is arbitrary), why would you try to get the 0.7V from that spot? Is this a conventional-current and electron-current issue; are electronics engineers thinking in electron-current perspective? T/Y

    Reply
      • “spot” mean in connecting the base is connected to the 9V negative; why didn’t you draw up to the positive side (with a resistor to get down to 0.7V)?

        Reply
        • Ah okay, you mean why did I use an extra battery instead of just getting the 0.7V from the 9V battery? That was just to try to keep concepts separated.

          Best,
          Oyvind

          Reply
  52. Bipolar transistor more interesting .a small voltage at the base equal a huge current from collector to emitter.so without the applied voltage at the base no output current.
    Thanks a lot.

    Reply
  53. Hi!

    Some really good blog articles you have here. Simple, but convey the basics.

    A few things you might share with your readers (maybe you did in the video; I haven’t had a chance yet to watch it, only to read the text):

    – On a schematic, if the arrow in the transistor is Pointing iN Proudly, it’s a PNP transistor; if the arrow is Not Pointing iN, it’s an NPN transistor.

    – You mentioned the difference between PNP and NPN transistors in the comments; you might want to move that info to the main blog article. NPNs are activated when a positive (relative to the other two terminals) voltage is applied; PNPs are activated when a negative (relative..) voltage is applied.

    – Think of a transistor as a large water pipe with a small control valve. The water comes in at the top “collector”. The small valve “base” controls how much water gets through the pipe to be emitted out of the “emitter”. A small water flow at the base controls the much larger main water flow from collector to emitter. This small water flow into the base joins the main stream to come out of the emitter.

    – Some transistor designs are better suited to a fully-on or fully-off flow-rate, like the flush handle on a toilet, and some transistor designs are better suited to controlling a range of flow-rates, like the bathtub faucet that can be turned on at a dribble or at full-force or anywhere in-between. The former work best as on-off switches, and the latter work best as amplifiers, but in a pinch, with the right external circuitry, you can probably stretch the design of either type to sortta emulate the other’s capability.

    – In the case of an amplifier-type transistor, the base (control-knob) is usually set (or “bias”ed) to rest at about the half-way position. This is usually done by bleeding some of the current from the collector line through a resistor. Imagine a small pipe coming off the input end of the main pipe (the collector) running to the base “control knob”. This pipe is sized so that the valve is kept at its half-way position. (The size of the pipe corresponds to the size of the resistor; a larger pipe will let more flow through to open the control-knob wider.)

    – Now that the amplifier-type of transistor has been biased to let about half of the main current flow through the transistor, we can connect an alternating current flow, say, from a microphone, to the base as well. This partly-positive-voltage/partly-negative-voltage flow from the microphone will add to or remove from the bias-pipe’s flow, thereby creating a corresponding increase/decrease, only much larger, in the main current. Thus, the tiny signal from the microphone is amplified as larger fluctuations in the main current. Without that bias signal set to the half-way point, the negative-voltage parts of the microphone signal would just bottom out the main flow to nothing, and you’d only amplify the positive-voltage portions of the microphone signal.

    But regardless of these things, you’ve done a great job at explaining the basics!

    Reply
  54. So if the current from the 0.7 V source doesn’t exceed 0.7 V, what path does it follow? Does it just go to the ground of the 0.7 V source or go into the resistor?

    Reply
  55. I’m surprised transistors haven’t used a description of “normally open” (NO) and “normally closed” (NC) for the designer’s recognition rather than retaining the deep scientific back-thinking of NPN and PNP doping; It seems a WHOLE lot more intuitive. I’ve been trying to catch up on this stuff for YEARS now so I am able to (attempt) to fix things, which I have been able to do. I also see how “switch-mode” has become a whole separate frequency field (as compared to constant-DC circuits and AC).

    Your excellent plain-english explanations are very much appreciated as I’ve seen the sophisticated fancy laboratory talk overcomplicate practical understanding.

    Reply
  56. Hi Oyvind,

    One main thing that I do not understand about the transistor is why you would choose to use it over just connecting a pushbutton directly? In your video above, you add a transistor and have the circuit open on the left of it, and implied many things could be connected to the open lines to activate the transistor (and thus the LED), such as a pushbutton or light activated switch… But why have a transistor at all? Why not just hook the pushbutton right after the LED which will turn on and off the circuit? Why introduce this “middle man” transistor?

