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How to choose a replacement for a bipolar transistor

There are many bipolar transistors, and most of them have many analogues, which are similar in their parameters, so that the search for the replacement usually does not cause any difficulties. Of course the best variant is to change the burned transistor to the same, but if it’s impossible to get it, there won’t be any difficulties to choose an analogue. These are the things that should be done for this:

  1. To find out the transistor’s name. If it is an SMD device – its code should be decrypted in the section of SMD-codes 🔗.
  2. To analyze the circuit of the transistor (bundling scheme).
  3. Find a datasheet of the faulty transistor and enter its main parameters in the analog search form.
  4. Looking through the proposed transistors’ datasheets, choose the most suitable analogue in the parameters, given the modes of its operation in the device.

What should you pay attention to?

Opening PDF-Datasheet, first of all we shall find out the transistor type - bipolar or field-effect, p-n-p or n-p-n, body frame type, pinouts location.

From the numerical parameters, first of all it is the maximum current and voltage. The transistor-replacement maximum current and voltage must be greater than or equal to the original.

For a bipolar transistor, an important parameter is the hFE current transfer coefficient. If the transistor is in the key circuits (demands on-off) hFE must be more than or equal to the required coefficient. If it is in the analogue boosters or in similar devices, it has to be close to hFE. In impulse power supplies, the analogue transistors should be chosen with the close hFE (it will be necessary to change also the working transistor standing in a pair).

It is necessary to check the temperature mode (heating) of the transistor after switching on the device. If the transistor is excessively hot, the problem may be both in the transistor, or in the non-working elements of its bundling.

The main parameters of bipolar transistors decoding

Semiconductor material: most transistors will be germanium or silicon. Other types are not used in conventional devices. Taking this parameter into account, the transistors’ bundling will be created.

Polarity: when you install a transistor of the other polarity, it fails.

Pc - Maximum power capacity: it is necessary to make sure that the selected transistor can dissipate enough power. This parameter depends on the maximum operating temperature of the transistor - when the temperature increases, the maximum power capacity decreases. If the dissipation power is insufficient – the other characteristics of the transistor become worse, such as a sharp increase in the collector current, which leads to even greater heating and the output transistor failure.

Ucb - Maximum permissible collector-base voltage, defined by the breakdown voltage p-n junction value. It depends on the collector’s current and transistor’s temperature.

Uce - Maximum permissible collector-emitter voltage. It is necessary for Uce to have a third more a collector circuit voltage. If the scheme demand is a relay coil, it is necessary to provide protection against the transistor overvoltage, such as diode.

Ic - Maximum DC collector current. The transistor current is also taken with a reserve of not less than 30%. Its value depends on the transistor temperature or the environment.

Tj - PN-junction maximum temperature. This parameter is important to consider if the transistor is operating in extreme conditions, for example in a car, where its temperature can reach 100 degrees.

ft - The boundary frequency of the current transfer coefficient - frequency at which the current transfer coefficient module in the scheme with the common emitter tends to one. This parameter is important because with increasing frequency of the input signal, the boosting coefficient decreases.

Cc - collector junction capacity. The transistor speed depends on this parameter (so, the lower, the better).

hfe - Static current transfer coefficient - the ratio of Iс collector current to the Ib base current.

Only the most important transistor parameters are described above. The manufacturer specifies many additional parameters in the datasheets: collector-emitter saturation voltage, maximum permissible collector impulse current, reverse emitter current, maximum permissible base current, etc.

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