The Working Principle Of Optical Coupler

1)The working principle of optical coupler is that the photo-coupler produces optical current due to photoelectric effect, which is induced from the output of the photon and realizes the conversion of electro-light-one-electricity.

2)Optical coupler (coupler, OC) is also known as photoelectric sensor, or photo coupling for short. The photo coupler transmits electrical signals using light as a medium. It has a good isolation effect on input and output electrical signals, so it is widely used in various circuits. At present, it has become one of the most widely used optical appliances. Optical couplers are generally composed of three parts: the emission of light, the reception of light, and signal amplification.

The input electrical signal drives the light-emitting diode (LED) so that it emits a certain wavelength of light, which is received by the optical detector to generate an optical current, which is then further amplified and then output. This completes the electrical-optical-electric conversion, thus playing the role of input, output, isolation. Because the optical couplers’ inputs and outputs are isolated from each other, the transmission of electrical signals has the characteristics of one-way, so it has good electrical insulation and anti-jamming ability. With the increase of the use time of the photo coupler and the decrease of the transmission ratio, it is necessary to provide sufficient driving current margin to Q2 in order to prevent control failure. There are many kinds of photoelectric couplers, commonly have photodiode, photoelectrical tripolar types, photoresist types, photo crystalline crystal, photoelectrical cases up to forest type, integrated circuit type, and so on.

3)Operating Characteristic

1, Common-mode: suppression ratio is very high Inside the photo coupler, because the coupling capacitor between the light-emitting tube and the photon is very small (within 2pF), the common-mode input voltage through the interpolar coupling capacitor has little effect on the output current, so the common-mode suppression ratio is very high.

2, The output characteristics of the photoelectric coupler output characteristics refers to a certain amount of light-emitting current IF, the relationship between the polarization voltage VCE added by the photosensitive tube and the output current IC, when IF-0, the light-emitting diode does not emit light, at this time the photo-sensitive transistor collector electrode output current is called dark current, generally very small. When if > 0, under certain IF action, the corresponding IC is basically independent of VCE. The change between IC and IF is linear, and the output characteristics of photoelectric couplers measured by the semiconductor tube characteristics graph are similar to between ordinary transistor output characteristics. Its test wire is shown in Figure 2, in which the D, C and E wires correspond to B, C and E poles respectively, connected to the instrument socket.

3, Isolation characteristics

a. In and out of the isolated voltage Vio (Isolation Voltage) photo coupling device input and output between the insulated pressure resistance value.

b. Isolated capacitor Cio (Isolation Capacitance): Capacitor value between the input and output of the optical coupler device

c. The in-and-out isolation resistance Rio: The insulation resistance value between the input and output of the semiconductor optical coupler.

4) Transmission characteristics:

1. When the current transmission ratio is specified to the operating voltage of the current coupler CTR (Current Transfer Radio) output tube, the ratio of the output current to the positive current of the light emitting diode is the ratio of the current transmission to the CTR.

2. Rise Time Tr and fall time Tf

Under specified operating conditions, the light-emitting diode input specifies the pulse wave of the current IFP, while the output tube outputs the corresponding pulse wave, from 10% to 90% of the forward amplitude of the output pulse, which takes time for the pulse rise time tr. From 90% to 10% of the amplitude of the edge after the output pulse, the time required is tf for the pulse to drop. Other parameters such as operating temperature, dissipation power, etc. are no longer described.

3. photo coupler can be used as a linear coupler. A bias current is provided on the light-emitting diode, and the signal voltage is coupled to the light-emitting diode by resistance, so that the photo transist receives a light signal that increases or decreases in the bias current, and the output current changes linearly with the input signal voltage. Optical couples can also work in a switching state, transmitting pulsed signals. When transmitting pulsed signals, there is a certain delay time between the input signal and the output signal, and the input and output delay times of photo coupling devices vary greatly from structure to structure.


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