    Reply
    • One example: Let’s say you want your outdoor lamp to turn on when it gets dark outside. And let’s say your light-sensor gives you a signal of 0.7V coming when it’s dark. Your lamp probably needs 110V or 220V to turn on, so the tiny signal coming from your light sensor can’t turn on the light. But with the transistor, this tiny signal can for example control a relay, which in turn controls the 110V or 220V power line to your lamp.

      Reply
  57. This is very good explanations on electronics. I’ve same problems as a beginner understanding how other articles and videos explained about transistor. Now I think I have understood how these things work.

    Thanks Obi-wan-kenobi

    Reply
  58. Yap I quite understand how transistor works now but the question is, is it possible to use just one power source (9vbattery) to power both the led and the transistor using resistor to bring the voltage down to 0.7v?

    Reply
  59. I think it would be good if you told why the 0.7 volts makes it start operating.

    And I think it would be good if the fact that Positive is ‘negative’ and Negative is ‘positive’ inasmuch as the Positive end of a power source has more electrons (i.e. is ‘negative’) and the Negative end has fewer (i.e. is ‘positive’) were told and explained.

    Unless that’s all wrong?

    That it was an initial unfortunate convention to state electricity ‘flowed’ from P to N when the electrons actually flow from N to P.

    I feel this becomes important in discussion of transistors because we are talking about electron flow one minute and the next minute using ‘conventional flow’ electricity to make it operate!

    Reply
    • Hey Arthur,

      Thanks for your feedback.

      About current flow:

      “Current” is an abstraction from what actually flows on a low level, and the convention is that current flows from positive to negative.

      If we look at what flows on a low level, it could be negative charge carriers (like electrons) that flow from negative to positive, or positive charge carries that flows from positive to negative.

      In a circuit, you can have both of these in action at the same time.

      I like to simplify, so that’s why I use conventional current flow from plus to minus.

      Best,
      Oyvind

      Reply
  60. Hi mate,

    Nice explanation, but I am confused about one thing. Isn’t the 9V (or whatever it is after accounting for the LED) sufficient to overcome the internal resistance of NPN junction directly? i.e. since we only need 0.7V to overcome the PN junction from the Base to Emitter, isn’t that essentially the same as requiring 1.4V (or thereabouts) from Collector to Emitter to achieve the same outcome, since the transistor is symmetrical?

    Or have I just made a huge mistake there by stating that the transistor is symmetrical?

    Many thanks,
    Seb

    Reply
    • Hi Seb,

      The current flow in a PN junction is from P to N. So even with 1.4V from collector to emitter, the current won’t flow without the help of the base.

      When you have current flowing in the PN junction from base to emitter, some “magic” happens in the connection from collector to emitter that makes it flow.

      (Lots of info exists on the “magic” part if you want to investigate that further)

      Best,
      Oyvind

      Reply
  61. 1. what happens if we gave a voltage more than 0.7 v to the base?
    2. what happens if we forward biased the collector and base ,and reverse biased the emitter ?

    Reply
  62. Your little LDR circuit is funny. When you have light, the circuit gives you more light. When you have dark, it’s even darker. So if you put the bottom between the base and the emitter and the resistor between the base and the power supply it should work the other way around. As you get more light the resistance across the LDR decreases and therefor the voltage drop decreases till you reach the point that the voltage at the base is lower than the 0.7V and the light goes out. As it becomes dark the resistance increases till you have the trigger voltage and the light turns on.
    I’ve just invented the automatic night light.

    Reply
  63. Dear sir, instead of giving YouTube link for videos why don’t you just upload videos on website as YouTube is distracting website and can be dangerous.

    Reply
  64. I’ve just seen your book on Ebay.

    “Electronics for kids”

    I’m a complete novice but can do smd repairs. I would like to understand more about how circuits work.

    Would this book be any good for me?

    Im 39 lol.

    I’m loving your website by the way.

    Reply
    • Hey Ollie,

      Absolutely. Don’t let the “for kids” title scare you away. I’m not holding back on the information, I’m just explaining using simple words so that also kids can follow along and learn.

      Best,
      Oyvind

      Reply
  65. I am trying to use a NPN resistor to repeat a digital signal from one circuit board to another. This is for the clock and data signals going from a microcontroller to a mic5841 driver chip. The signal needs to go to 40 boards, and so a fresh signal needs to be generated to be sent to the next board. So I put the original signal at 3.3 volts (from a Raspberry Pi) into the base, and I have 5 volts on the collector and I am expecting to get 5 volts on the emitter, but I only get 2.6 volts. I tried this with the 2n4400 and the 2n2369. After the first board, the signal on the base should be 5 volts, but if I apply 5 volts to the base, I still only get 4.5 volts on the emitter. I need to get 5 volts coming out. What can I do?

