Choosing Transistor Replacements
- summary of how to choose replacement transistors, ensuring the transistor replacements are able to operate satisfactorily within the circuit.
When working with electronics equipment, either to design, build or repair it is sometimes necessary to choose a replacement transistor. Either the type of transistor may not be to hand, or it may not be available. Fortunately it is normally possible to use a replacement transistor type as there is often a considerable degree of overlap between the specifications of different types of transistor, and by looking at the basic specifications it is normally possible to choose the correct transistor replacements.
This explanation is focussed on bipolar transistors, but it is possible to apply similar logic to field effect transistors to ensure that suitable replacements can be found.When looking for suitable transistor replacements it is necessary to look at the main specifications for the transistor. Once the transistor specifications and parameters have been ascertained, it is possible to check for other replacement transistor types with similar parameters that will be able to operate within the circuit in question.
When considering any possible replacement transistors, it is necessary to look at a variety of parameters. These will include the basic parameters of the transistor operation performance. They will also include the environmentally related parameters, and the physical parameters. All these need to be taken into account when choosing a suitable replacement transistor.
Looking at the basic transistor parameters
When looking for a suitable transistor replacement some of the basic transistor parameters that need to be considered include the following:
- Semiconductor material used: Most transistors will either be germanium or silicon. Other types are normally only used in very specialist applications. It is important to know what type the transistor is because there is a difference in the base emitter forward bias voltage drop. For germanium it is around 0.2 - 0.3 volts and for silicon it is around 0.6 volts. The circuit will be designed around a particular voltage drop.
- Polarity: It is absolutely imperative to find out whether the transistor is either NPN or PNP variety. Install the incorrect type and it experience the inverse of all the voltages it would expect and is likely to be destroyed.
- General application: Although it is not always necessary to exactly match the intended purpose for the transistor, a variety of areas of its performance will be tailored to its intended applications. Possible application types may include: switching, analogue, low power, RF amplifier, low noise, etc. Put in the correct type and it may not perform well. For example a low power general-purpose transistor is unlikely to work well in a switching application even if it has a high ft or frequency limit.
- Package and pin-out: Transistors have many packages. It is often necessary to match the replacement transistor package as closely as possible to enable the transistor to physically fit. Also the package may give an indication of other parameters.
- Voltage breakdown: It is necessary to make sure that the transistor is able to withstand the voltages it is likely to see. Transistor parameters such as Vceo, etc need to be checked.
- Current gain: , The current gain parameter of a transistor normally has a very wide spread. This is normally quoted as Β or hfe. Although they are slightly different, for all circuit equivalences of this nature these transistor parameters are the same. Choosing a replacement transistor with approximately the same current gain is necessary. Normally it is not a problem to choose a replacement transistor with a higher gain. Often a lower current gain may be acceptable.
- Frequency limit: The upper frequency limit for a transistor is normally quoted as its ft. It is normally important to ensure that the transistor can meet any frequency limits.
- Power dissipation: It is necessary to ensure that the replacement transistor can dissipate sufficient power. Often the package type is a good indication of this.
These are the main parameters that are of importance in most applications, but be on the look out for any other transistor parameters that may need to be included in the selection of the replacement transistor.Picking a replacement transistor
When choosing a suitable replacement transistor for use within an electronic circuit, there are several stages that must be considered when making the choice. These can be progressed in a logical order to narrow down the choice and enable the best alternative for the replacement transistor to be made.
Step by step instructions:
Once the choice of replacement transistor has been made, then it can be installed in the circuit, and the performance checked. In most cases it will operate satisfactorily, but occasionally there may be a problem. If this is the case, it is necessary to re-visit the way in which the choice of the replacement transistor was made and see if any mistakes were made or look for other parameters that may affect the operation of the transistor circuit.
What if I can't find the original transistor details?
Sometimes it is very easy to find out the parameters of a particular transistor as it may be possible to find them on the Internet or in a transistor data book. If this is not possible, either because the markings are not visible, or the data cannot be found, then not all is lost. It is still possible to find out a lot about the transistor from its package and also the circuit in which it is being used. In this way it is usually possible to find a suitable replacement transistor. The step by step instructions below should help the essential parameters of the transistor to be discovered.
Step by step instructions:
These instructions are set out in an approximate order of the most significant parameters first followed by the less significant ones:
Choosing a replacement transistor is normally quite easy. There is a huge number of transistor types available, and the specifications of many types of transistor overlap, making the choice of a replacement transistor quite easy in many instances.
Being able to choosing a replacement transistor can be very useful if the exact transistor type is not available easily. It is quite likely that a similar one may be available to hand, or possibly from a local stockist. In either case, it is useful to be able to choose the replacement transistor with a good possibility of it being able to work.