This is especially true when using the optocoupler, considering the relatively large range of current transfer ratio CTR) found in most devices. Since the input, or feedback, signal in industrial control systems may be ac or dc and may range from low, volt, levels to 110 or 0 volts ac, the design of such a threshold switching system can become more than a trivial problem. is often desirable, if not mandatory, to establish some current or voltage switching point or threshold at which the input signal is considered true. Of these methods, the optocoupler provides an ideal combination of speed, dc response, high common mode rejection, and low input to output coupling capacitance. Traditional methods of isolation involve the use of such devices as capacitors, relays, tranformers, and optocouplers. In these situations, the designer must resort to solutions which will provide isolation between the logic system and the input or output function. These problems have always been particularly evident in the field of industrial control where the electrically noisy environment produced by motors, power lines, lightning and other sources of interference may mask the desired signal, and in some cases even result in the destruction of the logic control system itself. 1 Threshold Sensing for Industrial Control Systems with the Interface Optocoupler Application Note 100 Introduction The use of electronic logic circuitry in most applications outside of a controlled environment very quickly brings the design engineer into contact with the problems and hazards involved in interfacing between the logic function and the controlled function.
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