In my previous article, Part Assortments: Bipolar Junction Transistors, I talked about buying bipolar junction transistor assortment kits from online marketplaces to stock up my prototyping supplies. Several of the transistors in these kits have long been obsolete, and they seem to be distinguished by having fewer options for MOSFET assortment kits, at least in the United Kingdom.

In this post, we’ll look at building our own assortment kit of through-hole BJT and MOSFETs which are very well stocked by major suppliers. I’m working under the expectation that if Digi-Key, Mouser, and the like have hundreds of thousands if not millions of a particular part in stock, it’s likely to be a popular part and not about to go obsolete. We want to choose active, commonly stocked parts, as we might need to use them in volume in a design. If we prototype a great new product on a breadboard, we want to ensure that we can build it as designed, rather than having to substitute in more modern components, which may have different performance parameters. Even if you are not going to mass produce a board based on your prototype, it's nice to be able to build a circuit board using components that are available outside of part assortment kits.

Ideally, the components we select here should be available in both through-hole, for prototyping, and surface-mount, for production. If you have any suggestions for parts you regularly find handy, leave a comment below!

Bipolar Junction Transistors

Before we get to selecting parts, let’s quickly go over the important specifications for a bipolar junction transistor and what they mean. There are many other specifications which may be considered important or even critical depending on your design, yet typically, these are going to be the most common parameters you will be selecting a device based on.

We’ll also be looking at a smaller range of components than your typical 10, 12, and 24 value assortment kits, as many of those components have very similar ratings. We’ll select a small quantity of common components with good specs that will be versatile in many designs.

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 is going to have little bearing on your circuit.

If your circuit will 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 metric to use for deciding between similar transistors.

Selecting Components

I selected the components below by looking first at the most stocked NPN and PNP components on Mouser, Digi-Key, TME and Farnell UK. After finding a range of transistors that offered a wide variety of voltages and currents, I then filtered my options by choosing transistors also available in a through-hole package. A couple of options well stocked in surface-mount variants only had obsolete through-hole components available, so an alternative option had to be found.

NPN Transistors

PNP Transistors

Metal Oxide Field Effect Transistors

Whilst bipolar junction transistors used to be pretty great, field effect transistors have surpassed them in terms of popularity and application. Currently, they are slightly more expensive than BJTs, but that gap continues to close. Furthermore, a FET does not need the current limiting resistor which the base pin of a BJT requires. When it comes to controlling large currents or voltages, FETs have the upper hand, as they do with all efficiency related aspects of design. For many applications, choosing between a FET and a BJT will not make too much difference in the final application.

For MOSFETs, we’ll look at stocking two ranges of components: small signal devices such as the transistors we looked at previously, and high current MOSFETs. As MOSFETs have a lot less standardization and there are a wide variety of devices in a range of packages to suit every need, we will only be looking at through-hole components for the MOSFETs. Typically, only relatively old components are available in both through-hole and surface-mount options with similar specifications, and we want to be able to work with the most stocked, modern devices.

As with bipolar junction transistors, there are a range of specifications you might consider when selecting a FET. These specifications are what you might consider when using a FET under several kilohertz. If you are looking at using a FET to switch a load at high frequency, consider taking a look at the post Making Big Motors Move, and looking at the FET specification guide contained within instead of the one below.

The specifications will vary between manufacturers and packages, and this variance is typically far more for MOSFETs than it is with BJTs.

Vdss - Drain to Source Voltage

As with the BJT, ensuring the voltage rating of the FET is greater than any voltage expected on in your circuit is the most basic specification to consider when selecting a MOSFET for your project. FETs are particularly sensitive to overvoltage events, so if your device has inductive loads, they should be clamped such that no voltage higher than the Vdss of the chosen FET can exist in the circuit.

Id - Current - Continuous Drain

Another basic specification to consider is the current that the load the FET is controlling will consume. The current through the FET should be below its rated current, including inrush/startup loads. It is worth noting that the specifications for continuous drain current may have notes attached in the datasheet, such as associated copper area, or ensuring the total power dissipation is within the specified parameters.

Rds(on) - Resistance Between Drain and Source When On

When the FET is conducting, it has a small series resistance. This resistance will cause it to heat up, and, depending on the application, may be a major factor to consider when selecting a FET. If you are using the FET to drive a motor or a high power LED for example, the small series resistance of the FET could cause a significant temperature rise in the device. If you are using the FET for amplification or to control power to a relatively small load, there may be very little heating involved, and the Rds(on) parameter of the device becomes irrelevant to the design.

If you expect to have a limited area around the FET in you final design for copper pour to act as a heatsink, and expect to be switching currents over 20% of the Id of the FET, you will need to prioritize the RDS(on) specification so the FET generates as little heat as possible.

Vgs(th) - Gate Threshold Voltage

FETs begin to conduct between the drain and source once the gate’s threshold voltage is reached. If you are driving the FET from a microcontroller pin or low voltage source, you will need to make sure the threshold voltage is below the voltage you are able to supply.

If you are worried about the Rds(on) parameter and power dissipation, you will want to pay close attention to the gate voltage required to reach the minimum Rds(on). As gate voltage increases above the switching threshold, resistance will drop. You may look at the specification for Drive Voltage (Max Rds(on), Min Rds(on)) which will list two voltages, the voltage for maximum Rds(on) and the voltage for the lowest possible Rds(on).

Pd - Power Dissipation

As with the BJT specifications, you need to ensure the total dissipation of the FET in your application does not exceed its rating, or it could overheat and fail.

N-Channel Small Signal MOSFETs

P-Channel Small Signal MOSFETs

N-Channel Power MOSFETs

P-Channel Power MOSFETs

How Many Do I Require?

I’ve tried to provide a very broad range of specifications in the selections above, which means some of these devices are relatively expensive. The part assortments I purchased had between 10 and 35 of each component, however, unless I’m doing something horribly wrong, I wouldn’t expect to need more than 4 or 5 of each transistor to be able to complete a design. For the cheaper components, I tend to prefer buying the first price break quantity as the price per unit can be considerably lower. For the more expensive components that have a very high specification parameter such as voltage or amperage, I feel just two devices would be sufficient for most people. Really, one would be sufficient, but if something bad happens to it, such as electrostatic discharge, it's always nice to have a backup to prevent your design process from being stalled waiting for another component to arrive.

Sometimes, it’s nice to have a large range of components available. However, most of the time, you may find it more cost effective to have a smaller quantity of more specific parts which are readily available and in active production.

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