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Which common transistors should you keep in stock?

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Common transistors for hobbyists

There's nothing worse than being desperate to finish a project or try out some idea that you've just had and not having the components you need. You can keep common resistor and capacitor values but you can't stock every transistor.There are a huge amount of different transistors available with many different specifications. Sometimes you have to use the correct transistor, however there are many occasions where a general purpose transistor will do.

If you look at a circuit and it contains a transistor, more often than not a general purpose transistor will suffice. If you’re not experienced enough or not sure, read the details of the project and they will usually tell you if a specific transistor is called for and why. If they don’t mention its specific use its use it’s probably down to the fact that a general purpose transistor will do.

Typical or popular transistors

You obviously need to know if it's a NPN or a PNP type.
The two types that I reorder and make sure I keep in stock are the 2N3904 and the 2N3906.

2N3904

The 2N3904 is an NPN type and has the following specifications.

Symbol

Parameter

Value

Unit

VCBO

Collector -  Base Voltage

60

V

VCEO

Collector - Emitter Voltage

40

V

VEBO

Emitter - Base Voltage

6

V

IC

Collector Current

200

mA

Ptot

Total Dissipation

625

mW

2N3906

The 2N3906 is an PNP type and has the following specifications.

Symbol

Parameter

Value

Unit

VCBO

Collector -  Base Voltage

60

V

VCEO

Collector - Emitter Voltage

40

V

VEBO

Emitter - Base Voltage

6

V

IC

Collector Current

200

mA

Ptot

Total Dissipation

625

mW

Understanding transistor specs.

As you can see the specifications for the 2N3904 and 2n3906 are the same. The 2N3906 is a PNP version of the NPN 2N3904.

There are a whole lot of numbers associated with transistor specifications. What I have included here is a simplified version. If you look at everything it just becomes a jumbled mess. Things like maximum operating temperature are not things you're going to encounter in normal hobbyist conditions. Same with storage temperatures, you can't store them at a temperature below -65C. So don't go taking them on any polar expeditions you may have planned. Also the 150C may be a problem if you're planning on storing them in a oven.

Seriously though a lot of transistor specs won't be of much relevance so i'll try to show you what the most important ones are to look out for. 

Not surprisingly small transistors can't handle the current and power that a large transistor can. A large transistor that has to be mounted on a heat sink can obviously handle more power.

​Looking at the specifications the IC is the current that the transistor can output and related to this is the power dissipation Ptot. So as you can see you can only drive 200mA which is fine for things like LED's and such.

As NPN transistors are the most common transistors, I'd recommend that you keep more of them in stock as you’re more likely to use them. I keep them on a three to one ratio. You certainly won’t break the bank as they are very cheap.
I don't keep any other more powerful transistors in stock anymore as I used them so infrequently. For the amount of times that I needed them I now only keep the 2N3904 and 2N3906.

If you want to know how to test a transistor you can read about it here.