    Reply
  66. Mr.Dahl. You make me understand how transistor works better than before!!! Thank you so much!!!

    Sometimes, i think that good teachers like you, Mr.Dahl, are much better than the boring and complex electronics books!!!

    Reply
  67. Transistors are used in circuits to control high current/voltage. Eg, in big industrial machines transistors are used in circuits for controls. Relays/contactors which then controls higher voltage.

    Reply
    • Does it matter where you put the resister? I assume that the triangular arrow (on the emitter side of the transistor diagram) is the direction of the current, putting the resister after the led.

      Reply
      • I doesn’t matter if you place the resistor before or after the LED. But you have to place it on the collector side of the transistor.

        Reply
  68. You can imagine transistor like three inductance coils connected by triangle. For example: coil#1(Emiter-Base) have 44turns, coil#2(Base-Collector) have 10turns, coil#3(Collector-Emiter) consist of two subcoils with 22 and 66 turns connected by parallel. Imagine that coil “Emiter-collector” have additional electrical power.

    1)When you connect income conductors to coil#1(Emiter-Base) you can get powered voltage on outcome conductors from coil#2(Base-Collector).
    2)When you connect income conductors to coil#2(Base-Collector) you can get powered amperage on outcome conductors from coil#3(Collector-Emiter).
    3)1)When you connect income conductors to coil#1(Emiter-Base) you can get powered voltage and powered amperage on outcome conductors from coil#3(Collector-Emiter), don’t forget about additional power here.

    P.s Semiconductors have no electron-hole conductivity, but have immobile ion coductivity. An electrons located on positive ion strive for aproach to core, an electrons on negative ion strive for distance from core. Electric fields can go only on electrons. When “minus” and “plus” electric fields expand its ions, electric fields can go further, when electric fields squeeze its ions they can not go furter(because big distance between ions) and electric current will block.

    All types of transistrors work by coil transformer principle, where both coils is not equal. In PNP transistror both P-areas in is not equal(by mass or consist) too.
    Magnetic field of Base can expand ions of Both p-areas(join it together), and allow to flow electric fields between its.

    Aforementioned example with three coils is just accurate analogy, really transistor consist base plate, with nearly located emiter and collector droplet on it. In cross section it is a triangle.

    Reply
  69. My tv broke, and I’m going to fix it. You are a talented story teller and your way of explaining this story is wonderful. Thank you so much! I am making great progress in my understanding

    Reply
  70. You sad min voltage is 0.7V. But what is max voltage? What if i connect 1.5V battery between base end emitter to control larger cirtuit? And how large are we speaking here? How many Amps/ Voltage?

    Reply
  71. Have zero knowledge about programming and engineering but got interested in Arduino projects … so when you write a program to do certain actions, it actually turn the transistor on and off or …?

    Reply
  72. hi can you tell me please, in a single phase motor using a transistor, how is the current switched on and off repeatedly to get a full rotation of the motor, im not sure if im asking the right question, but i recentlyt watched a home made motor video, using 2 coils and a magnet and a transistor was wired into the circuit, and in my underswtanding of a motor you need countering forces north south north south to get the magnet to turn otherwise it would just move slighly and not rotate, i understand the transister can switch on or off but how does it do it repeatedly on off on off and so on. thanks

    Reply
  73. “In a standard NPN transistor, you need to apply a voltage of about 0.7V between the base and the emitter to get the current flowing from BASE to emitter.”

    I think you mean COLLECTOR instead of BASE?

    “In a standard NPN transistor, you need to apply a voltage of about 0.7V between the base and the emitter to get the current flowing from COLLECTOR to emitter.”

    Reply
  74. i have a question why mosfet use voltage to control amplifier so i think we need use resistor series with transistor why it is parabeel?

    Reply
  75. explained very well. could you please also explain in what situations
    would we use MOSFET instead of a regular BJT transistor?

    Reply
  76. Dear Author

    So I assume the Gain of the Transistor resulting in a LARGER CURRENT flowing from the Collector to Emitter is actually the MAXIMUM CURRENT OR the MAXIMUM COLLECTOR COLLECT the Transistor can support?

    Reply
    • No, the gain is the relationship between the base-to-emitter current and the collector-to-emitter current.

      Ex if the gain is 100 and the base-to-collector current is 5 mA, then the collector-to-emitter current would in theory be 500 mA. But it might be that your transistor only supports 100 mA, so you would fry the transistor.

      The maximum current through the transistor has more to do with how much heat that is generated and how much heat the transistor is able to handle.

      Reply

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