Comments: Transistors

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  • At one point you use a PNP transistor as a high-side switch for a 12V motor. The diagram shows 0V = on, and 5V = off. Then the text referring to it says: “This circuit works just as well as the NPN-based switch, but there’s one huge difference: to turn the load “on” the base must be low. This can cause complications, especially if the load’s high voltage (VCC in this picture) is higher than our control input’s high voltage. For example, this circuit wouldn’t work if you were trying to use a 5V-operating Arduino to switch on a 12V motor. In that case it’d be impossible to turn the switch off because VB would always be less than VE.”

    This is pretty confusing. This text seems to be saying that the circuit in the diagram won’t work if you use a 5V Arduino, but the diagram itself says it works with a 5V control and 5V = off. I’m pretty sure there’s a small wording change that would make this clear, but the way it’s written now the wording and diagram seem to contradict each other.

  • What does Vb, Vc, and Ve mean, does it mean voltage across the start of the junction to the end of it?

  • This tutorial was very helpful for me as an inquisitive beginner.

    Any chance of getting another one that covers MOSFETS in similar fashion? I have gathered they have some benefits over BJT’s as high side switches, but I am having a hard time understanding the details.

  • An easier way (at least for me) to remember the direction of the arrow of the emitter is to remember that it points to the conventional current flow: from Positive to Negative. So that means NP->N and P->NP. This works for me.

  • Some sources say voltage is the difference in electric potential between two points. So when this page says the voltage at one point has to be greater than the voltage at another point, does it mean the electric potential has to be greater?

  • Why doesn’t the voltage from the collector to the emitter have to be forward-biased in order for a NPN transistor to enter saturation mode?

  • Nice tutorial. It could be improved a bit though. The other commenters have given very reasonable suggestions as to how to do this (e.g. a bit more info on the voltage drop across the LED in the animated switch as well as @bokubob’s comment about the confusing image). Also, it seems there’s another small mistake with the AND-gate: “If either transistor is turned off, then the output at the second transistor’s collector will be pulled low.” The output of the 2nd transistor is the emitter, as it is an NPN-transistor.

  • Thanks for these awesome tutorials!

    In the animated switch image (switch-animated-600h.gif)… when the transistor is cutoff, how did you get a 1.3v drop across the LED, leaving Vce as 3.7v? I would have thought that the drop across the LED would be 0 (since there’s no current), so Vce would be 5v.

  • Great tutorial. I just want you to notice an excusable lack of basic informations and a little mistake.

    When you explain (very well) the astable multivibrator, you simply don’t cite R2 and R3 in the explanation, except than in the final formula. I think a newbie - me too, why not? - should know the very role of them, so do you? If they are so important to deserve a place in the formula, they should be mentioned as well in the text above (IMHO).

    And about the mistake, you say: “Here is the circuit inside an LM3558, a really simple op amp”. The link to LM358 is correct, but text says LM3558. Or is it some kind of weird mashup between LM358 and NE555?

    Thank you.

  • I think this sentence is confusing: “A PNP works in a same but opposite fashion. The base still controls current flow, but that current flows in the opposite direction – from emitter to collector.”

    Looking at the previous paragraph: “The NPN transistor is designed to pass electrons from the emitter to the collector (so conventional current flows from collector to emitter).”

  • You know what would be a freaking amazing beginner tutorial (video or article or whatever) is if Sparkfun (or someone else) did a complete point by point walk through (along with definitions and explanations) of a datasheet. You linked to the datasheet below in this tutorial so it would be a great extension of learning transistors to use that one… thanks!

    https://www.sparkfun.com/datasheets/Components/2N3904.pdf?_ga=1.138318832.1314341594.1422049611

  • Really enjoyed this, thank you. I’ve always been confused about WHAT the emitter and collector are in reference to, base!

    Small error in Applications I: Switches image https://cdn.sparkfun.com/assets/learn_tutorials/1/9/3/pnp-switch-motor_2.png The 5v supply should be 12v in that example (to match with the text)

    • Good catch. Thanks!

      • Now the graphic shows a situation that doesn’t do anything useful, but isn’t marked as such. I think it’s much more confusing now than with the original graphic.

  • Great tutorial! A minor typo in Operation Modes under Reverse-Active, where it says, ‘Current flows from emitter to current.’ Overall, a really effective teaching aid! - Leonard


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