The photo-electric sensor is called the sensor which reacts to change of illumination.
In photo-electric sensors 3 types of photoeffect are used (the photoeffect is understood as the phenomenon of change of properties of substance at change of its osveshchenyonost):
- the outer photo-emissive effect consisting that under the influence of light energy there is the departure of electrons (emisyosiya) from the cathode of the electron tube; the veyolichina of the emission current depends on cathode illumination;
- inner photo-emissive effect, sostoyayoshchy in what active soprotivleyony (conductivity) of the poluprovodyonik is depending on its illumination;
- the barrier-layer effect, заключающийся that between layers of the lit conductor and not lit semiconductor separated by the tonyoky insulating layer there is the electromotive force the size of kotoyory depends on illumination.
Photo cells with the outer photo-emissive effect represent the vacuum or gas-filled lamp with the cathode from the photosensitive layer.
The scheme of connection of the photo cell with the anodyony battery is provided on fig. 1. The anode A and the cathode K of the photo cell F are put into the stekyolyanny cylinder from which air is pumped out (at vacuum fotoyoelement) or which after pumping of air is filled with razrezhenyony gas — argon (at gas-filled photo cells).
When the luminous flux falls on the cathode covered with the active layer, the part of the radiant energy absorbed by the cathode is reported to electrons and electrons take off from the cathode. This phenomenon is called fotoyoelektronny emission. To use this emission, between the anode and the photocathode electric field, napravlyayuyoshchy electrons to the positively charged anode is created. When deystyovy light stops, current in the photo cell disappears.
Photo cells like TsG (oxygen-caesium gas-filled) belong to industrial types of photo cells with the outer photo-emissive effect, (ACV (antimony-caesium vacuum type).
Work of photo cells is defined by their characteristics. Rasyosmotrim some of them. The line representing dependence of photocurrent of the photo cell on tension on the anode is called the volt-ampere characteristic.
The light characteristic of the photo cell is called the photocurrent zavisiyomost from the luminous flux falling on the photocathode.
The light characteristic defines sensitivity of the fotoyoelement. The sensitivity of the photo cell is the photocurrent veliyochina relation in microamperes to the size of the luminous flux in lumens which caused this current. The photo cell reacts to the intenyosivnost of the luminous flux and its frequency therefore its sensitivity is separated on integral (on intensity) and spectral (on frequency).
Integral sensitivity of the photo cell is called the size of the current of the external photoelectric effect created in the fotoeleyoment by all luminous flux (from ultra-violet to infrakrasyony beams inclusive).
The spectral sensitivity of the fotoyoelement characterizes its ability to react to light fluctuations of one frequency (i.e. the certain wavelength).
In vacuum photo cells the plate current is caused only by the electrons which are taking off from the photocathode, and the light harakteriyostika of such photo cell is linear (straight lines 1 and 2 in fig. 2 and). In gas-filled photo cells current is created not only by the elektroyona which took off from the cathode but also the electrons and ions which are turning out as a result of gas ionization, to these объясняется nonlinearity of their light characteristics (curves 3 and 4 in fig. 2 and).
In fig. 2 and fototoki the luminous flux F - in lumens is expressed in microamperes, and.
In gas-filled photo cells the available molecules of gas create the opportunity to use ionization for increase in photocurrent that is visually visible from comparison of volt-ampere characteristics (fig. 2), the gas-filled photo cell (kriyovaya2) and vacuum (the curve 1).
The sensitivity of the gas-filled photo cell is more than chuvyostvitelnost of the vacuum photo cell. For example, at the nomiyonalny operating voltage of 240 V the integral sensitivity of the vacuum photo cell of the STsV-4 type is 100 ¼¬Ç/lm, and the gas-filled photo cell of the TsG-4 type - 200 ¼¬Ç/lm.
Use of photo cells in schemes of automatic equipment demands use of amplifiers with very big amplification constant. Photo cells with the internal fotoeffekyot (photoresistance). The phenomenon of the inner photo-emissive effect consists that as a result of light absorption in the semiconductor additional free elektroyona appear thanks to what the conductivity of substance increases, and its soproyotivleniye decreases.
Photoresistance (fig. 3) soyostoit from the light-sensitive layer of the Itolshchinoy semiconductor about 1 micron applied on the glass or quartz plastinyoka 2. On the surface of the semiconductor tokosjemny electrodes are strengthened 3 (usually gold). The element, sensitive to light, with tokosjemny electrodes is mounted in the plastic body so that the acting electrodes provided inclusion of the fotoyosoprotivleniye in the scheme via the special panel. The appearance is given in fig. 3 and photoresistance dimensions like FS-K1 are given.
Photoresistance released by the industry have the following standard designations: behind the letters FS designating photoresistance there are letters and digits concerning composition of material and photoresistance design. So, fotoyosoprotivleniye from sulphurous lead, except letters FS, have obozyonacheniye And, from sulphurous bismuth - B, from sulphurous cadmium – To.
Work of photoresistance is that at the osveshcheyoniya electrical resistance sharply falls and, therefore, current in the electric circuit in which it is included fotosoprotivleyony, increases. Current passes through photoresistance included in the chain and in the dark, but when lighting current size sharply increases. The measure of sensitivity of photoresistance is the difference of currents in the dark and on light, carried to the size of the svetoyovy flow falling on photoresistance.
It is necessary to emphasize that the sensitivity of fotosoprotivleyoniya is many times more than sensitivity of photo cells with vneshyony photoeffect. The integral sensitivity of some photoresistance, for example FS-KM2, at the largest dopustiyomy tension is 3000 — 10000 ¼¬Ç/lm.
The main characteristics of photoresistance are: spectral which characterizes sensitivity of the fotosoproyotivleniye at action of the radiation of the certain wavelength on it; light which characterizes sensitivity of fotosoventilny photo cells it is rather big as the system of the electrodes separated by the thinnest barrier film forms considerable capacity.