The difference between photodiodes and phototransistors

Photodiode

Photosensitive diode is a PN junction type photoelectric device, also known as photodiode, has a unidirectional conductivity, can be converted into an electrical signal detector device. pn combined with reverse voltage, reverse conduction in the light, the material used in the photosensitive diode is generally silicon.

There are generally two types of photodiodes, the ZDU type and the ZCU type. Generally used is the ZCU type, which is fully sealed, metal shell, the top has a glass window. Photodiodes are characterized by small size, light weight, long service life and high sensitivity.

The main parameters of the photodiode

① Maximum operating voltage Umax: the reverse current of the photodiode does not exceed 0.lμA reverse maximum voltage value.

② photocurrent IL: when the photodiode is subject to a certain light, plus the current value of the normal reverse operating voltage.

③ Dark current ID: the photodiode will increase the reverse leakage under normal operating voltage without light.

④ Response time Tr: the time required for a photodiode to convert a light signal into an electrical signal.

⑤Photosensitivity: indicates that the photodiode is sensitive to light.

Application of photodiode

There are photodiodes PN junction type, PIN junction type, avalanche type and Schottky junction type there are four types, of which the avalanche type and Schottky junction type made of silicon material are widely used. junction type of PN photodiodes. Photodiodes are mainly used for automatic control. For example, optical coupling, optical readout devices, infrared remote control devices, infrared anti-theft, automatic control of street lights, process control, encoders, decoders and so on.

Detecting photodiodes

①The difference between the photodiode pins can usually be distinguished by directly checking the length of the photodiode pins: the length of the pins is positive (P-pole extreme), and the short pins are negative (N-pole).

② For pipes marked with colored dots or keys, one pin near the mark is positive and the other is negative.

③ The difference between the multimeter is to put the multimeter in the Rx1k block, block the light window of the tube with a baffle, with the red and black pens against the measured resistance value of the two times, the resistance value of the larger of a measurement (reverse resistance), the red pen connected to the pin for the positive, the black pen connected to the pin for the negative pole.

Phototransistor

The structure of photosensitive triode is similar to ordinary crystal triode, and there are two inside.PN Its emitter junction has the same photosensitive characteristics as photosensitive diode, and it can get current gain through collector junction and ordinary transistor, so the sensitivity of photosensitive triode is higher than that of photosensitive diode. Converts an optical signal into an electrical signal. It also amplifies the signal current, that is, it has an amplifying effect. The material used for photosensitive triode is the same as that of photosensitive diode, which also has an amplifying effect. two types of PNP and NPN.

Main parameters of photosensitive triode

① Photocurrent IL: Under the specified operating voltage, the current flowing through the photosensitive triode is called photocurrent IL. the larger the photocurrent is, the higher the sensitivity of the photosensitive triode.

② Dark current ID: When the voltage between the collector and emitter is a specified value, the leakage current through the collector is called the dark current of the photosensitive triode without light.

③ temperature characteristics: the temperature of the photosensitive triode dark light flow ID and photocurrent IL all of which have an impact.

④ Volt-ampere characteristics: the volt-ampere characteristics of the photosensitive triode refers to the phototransistor voltage and photocurrent IL their relationship under a given light.

⑤ Maximum operating voltage VCE: Collector current without light under the collector current IC in the specified permissible value, the voltage drop between the collector and emitter is called the maximum operating voltage.

⑥ Maximum power PM: Maximum power refers to the maximum power that the photosensitive transistor can withstand under the specified conditions.

Applications of Photosensitive Triodes

As the photosensitive triode has current amplification, it is widely used in brightness measurement. For example, speed measurement, light source switching circuit, in the photoelectric isolation occasions, photocoupler by the photosensitive triode and light-emitting diode composition, known as the photocoupler. Optocoupler to light as a medium to transmit signals, the output of the electrical signal has a good isolation effect. It is worth noting that phototransistors do not usually lead to the base, but some phototransistors are commonly used for temperature compensation and additional control.

The difference between photodiode and phototransistor

1, infrared photoelectric devices are photosensitive diodes and photosensitive triodes. The former consists of infrared photoelectric devices. pn The latter is made of semiconductor manufacturing process with a structure, the latter is made of semiconductor manufacturing process. npn or pnp structure, both of which have light, both produce photocurrent;

2, the photodiode pin is divided into positive and negative two, but the photosensitive triode is generally only C pole and E pole, the same appearance, it is difficult to distinguish, pay attention to the model;

3, photosensitive triode and photosensitive diode can feel the light signal is converted into an electrical signal, but the basic area of the photosensitive triode is to receive light, so the basic area is larger than the ordinary triode, the conversion of photocurrent is tens of times or even hundreds of times the photosensitive diode;

Photodiode photocurrent is small, the output characteristics of good linearity, fast response time; photosensitive triode photocurrent is large, the output characteristics of poor linearity, slow response time.

Therefore, the photosensitive transistor is generally used for switching circuits that require high sensitivity and low operating frequency, and the photosensitive diode is used for the linear relationship between photocurrent and illumination or high-frequency operation.

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