What are the Basics of Transistor Substitution?

Transistor substitution is often necessary when a specific transistor called for in an electronics design is not available. When attempting a transistor substitution, the operational and physical characteristics of the transistors should be carefully compared. Depending on the application and type of the original transistor, the basic areas of concern are the voltage, power, current, switching speed and amplification characteristics of the substitute. Other areas that may also be important include the lead locations on the transistors and the mounting options.

The first factor that should be considered when substituting a transistor is the charge of the transistors. A transistor that has a positive-negative-positive charge (PNP) must be substituted with a PNP type transistor. Likewise, a negative-positive-negative (NPN) transistor must be substituted with an NPN transistor.

All transistors need to be able to dissipate a given amount of power, though just how much power varies with every application. Power dissipation characteristics are typically identified in watts or milliwatts. A substitute transistor should be able to dissipate at least the same amount of power as the original. A transistor of a greater wattage rating is suitable if the transistor’s wattage ratings include the full range of the original.

The voltage rating of the original transistor must be matched when pursuing a transistor substitution. Measured in volts or millivolts, transistor voltages can vary, as can the voltage suitable for the different components of the transistor. Voltage can also vary depending on the application of a transistor. The substitute transistor must either match or exceed all of these characteristics of the original.

Operational current, measured in amps or milliamps, must be comparable between the original transistor and its substitution. Both transistors should also have similar minimum and maximum current-carrying capabilities. Some transistors have minimum and maximum current-carrying abilities at different voltages. These too must be comparable.

If the transistor is used in a switching application, the speed at which the original transistor switches needs to be identical in the substitute. Switching too slowly or too fast could cause problems for other components in the circuit. Some transistors also have a specific voltage for switching applications that must match.

Transistor substitution for amplification applications can be tricky. The replacement must have voltage, current and signal-to-noise ratios identical to the original. Additionally, different types of inputs can trigger different types of outputs in different transistors. All of these parameters must be identical for a substitute to function as well as the original transistor.

Other considerations for transistor substitution relate to the physical characteristics of the transistors. Either they should have identical collector, emitter and base orientations, or there needs to be enough space to reposition the substitute’s leads without shorting them together. There also needs to be enough physical space for the substitute to fit on the circuit board. Finally, some transistors mount with a screw or small bolt. These should be substituted with identically mounting transistors.