The best transistors to keep in your stock of parts

When you need transistors for a project there are a multitude of choices. This can make answering the question “What transistor should I use?” potentially very confusing. Fear not, before wading through spec sheet after spec sheet consider using one of these general purpose transistors that are included in the ADALP2000 Analog Parts Kit.  Every engineer’s stock of go-to parts should have a few of each.

Transistors are one of the most versatile discrete components in electronics. In digital systems they switch signals on and off while in analog systems they are used to amplify signals. For many projects, they are used to switch on and off a load that requires current much larger than the I/O pin of a microcontroller or microprocessor could supply. This can often be accomplished with either a BJT or MOSFET, depending on the load current and voltage you need to switch.

In short, the ones we recommend are:

    BJT, NPN, 2N3904

    BJT, PNP, 2N3906

    Power NPN, TIP31

    Power PNP, TIP32

    N-Channel MOSFET, ZVN2110A

    P-Channel MOSFET, ZVP2110A

    Power N-Channel MOSFET, IRF510

There are European and Japanese equivalents that may be more available if you are located in that part of the world. For NPN look at the BC547 or 2SC1815, for PNP the BC557 or 2SA1015.

Here are some more details on each of the ones on our list.

BJTs

Bipolar transistors come in small packages. With Vbe around 0.7 volts, the base can be driven with a high value resistor directly from CMOS I/O pins and they are generally rather low cost. There are two variants, the NPN and PNP. These are the workhorses of most control circuits, for low current applications. Commonly found as 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, with the emitter connected to ground as shown in the LED driver example shown in figure 1.

Figure 1, NPN as low side switch

A common NPN transistor 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 many small DC motors, relay coils or LEDs.

2N3904 from ALP2000

#2 PNP – 2N3906

For high-side switch circuits, you need a PNP type transistor. A high-side switch is where the emitter is connected to a positive voltage supply and the load is placed between the collector and ground. Since a 2N3904 was included for the NPN, its complement: the 2N3906 is included. It is also rated to 40V and 200mA.

Figure 2, PNP as high side switch

2N3906 from ALP2000

#3 Power – TIP31, TIP32

One of the advantages of BJTs is that they are easily driven from a CMOS logic I/O pin. The beta of the NPN TIP31 and PNP TIP32 can be a low as 25 which might not be high enough when driving heavy load currents. When configured as a “darlington pair” with a 2N3904 or 2N3906, they can provide significantly higher current capability than single transistors. The TIP31 can handle as much as 5A when in a relatively large TO-220 package. (You sometimes see the same package used for linear regulators.)  If you want to drive that much current, don’t forget to use a heat sink.

TIP31 and TIP32 from ALP2000

#4 MOSFETs

Small signal MOSFETs like the ZVN2110A and its complement, ZVP2110A can have advantages over BJT transistors in that they do not require current to turn them on, only voltage on the gate. This can be ideal when making high input impedance buffer amplifiers. The threshold voltage for the ZVN2110A can range between 0.8 and 2.4 volts, and from -1.5 to -3.5 for the ZVP2110A, which is considerably larger than a BJT Vbe and vary much more device to device which can make using them as analog amplifiers more tricky.

ZVN2110A, ZVP2110A from ALP2000

However, many MOSFETs do not work at “logic levels” meaning they typically need 10-15V on the gate to switch completely on. Such transistors with a large Vth can’t be driven by a CMOS logic level 5V output.  Which means, forget using these directly with microcontrollers or single board computers.

When you have to drive a lot of amps of current, Power MOSFETs are awesome. They require no gate current to drive them, only voltage which makes them ideal for switching high currents efficiently.

#5 Power N-Channel – IRF510

These transistors are rated for 100V and 5.6A. Not milliamps, but Amps. (Though, you will need to use a heat sink). The gate threshold voltage minimum to maximum is 2V to 4V so these might be considered “logic level” compatible and can be driven from 5V logic levels. Although, probably not all the way to their minimum specified Ron. This is why the IRF510 was included in the parts kit.

IRF510 from ALP2000

In Conclusion

These general purpose transistors will cover a wide range of uses. Other similar devices could be used equally well. Having a few of each in your stock pile of parts will come in handy for just about any project. Leave your suggestions below on which transistors you find most useful.

Doug

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