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Uploaded on Nov 29, 2010
Digital TransistorsIn different circuits we often use transistor keys or level transformers on bipolar transistors which are controlled by logic signals. At that it is impossible to get along without resistors. This increases the total quantity of the elements in the circuit and square of printed-circuit cardboard and complicates its mounting as well.Some foreign manufacturers have been producing the so called digital transistors for a long time.These devices became popular very quickly.You can see them on the cardboards of TV sets, video recorders, home music systems.The digital transistor has two resistors inside of the housing. This allows you to control these transistors directly from TTL or CMOS of logical levels.Actually this transistor is an inverter with open collector. Internal resistors are thin-film components which are fully insulated.This makes it possible to supply voltage of any polarity without any side effects. There are the digital transistors with different values and rated values proportion of internal transistors. In some versions one of the resistors may be absent.The type of circuit, ratings, and decryption can be defined by the digital transistor designation.Digital transistors are produced in various housings for normal and surface mounting.Though the digital transistors can be operated in active mode, they are mostly operated in the switching mode. As an example we can take several circuits with application of such transistors.This circuit provides source current for 5 volt load. It can be used in those cases when the microcontroller must cut off supply in some part of the circuitor control any other loading which has one grounded lead. The transistor will be opened when the low logical level appears on the microcontroller port. This circuit is suitable for the microcontrollers of MCS 51 line. Their ports turn into high logical level state during actuation of "reset" signal. So there can be a false load starting.This circuit performs the same functions but is controlled by high logical level. It is suitable for example, for microcontrollers of AVR line, where during "reset" signal action the ports turn into the third state.This circuit shows control of the higher voltage load which has one lead connected to the power supply source. The circuit is controlled by high logical level. This circuit allows to switch negative voltage to the load. The circuit is controlled by low logical level. This circuit is a simple TTL transformer of the levels into RS interface levels. For the receiver the digital transistor with divisor 1 to 0.25 is used. This excludes backfire of the emitter junction under the input voltage of 12V.