Avionics News May 2019 - 54

THEORY & PRACTICE
Continued from page 53

cutoff, no current will flow, and the lights will be off.
As the resistor is adjusted to increase the resistance
to ground, the transistor will turn on and allow progressively more current to flow until the saturation point is
reached. At that point, nearly the full bus voltage will be applied to the lights, and they will be at full brightness. When
adjusted between saturation and cutoff, the transistor will
be doing most of the work and will be able to dissipate heat
safely into the heat sink. The adjustment knob and the panel
will remain cool.
When working with transistors, technicians should remember that the best testing device is the circuit in which the transistor is installed. If there is sufficient voltage at the base for
the transistor to be in saturation, then there should not be any
significant voltage drop from emitter to collector. If you see a
voltage drop, then the transistor is open and must be replaced.
If the voltage across the base emitter junction is less than 0.6

FIGURE 2: A simple light dimming circuit using a potentiometer
and a bipolar NPN transistor (Illustration by Thomas Inman)

volts, then the transistor should be in cutoff mode. All available voltage should be dropped from emitter to collector. If
this is not the case, then the transistor is shorted and must be
replaced. The base to emitter voltage should never be more
than 0.6 volts. If the voltage across the emitter to the base
reads significantly more than 0.6 volts, then that junction is
open and the transistor must be replaced.
If the transistor is out of the circuit, then it can be tested
with an ohm meter. The technician must be careful about
scale selection. An analog meter set to RX1 will overload
the transistor, causing it to fail. Electronic meters may use

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Avionics News May 2019

Table of Contents for the Digital Edition of Avionics News May 2019