The two most popular kinds of transistors you might use for a project are either BJTs or MOSFETs.  With so many options, it was tough to pick just a few.  

We covered the 2n3904 in this video.  What BJTs are your favorites and why?  Leave a comment below.

Transistors are electronic switches that are used to control things that would kill an Arduino, Raspberry Pi, or Beaglebone.  There are two major types known as a BJT and MOSFET.  This is part 1 of a two part series, which looks at BJTs first.

Learn what they are and how to use them.

Don’t forget to subscribe to the AddOhms YouTube channel to know when Part 2 on MOSFETs is released!

Episode Notes

Suggested links for Ep10:

Inside a NPN BJT has an awesome picture of the die inside of a Fairchild S9014 BJT NPN. has a ton of other great images, check out their bare die pictures.

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  1. Carlos Silva Reply

    I James, this is Carlos, from Portugal, do you have any information about a Mosfet ref. CMU06N02N 1209? It’s a Mosfet to control a DC motor for an RC unit.Thanks in advance and congratulations for your excelent way of teaching.Regards.Carlos 

  2. Rusy Shackleford Reply

    Thanks for the video. I am confused though, it seemed you assumed HFE to be 100, even though the spec sheet had many values listed depending conditions. For example 100 HFE was for IC = 10mA and VCE = 10V.

    From the video I don’t see calculations made for IC or VCE so how can you be so sure its 100HFE? I understand HFE is a bit of a moving target. However unless I am missing something (which I’m sure I am) HFE is critical to determining how much current you will get between ICE. I’m struggling with finding a way to find out what HFE is for a given input current and desired output current.

    For example, if I have my Raspberry Pico which has 3.3 V GPIO lines and I connect one to the base and I want to have 20mA available to me at the collector so I can “max power” an LED how can do this, if I dont know what HFE is? if I guess 1000 say in this case the LED would be toasted.

    Is there a way using a multimeter and/or bench top power supply to determine what R values I need to use to get to 20mA? HFE is spread from 35 to 500 depending on the BJT. So Without guessing HFE how can I find out. I’ve ordered a tool that will measure it but I want to know how it can be done using just a bread board, some wires and an LED or another device? I’ve been searching for a good answer on this all over and cannot find it. HFE is assumed in almost everything I read. There has to be a way to test this without blowing through LEDs until I find a combination that hits 20mA. I know my problem is mostly going to go away when I get my measuring device (Peak Atlas DCA) which will help with other devices as well. Thanks.

    • You can’t use the transistor to limit the current for an LED. You still need a current-limiting resistor.

      When using a BJT as a switch, you really don’t care what its hFE is since you’re driving into saturation anyway.

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