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Ham Radio Store Choosing Transistor Replacements When repairing a circuit, or even building a new one it is often not possible to find the exact electronics component - we tell you how to choose a suitable replacement. 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 other electronic components including 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. BC Plastic leaded transistor Looking at the basic transistor parameters Fortunately many transistors used in electronic circuit design are general purpose types.
Their specifications are not particularly exacting and a variety of general purpose transistors could be used. Today, the performance of even general purpose transistors is exceedingly high and they can be used in a variety of applications.
However a much closer look must be taken at transistors that fulfil a more exacting role. Their specifications need to be examined more closely to ensure that any substitutes will have a similar specification. 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.
The circuit will be designed around a particular voltage drop. Install the incorrect type and it experience the inverse of all the voltages it would expect and is likely to be destroyed. 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. 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. Transistor parameters such as Vceo, etc need to be checked.
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. It is normally important to ensure that the transistor can meet any frequency limits. 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. As bias voltages and other features are different it is necessary to select a replacement transistor with the same material.
The replacement should have the same application if possible. Differences in the package for small signal transistors is not normally an issue, but for larger ones where there may be heatsinks, etc involved, different packages can cause significant issues.
Also where the pin connections are different, care should be taken to ensure that the right pins are taken the right connections. For many transistors the pinout is EBC, but there are other configurations for the pinout that can easily trap many people.
Current gain values normally vary widely even for transistors of the same type so some variation will be acceptable. Choosing a replacement transistor with a similar can style will often mean that both transistors have a similar power dissipation. These are normally required when transistors are used in specialist applications.
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. 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: Is it a transistor? This may appear to be an obvious question, but occasionally some devices may appear to be a transistor at first sight.
It may be a field effect transistor, a Darlington transistor or even some other form of device. Alternatively, sometimes small voltage regulators are contained in packages similar to that of a transistor. Other devices may also appear in what may appear to be transistor packages at first sight. Careful examination of the application will enable this to be verified. It may be possible to discover this in a number of ways.
If the original transistor is still working then this can be discovered by measuring the voltage across the base emitter junction when it is forward biased. This should be about 0. Alternatively it may be possible to ascertain the type by looking at other transistors in the circuit. Often the same technology will be used throughout the equipment. This is not always true so beware! Look at the specifications for other transistors in the same packages and this will give a good guide.
Those packages designed for mounting on heatsinks will be more variable because they can often dissipate more power dependent upon the heatsink. It is best to be more cautious with these packages.
High power transistors often offer lower gains - older power transistor types may be as low as 20 - 50, whereas the smaller transistors may offer gains anywhere between 50 and Look at the components in the circuit and the function of the circuit. It is usually possible to estimate the frequency of operation. Then take this and choose a replacement transistor that can easily operate at this frequency. This is particularly true for specialised circuits where some specific performance features may be critical.
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.
It can often help to check the stock of local stockists or reputable electronic component distributors. It is often necessary to select a transistor that can be obtained quickly and easily. Checking what might be available with a stockist or electronic component distributor will help make the final decision. 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.
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