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 of these 4 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.
In short the 4 I recommend are:
Here’s some more details on each of these.
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
NPN Transistors are used in low-side switch circuits. This means whatever you want to control is connected between the high voltage and the collector of the transistor.
A common transistor I use is the 2N3904. You can easily switch big 12 or 16V loads with this 40V transistor. The current is rated at 200mA which 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 is placed between the emitter and ground. Since I recommended a 2N3904 for the NPN, I will suggest its complement: the 2n3906. It is also rated to 40V and 200mA.
#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 5A when in a relatively large TO-220 package. (You sometimes see the same package used for LM7805 linear regulators.) If you want to drive that much current, don’t forget the heat sink!
When you have to drive a lot of amps of current, MOSFETs are awesome. However, most do not work at “logic levels” meaning they typically need 10-15V to switch them on. So most can’t be driven by an Arduino I/O pin’s 5V output. Which means, forgetting about a Beaglebone or Raspberry Pi.
#4 N-Channel (Logic Level) – FQP30N06L
These killer-transistors are rated for 60V and 30A. Not milliamps. Amps! (Though, you’ll need a heat sink!) They cost nearly 2X what a TIP120 costs, but they drive way way more current. The best part? They are “logic level” compatible and can be easily driven from 5V! This is why I keep a pile of FQP30N06s on hand.
These 4 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: Originally, I mixed up high-side and low-side switches, it is fixed now.
Update: I’ve added a quick note about the European alternatives for NPN and PNP BJTs.