3-33
Figure 3-36.—Dc blocking with a coupling capacitor.
Figure 3-37.—Optical coupler.
The optical coupler is suitable for frequencies in the low megahertz range. The photodiode type
shown above can handle only small currents; however, other types of couplers, combining
phototransistors with the SCR, can be used where more output is required. Optical couplers are replacing
transformers in low-voltage and low-current applications. Sensitive digital circuits can use the coupler to
control large current and voltages with low-voltage logic levels.
Q19. What type of bias is required to cause an LED to produce light?
Q20. When compared to incandescent lamps, what is the power requirement of an LED?
Q21. In a common anode, seven-segment LED display, an individual LED will light if a negative
voltage is applied to what element?
Q22. What is the resistance level of a photodiode in total darkness?
Q23. What type of bias is required for proper operation of a photodiode?
Q24. What is a typical light-to-dark resistance ratio for a photocell?
Q25. What semiconductor device produces electrical energy when exposed to light?
