Use a photoelectric-FET optocoupler as a linear voltage-controlled potentiometer

摘要

You can use a photoelectric FET as a variable resistor or a potentiometer in combination with a fixed resistor. The H11F3M photoelectric FET has an isolation voltage of 7.5 kV, enabling you to safely control high-voltage circuit parameters. The nonlinear-transfer characteristics of these devices are problematic, however (Figure 1). To correct the nonlinearity, using a simple feedback mechanism as a potentiometer yields a linear response (Figure 2). This circuit uses two photoelectric FETs-one for feedback and the other for applications requiring an isolated potentiometer. You connect the inputs of the two photoelectric FETs in series to ensure the same amount of current for the input LEDs.