Part Assortments: Bipolar Junction Transistors

I moved to the United Kingdom not long ago, and am in the process of setting up a new home electronics lab. I sold all my components and test equipment as it was going to be too difficult to bring them with me. Recently, I’ve been buying parts assortments from online marketplaces to fill out drawers of components in order to make it easy to prototype a range of projects on breadboards for proving out designs before making a PCB in Altium Designer® or Altium Upverter®.

Many of these parts assortments are a mixed bag of results, some have some really great components, and some have parts which most manufacturers have considered obsolete for decades. In this post, we’re going to look specifically at bipolar junction transistors. While these are less popular components in modern designs, with MOSFETs taking their place in many circumstances, they can be very handy for certain applications in which they work better than a MOSFET might.

It was a real challenge to find key specifications for some of these components. Given the popularity of assortment kits in online marketplaces, I thought I’d collect all the specifications together to make it easy to find for other people in the same position.

TO-92 Assorted Transistors

In my assortment kits, and all the other assortment kits I considered, the parts are simply labeled with a part number, type (optional) and quantity. For example, 2N2222, PNP, 35Pcs. If you have looked at transistors before, you might recognize the 2N2222 as being one of the most common NPN transistors available, so the 2N2222 in my assortment kit which is in the top left most compartment (and therefore first in my mind) is labeled incorrectly for either type or part number—not a great start! I’m going to be working purely from the part number, ignoring whatever type is printed on the label.

Historically, these component part numbers are very common, and as such tend to be registered with JEDEC. This ensures that certain rated values will be met by all components with that part number, regardless of the manufacturer. The exact specifications vary somewhat between manufacturers, packaging and specific variations. As these component assortments do not have a specified manufacturer, I will be working from what datasheets I can find for TO-92 components, so there may be some variation with regard to actual performance of the components in your assortment.

In the tables below, the Availability column refers to how readily stocked the part in the same package (TO-92) through distributors. This could influence whether you prototype a design with that component, since you usually want to avoid low availability components in your designs. Some components are also available in alternate packages, however, these may have different maximum specifications. Here is how I’m classifying the availability:

• Common: Available at major North American and European distributors.
• Uncommon: Available at smaller distributors online with a low minimum order quantity.
• Unavailable: Only available through stock clearance distributors with large minimum quantity, or not available at all.

Before we get to the table of parts, let’s quickly go over the important specifications for a transistor and what they mean.

Vce - Voltage - Collector to Emitter Breakdown (Max)

As long as the maximum supply voltage the transistor will experience is lower than Vce and there is no circuity which could create high voltage transients (such as inductors, motors, solenoids), this specification will have little bearing on your circuit.

If your circuit is going to have inductive loads which could create transient voltages higher than Vce, you should place a TVS diode next to the source of the transient spikes to clamp the maximum voltage to lower than Vce of the transistor.

Ic - Collector Current (Max)

Ic is the maximum current you can pass through the transistor, as long as it is within the power limit of the device.

Pd - Power (Max)

Pd is the maximum power dissipation the device is capable of. The TO-92 package which most assortment kits use is typically limited to around 625mW, but some devices will have significantly lower or slightly higher maximums. Your design should ensure you stay within the power rating of the device, or it may overheat and either fail, or have degraded performance.

fT - Transition Frequency

This is the most commonly used parameter with which to compare the frequency response of a bipolar junction transistor—the frequency at which its short circuit current gain drops to unity. It's probably unlikely that you approach the transition frequency with a project built on a breadboard, however, if you need a fast response from the transistor, it could give you a decisive measure to use for deciding between similar transistors.

NPN Transistors

PNP Transistors

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