Interfacing Microcontroller: Darlington Transistors

In this video I’ll tell you what is darlington transistors, how to use them to switch medium current loads with microcontroller and what downsides they have.
Also you’ll find out about darlington arrays. Of course there would be small demonstration of controlling different loads at the end of this video.

In case if you don’t know how to use an ordinary bipolar transistors, check out this two videos:
How to design a transistor circuit that controls low-power devices from Ben Krasnow
SparkFun According to Pete 7-5-11: Diodes and Transistors

Schematics & datasheets:

All photos are clicable!


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  1. Mike says:

    Nice tuturial. I finally understand what’s the hFE is about. Thanks!

  2. Ignacy says:

    Excellent video, thanks!

  3. George says:

    Great tutorial! And nice demos at the end!

    Can you explain what the R1 and R2 resistors in the darlington configuration do, i.e., how the work and what problem they fix? Thanks!

    1. admin says:

      Both R1 and R2 are there to improve switch-off speed of transistors.
      Well, R2 is more important, because Q1 cannot switch off Q2 really fast without it. But R1 in most cases is not so mandatory. It depends on what you driving darlington with. If it’s microcontroller, than in most cases you don’t need R1 because you usually switch mcu pin back and forth from sink (GND) to source(VCC), but not in tri-state (high impedance state, when port has very high input resistance almost like open circuit) and that’s why mcu can switch-off Q1 as fast as switch it on without R1.

  4. patb says:

    Thanks for another great tutorial. You could also explain maybe why do we need those protection diodes for inductive loads in one of the future vids.

    Fantastic work! Can’t wait to see your next tutorials.

    1. admin says:

      You cannot momentarily change voltage across capacitor. But across inductor you cannot momentarily change current! It will resist to fast changing current.
      So when some current is flowing through inductor and you all of a sudden opening the circuit (disconnecting power from inductor) it will try to resist to changing current flow by raising the voltage as high as it can but with reverse polarity. And this voltages could be hundreds of volts, which is most likely well over your transistor reverse breakdown voltage limit. So, it will instantly brick the transistor.
      And that’s why you need to put diode across inductive load in reverse. So, when disconnecting power from inductor, current will flow through that diode, and not the transistor.

      But these darlingtons, they have protection diode across collector-emitter, so they are protected from that, but instead current from inductor can flow through power supply or through other circuitry on the same power rail causing in best case a lot of noise.

      1. patb says:

        Thanks for the explanaition. I know that these are absolute basics, but as you can see not everyone knows about it. How about a video about inductors only? Just an idea for your next tutorial.


        1. admin says:

          Thanks for idea, but it’s already been done. There’s a lot of videos on inductors out there.
          For example this one: http://www.youtube.com/watch?v=NgwXkUt3XxQ

  5. kiT says:

    Very useful info on darlingtons! Thanks!

  6. Jason says:

    Is there a way I can use this principle to vary the output voltage.

    I am trying to control a 16V output for a slot car (scaletix) to be controlled from a PLC.

    I have a PLC with either 0-10V (@24mA max) output or 0-20mA output from an analog.

    I would like to vary the PLC current and have the voltage change on the output to speed up and slow down the car.

    Is this possible?

  7. Mark says:


    I need to switch a lightbar using microcontroller with refference voltage 12v and the light draws current up to 8A. So when the microcontroller sends logic “1″ or 5v , then one of the light on the light bar should glow (may draw 1.5a current) and there are many lights on the lightbar. When all the lights on the lightbar glows, it will draw approximately upto 6A. For this i need a switching circuit, which transistor would u suggest and if possible can u send me the circuit please. First I used the combination of NPN & PNP transistors but didnt work. Then I used N channel Mosfet and darlington PNP combination, it worked but the darlington is getting heaten up. So kindly help. .

    Thank you

  8. tatticus says:

    Are the relays really needed in the example shown in the video with incandescent lights?

    1. JumperOne says:

      The other way to drive those lights would be using triacs or IGBTs or even bipolar transistors. But relays – is a simplest way. They’re easy to use and they isolate low voltage from HV side.

  9. Pietro says:

    Perhaps you already said that in other post or video but, what microcontroller you used in this project? what is homebrew looking devboard you use there? Good tutorial man, great work!

  10. wail chalabi says:

    thanks a lot for this tutorial i really aprciate it :)

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