What's the Missing Component?by Forrest M. Mims III
The circuit in Figure 1 drives a near-infrared laser diode with a stream of fast rising, 20 amp pulses and a duration of about 50 nanoseconds.
R1 controls the pulse rate. C1 controls the pulse duration.
Figure 1. High-current pulse generator for driving a laser diode with very brief pulses.
What is the component concealed behind the puzzle piece? Before revealing the answer, here are some hints:
R1 and C1 form a classic RC circuit in which C1 is charged through R1. C1 is quickly charged to the supply voltage, and this applies more than 100 volts to the mystery component.
By now it should be evident that the mystery component is some kind of switch that allows the voltage on C1 to discharge through the laser diode. But what component can switch more than 100 volts? And how is it triggered?
A power FET is an almost ideal 3-terminal switch, for it has a very low resistance. But a power FET will not work in this circuit.
Certain kinds of miniature switching vacuum tubes might work in this circuit. But the mystery component is a solid state device.
Click here for the answer.