The 4 best transistors to keep in your parts kit

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When your project needs a transistor, there are tons of choices.  Which makes answering the question “Which transistor should I use or buy?” a daunting task.  Fear not, before wading through spec sheet after spec sheet, consider one of these four general purpose transistors.  Every electronics enginerd’s toolbox should have a few of each.

Transistors are one of the most versatile discrete components in electronics.  In digital circuits, they switch on and off while in analog circuits they are used to amplify signals.  For most projects, they are used to turn on a load that would kill the I/O pin of a microcontroller or microprocessor.   For most circuits either a BJT or MOSFET can be used, depending on the load current you need to switch.

[Edit Note]  Jan (comment below) points out that there are European Equivalents that may be more available for those located in that part of the world. For NPN Check out the BC547, for PNP the BC557.

Here are some more details on each of these.

Best Transistors: BJTs

Bipolar transistors come in small packages, can be driven by I/O pins directly, and are VERY cheap. There are two variants, the NPN and PNP. These little guys are the workhorses of most control circuits, for small current applications. You’ll commonly find through-hole parts in the 3-pin TO-92 style package.

#1  NPN – 2N3904

You can find most often NPN Transistors in low-side switch circuits. This configuration means whatever you want to control is connected between the “high” voltage and the collector of the transistor. Check out this post for more information on low-side vs. high-side switches.

A common transistor I use is the 2N3904. You can easily switch big, like great than 12 volt loads with this transistor’s max 40 volt rating. Its current rating is only 200mA, but that is enough for most relays.

2N3904 from Mouser

#2  PNP – 2N3906

For high-side switch circuits, you need a PNP style BJT. A high-side circuit is where the load sits between the transistor’s collector and the circuit’s ground. Its emitter connects to the “high voltage.” Since I recommended a 2N3904 for the NPN, I will suggest its complement: the 2n3906. Like the NPN, it has the same max voltage and current ratings: 40V and 200mA. Check out this post for more information on low-side vs. high-side switches.

2N3906 from Mouser

#3  Power – TIP120

One of the advantages of BJTs is that they are easily driven from an Arduino or Raspberry Pi I/O pin. When configured as a “Darlington pair,” they can provide significantly higher current capability than single transistors. The TIP120 is a Darlington pair that can handle as much as 5 amps when in a TO-220 package. You sometimes see the same package used for LM7805 linear regulators. If you want to drive that much current, do not forget the heat sink!

TIP120 from Mouser

Best Transistors: MOSFETs

When you have to drive many amps of current, MOSFETs are fantastic. However, most do not work at “logic levels,” meaning they typically need 10 to 15 volts to switch them on properly.  Such a high voltage is tough to reach for an Arduino’s 5 volt I/O pin, let alone a Beaglebone or Raspberry Pi.

If you are new to MOSFETs, check out my MOSFET video tutorial (scroll to bottom) and this post on dispelling MOSFET myths.

#4  N-Channel (Logic Level) – FQP30N06L

These workhorse transistors have a max 60 volt and 30 amp rating. Not milliamps. Amps! (Though, you will need a heat sink!) They cost nearly 2X what a TIP120 costs, but they drive way-way more current.  The best part?  With a “logic level” compatible Vgs-threshold, an Arduino can easily drive them with its 5.0 volt output pin. These properties are why I keep a pile of FQP30N06s on hand.

FQP30N06L from Mouser

FPQ30N06L from Amazon

Conclusion

These four general purpose transistors will cover a wide range of uses. Having a couple of each in your box will come in handy for nearly any project.  Leave a comment below on which transistors you keep on hand.

Update:  I’ve added a quick note about the European alternatives for NPN and PNP BJTs.

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70 thoughts on “The 4 best transistors to keep in your parts kit”

  2. HELLO MY FRIEND! could you help me….
    l would like to ask you for a type of transistor that can switch up a consumer of a 2A 3v by a digispark pin of a 3.3 v !
    the supplier for the both of them is a3.7 lipo !

    it is for model of airplane so I need a smd transistor and a “regular” too but a small or light one !

    Thanks in advance

    Reply

  5. HELLO … ! you have a great site !!
    i wanted to ask about BJT tutorial – you finished of a saying that the transistor is forcing to current from the co. to emi. to be 100 mA , and you get 6 v battery , then I’m not understand – what now this circuit got forced to be in v and I and what is the analyse that i do now for enter another consumer than a motor , lets say another motor (my motor don’t have any v properties , i just found that it draws between 60 to 200 mA on 5 v ?

    Reply

    • Current is not forced at any time. Current is allowed to flow. The “beta” of the transistor is the ratio of current between Base/Emitter and Collector/Emitter. If the transistor has a beta of 100, and the current from the Base to Emitter is 1mA, then up to 100mA will flow between the Collector and Emitter. Beta varies from transistor to transistor and it varies depending on the amount of current flowing (which makes it sometimes difficult to predict.)

      Reply

    • For NMOS driven by 3.3V (1.8V also), smd package, I’d use Si2312 [email protected]
      VGSth=0.8V, Ciss=740pF IDSmax=4.2A.
      We should use these when the in/out capacitance is not a problem. In my current project, I need very small capacitance between drain/collector and source/emitter, SOT-23, so I’m limited to a NPN BJT instead.
      MMBT2222, BCW66, BCX19 all look good.
      Notice that the current gain of BJT depends on the target Ic. For my design, Ic would be 150mA, hence the above devices are suitable.

      Reply

  9. Thats a good list!

    The european versions of the 3204 and 3906 transistors are like the BC 547 and BC 557. These are most common here (theres a huge list which compares BC and 2N types)

    Anyway I´d like to add the BS170 low power mosfet to the list. I use it all the time instead of a low power NPN. Not logic level but have a look at the datasheet, DS-resistance is low enough @5V gate voltage…

    The two high power logic level mosfets I always have at hand are:

    IRLU 024N (55V, 17A; cheap and SMALL PACKAGE)
    IRL 3803 (30V, 120A; quite expensive)

    Reply

  10. Your description of #1 is a Low side switch, not a high-side switch.

    Look at Linear Tech data sheet for LT1089. It shows an NPN with collector tied to the positive rail, and the load is from emitter to ground.

    Have a look at the block diagram for LTC4446. Control of the transistor that is connected to the positive rail is the high side, the transistor that is tied to ground is the low side.

    Opposite mistake on #2. Your description is a high side switch, not low side.

    Thanks for #4, good find.

    Reply