_______________________________________________________________________ Special Devices
SILICON CONTROLLED RECTIFIER - a four-element, solid state device that
combines characteristics of diodes and transistors. A signal must be applied to
the gate to cause the SCR to conduct. When the proper gate signal is applied,
the SCR conducts or "fires" until the bias potential across the device drops
below the minimum required sustaining current flow. Removal of the gate
signal does not shut off the SCR. In fact, the gate signal is often a very narrow
voltage pulse or trigger. The SCR is ideal for use in situations where a small,
low-power gate can be used to turn on larger current, such as those found in
rectifier and switching circuits. SCRs are used extensively in power supply
circuits as rectifiers.
OPTOELECTRONIC DEVICES the two basic types are light producers or light
users. The LED is the most widely used light-producing device. When the
LED is forward biased it emits energy in the form of light. LEDs are used in
several configurations as digital equipment readout displays. The
PHOTODIODE, the PHOTOTRANSISTOR, and the PHOTOCELL are all
devices that use light to modify the conduction through them. The SOLAR
CELL uses light to produce voltage.
UNIJUNCTION TRANSISTOR - a three-terminal, solid state device with only
one PN junction. The area between base 1 and base 2 of the UJT acts as a
variable resistor. The emitter of the UJT acts as the wiper arm. The sequential
rise in voltage between the bases is called a voltage gradient. The UJT
conducts when the emitter is more positive than the voltage gradient at the
emitter/base contact point. There are many variations of the UJT which are
used in switching circuits, oscillators, and wave-shaping circuits.
23 June 2005
TC 9-62
3-39
Previous Page
