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Phototransistors are semiconductor optoelectronic devices. Phototransistors are usually bipolar transistors. The current in the phototransistor depends on the intensity of incident light. With bright lighting, the base-collector voltage is around 0V. In other aspects, phototransistors are similar to conventional transistors, so they are easy to implement and used in many applications.
A phototransistor has a photodiode at its base-collector junction. The current flow between the collector and the emitter is determined by the current generated by light (photons) falling on the base-collector junction (just like in the case of an ordinary transistor).Since phototransistor are light-sensitive devices, they are packaged in transparent housings and are visually similar to LEDs (3/5mm). A phototransistor can be either a two-lead (emitter-collector) or a three-lead device (where the base lead is brought out). They are available in both through-hole and surface mount (THT, SMT) housings, as well as in a miniature 0805 package.
Although phototransistors have shorter response time than photoresistors, they are inferior to photodiodes in this respect. They also need relatively intense lighting to work properly. Nevertheless, since they are characterized by their broad-spectrum sensitivity, they are ideal, among other things, as infrared receivers. Phototransistors are used as components of photoelectric sensors, as well as in automation and security systems.Phototransistors are used in optocouplers, i.e. devices that galvanically separates circuits, e.g., in telecommunications equipment or when using microcontroller outputs to control inductive loads (electromagnetic relays etc.). This is why optocouplers have related operating parameters to phototransistors.
Generally, phototransistors should be selected taking into account electrical parameters of bipolar transistors. Certainly, attention should be paid to the collector-emitter voltage and power (heat dissipation).There are several features specific to phototransistors or optoelectronic semiconductor devices. These include the viewing angle, which allows determining the optimal incident angle in relation to the component face; the dark current, i.e. the leakage current of the collector-base junction when the device is supposed to be off; the peak sensitivity wavelength (here it should be remembered that phototransistors are very sensitive to radiation, and this parameter only determines the peak sensitivity wavelength). In some applications, especially in the field of communication, the switch-on and switch-off times, which determine how long it takes for the phototransistor to reach the nominal conduction for a given light intensity, will be of great importance.Taking into account the specifics of a given project, the dimensions of the PCB, and even aesthetic considerations, attention should also be paid to the mechanical parameters, including the shape, diameter, and colour of the lens (note: it can be used as an IR filter, limiting the amount of light reaching the device). And, of course, assembly method.
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