Establishing Electrical Isolation in Circuits
In certain circuit scenarios, the need arises to create an electrically isolated connection. Optical components often play a pivotal role in achieving this isolation. Typically, in most optocouplers, a single light-emitting diode (LED), functioning as the transmitter, is paired with a single photodiode, serving as the receiver, and both are optically coupled within the same package. This arrangement effectively addresses the requirements for transferring digital levels, such as control signals for a thyristor, where only two distinct logical states need to be conveyed—either the LED is on or off. In such cases, achieving precise analog coupling is not essential.
Precise Analog Voltage Transfer
When the task involves the transfer of analog voltages, it becomes crucial for the input and output voltages to closely align with each other. To achieve this precision, the transmitter and receiver must employ similar components integrated into an analog circuit. Optocouplers like the type CNR200 and CNR201 from Agilent (formerly Hewlett-Packard) come equipped with all the necessary components to fulfill this function. These optocouplers contain a package housing two photodiodes and one LED, featuring optical coupling between the LED and one of the photodiodes. The schematic diagram illustrates how the transmitter’s LED is optically connected to the receiver’s photodiode.
Matching Transmitter and Receiver Characteristics
The additional photodiode, integrated into the transmitter, ensures that the transmitter amplifier’s characteristics align with those of the receiver. With a 5 V supply voltage, analog voltages within the 0 to 3 V range can be seamlessly transferred. The optocoupler provides an isolation voltage of 1000 V between its input and output, but the practical value achieved depends on the printed circuit board layout.
Analog Signals and Binary Representation
Analog signals embody continuous values, signifying they can take on an infinite number of different voltage levels. These signals are akin to floating point or fractional numbers, smoothly transitioning between an array of in-between values, such as 1.8V, 1.81V, 1.801V, 1.8001V, 1.80001V, and so on, extending infinitely.