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.
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.
#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.
#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!
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.
#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.
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.