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VTL5C1 Datasheet(PDF) 33 Page - PerkinElmer Optoelectronics |
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VTL5C1 Datasheet(HTML) 33 Page - PerkinElmer Optoelectronics |
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33 / 76 page  28 What Are Analog Optical Isolators? PerkinElmer Optoelectronics has been a leading manufacturer of analog optical isolators for over twenty years and makes a broad range of standard parts under its trademark VACTROL®. There are many kinds of optical isolators, but the most common is the LED/phototransistor type. Other familiar types use output elements such as light sensitive SCRs, Triacs, FETs, and ICs. The major application for these silicon based devices is to provide electrical isolation of digital lines connected between different pieces of equipment. The principle of operation is very simple. When an input current is applied to the LED, the output phototransistor turns on. The only connection between the LED and phototransistor is through light—not electricity, thus the term optical isolator. These optical isolators are primarily digital in nature with fast response times suitable for interfacing with logic gates. Rise and fall times of a few microseconds, faster for some isolators, are typical. The analog optical isolator (AOI) also uses an optical link between input and output. The input element is an LED and the output element is always photoconductive cell or simply photocell. Together, the coupled pair act as an electrically variable potentiometer. since the output element of the AOI is a resistor, the voltage applied to this output resistor may be DC and/or AC and the magnitude may be as low as zero or as high as the maximum voltage rating. Because the input will control the magnitude of a complex waveform in a proportional manner, this type of isolator is an analog control element. AOIs may be used in the ON-OFF mode but the fastest response time is only in the millisecond range. A level sensitive Schmitt trigger is required between the AOI and logic gates when used in digital circuits. The figure below shows the circuit diagram of a standard AOI. AOI Circuit Diagram Input Element Light emitting diodes used in AOIs are usually visible LEDs best matching the sensitivity spectrum of the photocell output element. LEDs are the ideal input element in most applications. They require low drive current and voltage, respond very fast and have virtually unlimited life. They are very rugged and are unaffected by shock and vibration. Since the LED is a diode, it conducts in one direction only. They must be protected from excessive forward current due to the low dynamic resistance in the forward direction. The forward characteristic of an LED typically used in VACTROLs is shown below. LED Forward Characteristics Output Element The output element in all PerkinElmer’s AOIs is a light dependent resistor (LDR), also called a photoconductor or photocell. Photocells are true resistors. These passive resistors are made from a light sensitive polycrystalline semiconductor thin film which has a very high electron/photon gain. There are no P/N junctions in a photocell, making it a bilateral device. The resistance of the photocell depends on the amount of light falling on the cell. For a given illumination, the amount of electrical current through the cell depends on the voltage applied. This voltage may be either AC or DC. Thus, the photocell is the ideal low distortion output element for an analog optoisolator. A complete discussion of photoconductive cells can be found in the first section of this book. |
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