Tutorial: How to design a transistor circuit that controls low-power devices

I describe how to design a simple transistor circuit that will allow microcontrollers or other small signal sources to control low-power actuators such as solenoid valves, motors, etc.

[mateoish]

I’ve watched dozens of transistor vids looking for exactly this info and none of the others put it so plainly and comprehensive. Thank you.

[brianmcdermott]

I've never watched a video that was as informative and clear. Your explanations are complete without getting overly complicated. Thank you so much for making these.

[AppliedScience]

When the transistor turns off, current flowing through the coil will cause a voltage rise on the collector. If the voltage rise is high enough to turn on the diode, current will flow through the diode and limit the voltage at the collector. The problem is that the diode cannot turn on instantly. It takes time for the diode to start flowing current. The capacitor smooths out the rising voltage, and gives time for the diode to start conducting.

[ssl]

For 30 years I've seen explanations of how to use a transistor and never "gotten" it. Bravo this is spectacular. The best explanation of a bipolar transistor I have ever seen in any medium.

[ovidiub]

I wasted weeks learning all you say in here, which I could have learned in ~20 minutes. Great video. Please add more tags to it, so you rate higher in search results.

[jimcashman]

One of the clearest explanations I have seen online. And as a newbie electronics hobbyist, I have looked at a lot!

[Burnitnow]

Dammit, I wish the internet was around when I was in school!! You kids are sooo lucky to have all this knowledge available instantly!

--OldGuy :)

[sfperalta]

Clear and well-presented. Amazingly, this circuit was pretty much the same as when I started studying electronics back in 1975, including the 2222 transistor! So for you younger students, the techniques outlined in this video will practically never become obsolete. We tend to think that the world is run on low-voltage, low-current technology (e.g. microprocessors, microcontrollers). But at some point real work requires higher voltages and/or higher currents, so studying transistor control circuits, like the one presented here, is very valuable knowledge.

[felippesilva]

Good Tutorial. Thank you for that. Just another thing:  You have to consider the power dissipation of the transistor itself,  due to the voltage drops across it. taking the numbers (0.2Volts and 56mA), you can say that the transistor dissipates like 11mW of power, which is quite low for that circuit, but if you have a larger current and also a larger voltage drops on the transistor, you could damage your transistor if you don't attach it to a proper heatsink.

[calyodelphi]

@2:54 Your description of the circuit symbols for NPN vs PNP BJTs is so ridiculously simple and intuitive
@7:00 So THAT'S what hFE is used for with BJTs!

This whole video is so informative it's helped me to make tons of sense of how BJTs work. You, sir, are a genius. <3

[TheChangospace]

How I wish I had friends as smart as this guy in my circle. I love channels that pass along wisdom to others....thank you, Sir!

[artswri]

Great video, very practical with enough math to do the job but not graduate-level deep dive to obfuscate things. A great tutorial for learning practical design. Thanks much!

[JonathanAnon]

This video should be shown to everybody starting electronics... No talk of doping and PN junctions, show them this first so that they get a picture of what they are doing.. Too many electronics courses start at the microscopic level.

[GarethJefferson]

So clear. So lucid. This is a great switching transistor 101. Thanks, really appreciate the work you put into this.

[rajanne]

Very easy to understand you have a lot of patience! I obtained some extra insights by watching your exposition that with 20 years of doing electronics i was able to obtain today!

[fabriziolavini]

I think i watched hundred of videos on the same subject.
This is the first time that i see someone explaining every single step, all others always take those for granted.
And abole all i appreciated the first part, when he described the several choices (possible components to use). All the designers make tutorials and never clear why they use a particular component among others and indeed they must know the reason (again take that for graanted).
I congratulate and thank you.

[AM-dnlk]

Why couldn't I have had you as my electrical theory teacher. What I learned in 21 minutes with you took me weeks with my teacher....and I probably still did not quite digest and assimilate it well enough. Thank you for your tutorials and lectures. You are educating the world.

[kwgm]

Nice job. I was trained as an EE, and in the lab I blew up dozens of 2N2222 transistors. Later, as an "advanced" student, I destroyed a few Op Amps before I got the hang of their biasing. In the late 1970's, a simple NPN transistor was inexpensive - maybe 25 cents at the school lab stock room. Op Amps were much more expensive in the 1970's -- maybe a couple of dollars -- so I learned to read the data sheet carefully before breadboarding those projects.

When in doubt, add a resistor! It's a good motto for any EE student or dabbler in the mysteries of electronics. One of the most valuable circuits a designer should know by heart is the voltage divider. Such a simple circuit, and so many fresh faces in the lab don't know it. Lesson 2, a little more complicated, is using a negative feedback circuit to automatically control current surging at the base and thought the transistor. Always protect your active components! I appreciated your trick to sink stored current around the solenoid, too.

Thanks again. I'm retired and haven't played with electronics in a long time. I've been looking around YouTube for interesting channels. and yours is practical without being pedantic. I'm a new member and will be looking in.

[richardmitchell]

First year electrical engineer student.
Thanks for this video, I think you are a great tutor.

[docfoot]

I learnt alot from this video, well shown and explained, and i am 65yrs and still learning, Wish i had someone like this to teach me in my younger days .Many thanks .