The pair "optical emitter - optical receiver" has long been used in electronics and electrical engineering. An electronic component in which the receiver and transmitter are located in the same housing and there is an optical link between them is called an optocoupler or optocoupler.
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Optocoupler device
The optocoupler consists of an optical transmitter (emitter), an optical channel and an optical signal receiver. The phototransmitter converts the electrical signal into an optical one. The transmitter in most cases is an LED (earlier models used incandescent or neon light bulbs). The use of LEDs is unprincipled, but they are more durable and reliable.
The optical signal is transmitted through an optical channel to the receiver. The channel is closed - when the light emitted by the transmitter does not go beyond the body of the optocoupler. Then the signal generated by the receiver is synchronized with the signal at the transmitter input.Such channels are air or filled with a special optical compound. There are also "long" optocouplers, the channel in which is optical fiber.
If the optocoupler is designed in such a way that the generated radiation, before reaching the receiver, leaves the housing, such a channel is called open. With it, you can register obstacles that arise in the path of the light beam.
The photodetector performs the inverse conversion of the optical signal into an electrical one. The most commonly used receivers are:
- Photodiodes. Usually used in digital communication lines. Their lineage is small.
- Photoresistors. Their feature is the two-way conductivity of the receiver. The current through the resistor can go in either direction.
- Phototransistors. A feature of such devices is the ability to control the transistor current both through an optotransmitter and through the output circuit. Used in both linear and digital modes. A separate type of optocouplers - with parallel-opposed field-effect transistors. Such devices are called solid state relays.
- Photothyristors. Such optocouplers are distinguished by increased power of output circuits and their switching speed; such devices are conveniently used in controlling elements of power electronics. These devices are also categorized as solid state relays.
Optocoupler microcircuits have become widespread - assemblies of optocouplers with strapping in one package. Such optocouplers are used as switching devices and for other purposes.
Advantages and disadvantages
The first advantage noted in optical instruments is the absence of mechanical parts.This means that during operation there is no friction, wear, sparking of contacts, as in electromechanical relays. Unlike other devices for galvanic isolation of signals (transformers, etc.), optocouplers can operate at very low frequencies, including direct current.
In addition, the advantage of optical isolation is the very low capacitive and inductive coupling between input and output. Due to this, the probability of transmission of impulse and high-frequency interference is reduced. The absence of mechanical and electrical connection between the input and output provides the possibility of a variety of technical solutions for the creation of contactless control and switching circuits.
Despite the limitation in real designs in terms of voltage and current for input and output, in theory there are no fundamental obstacles to increasing these characteristics. This allows you to create optocouplers for almost any task.
The disadvantages of optocouplers include one-way signal transmission - it is impossible to transmit an optical signal from the photodetector back to the transmitter. This makes it difficult to organize feedback according to the response of the receiving circuit to the transmitter signal.
The reaction of the receiving part can be influenced not only by changing the radiation of the transmitter, but also by influencing the state of the channel (the appearance of third-party objects, changing the optical properties of the channel medium, etc.). Such an impact can also be of a non-electric nature. This expands the possibilities of using optocouplers. And insensitivity to external electromagnetic fields allows you to create data transmission channels with high noise immunity.
The main disadvantage of optocouplers is low energy efficiency associated with signal losses during double signal conversion. Also a disadvantage is the high intrinsic noise level. This reduces the sensitivity of optocouplers and limits the scope of their application where work with weak signals is needed.
When using optocouplers, the influence of temperature on their parameters must also be taken into account - it is significant. In addition, the disadvantages of optocouplers include a noticeable degradation of the elements during operation and a certain lack of technology in production associated with the use of various semiconductor materials in one package.
Characteristics of optocouplers
Optocoupler parameters fall into two categories:
- characterizing the properties of the device to transmit a signal;
- characterizing the decoupling between input and output.
The first category is the current transfer coefficient. It depends on the emissivity of the LED, the sensitivity of the receiver and the properties of the optical channel. This coefficient is equal to the ratio of the output current to the input current and for most types of optocouplers is 0.005 ... 0.2. For transistor elements, the transfer coefficient can reach 1.
If we consider the optocoupler as a four-pole, then its input characteristic is completely determined by the CVC of the opto-emitter (LED), and the output - by the characteristic of the receiver. The pass characteristic is generally non-linear, but some types of optocouplers have linear sections. So, a part of the CVC of the diode optocoupler has good linearity, but this section is not very large.
Resistor elements are also evaluated by the ratio of dark resistance (with an input current equal to zero) to light resistance. For thyristor optocouplers, an important characteristic is the minimum holding current in the open state. The significant parameters of the optocoupler also include the highest operating frequency.
The quality of galvanic isolation is characterized by:
- the maximum voltage applied to the input and output;
- maximum voltage between input and output;
- insulation resistance between input and output;
- passage capacity.
The last parameter characterizes the ability of an electrical high-frequency signal to leak from the input to the output, bypassing the optical channel, through the capacitance between the electrodes.
There are parameters that allow you to determine the capabilities of the input circuit:
- the highest voltage that can be applied to the input terminals;
- the maximum current that the LED can withstand;
- voltage drop across the LED at rated current;
- Reverse Input Voltage - Reverse polarity voltage that the LED can withstand.
For the output circuit, these characteristics will be the maximum allowable output current and voltage, as well as the leakage current at zero input current.
Scope of optocouplers
Optocouplers with a closed channel are used where, for some reason (electrical safety, etc.), decoupling is required between the signal source and the receiving side. For example, in feedback loops switching power supplies - the signal is taken from the PSU output, fed to the radiating element, the brightness of which depends on the voltage level.A signal depending on the output voltage is taken from the receiver and fed to the PWM controller.
A fragment of a computer power supply circuit with two optocouplers is shown in the figure. The top optocoupler IC2 creates a feedback that stabilizes the voltage. The bottom IC3 operates in discrete mode and supplies power to the PWM chip when the standby voltage is present.
Galvanic isolation between source and receiver is also required by some standard electrical interfaces.
Devices with an open channel are used to create sensors for detecting any objects (presence of paper in the printer), limit switches, counters (objects on the conveyor, the number of gear teeth in mouse manipulators), etc.
Solid state relays are used in the same place as conventional relays - for switching signals. But their propagation is hindered by the high resistance of the channel in the open state. They are also used as drivers for elements of power solid-state electronics (powerful field-effect or IGBT transistors).
The optocoupler was developed more than half a century ago, but its widespread use began after LEDs became affordable and inexpensive. Now all new models of optocouplers are being developed (for the most part, microcircuits based on them), and their scope is only expanding.
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