Optocouplers - analog output; Mounting:THT
(2 108)Optical couplers combine an optoelectronic element (phototransistor) and a light-emitting element (usually a LED). They are used to isolate circuits, typically in controllers, to avoid interferences in digital circuit operation caused by currents and voltage spikes induced when activating/deactivating large consumers (e.g. inductive loads, such as electromagnetic relays).
Parameters and use of optical couplers with analogue outputs
Optical coupler operation and its implementation in a circuit is simple, as it requires only LED operation. The element leads are connected with a phototransistor emitter and collector. In this set-up, the base current depends on the intensity of light illuminating a photosensitive structure, so the component operation is not binary, but analogue. It enables a designer to take advantage of the basic principles of electronic circuit structure. Therefore, an optical coupler ensures considerable amplification and helps control consumers requiring relatively high-intensity currents for operation. Thus CTR, i.e. Current Transfer Ratio between the component input and output, is one of the key parameters. Selected component models feature the Darlington configuration to additionally improve optical coupler amplification.
Other important optical coupler properties include the actuation speed (time), which determines the delay between voltage application at the optoelectrical input and opening of a channel between the collector input and phototransistor emitter. Similarly, component manufacturers also specify the deactivation time. Such characteristics determine whether a component is suitable for a given application.
Optical couplers come in numerous enclosure formats, i.e. for through-hole assembly (THT) and surface-mounted (SMD), mostly housed in bodies typical for integrated circuits. Note that they are also available in multi-channel versions (2, 3 or 4 mutually separated channels). They frequently come with a separate lead shorted to the phototransistor base to facilitate independent control of the current flow at the output, circuit tuning, etc.
Optical coupler applications
As it has been mentioned above, circuit isolation is the most important feature of optical couplers, which is of vital importance most of their applications. Therefore, they help avoid potential damage caused by ripples and voltage spikes. This also makes them ideal for installation in circuits tasked with change voltage proportionally between the controller and the consumer. This property is used for signal transmission purposes, e.g. in controls for industrial and consumer automation systems, in particular to operate motors, servomotors and actuators. They can also be used for the logical level conversion. Because they can operate at high frequency, they are often used in telecommunications equipment, in particular in situations with a risk of significant interference affecting transmission lines. For example, this applies to state-of-the-art, centralised machinery parks where isolation between communication circuits (RS-232, RS-485, PROFIBUS, PROFINET etc.) and servers supervising certain equipment operation is required. Optical couplers are also similarly applied in Ethernet networks.
The isolating (separating) properties of optical couplers make them a good choice for feedback systems, where a sensitive circuit equipped with an A/C converter controls circuits in which significant interferences may occur, e.g. within analogue sensor lines. To this end, they are used, for example, in power supply modules to adjust the device power through constant voltage and current monitoring. They enable designers to create protective circuits for overcurrent protection, short-circuit protection, etc.
Moreover, it is worth noting here that optical couplers are essential components of numerous medical devices, e.g. vital signs monitors, ECG monitors, etc. In such applications, isolation between the power grid and components coming in contact with the patient is particularly important.
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