Variobot varikabo Instructions Manual page 41

3) Transistors
A transistor is a fundamental electronic amplifier
with three terminals: Base (B), Emitter (E), and
Collector (C).
The amplification factor
between the collector current I
the base current I at the input.
B
To control varikabo's motors using sensor signals,
so-called Darlington transistors are used. These
have a particularly high current amplification of
β
= 30000.
A motor and two LEDs together require a current of
about 30 mA (milliamperes). For this, a base
current of only 1 µA (microampere) is sufficient.
30 mA / 30000 = 0.001 mA = 1 µA
The configuration
of pins may vary
with different
manufacturers.
β
is defined as the ratio
at the output and
C
With a voltage between the base and emitter of
about U = 0.7 V (V = Volt), a transistor reduces
BE
the resistance between collector and emitter and
switches through.
Darlington transistors have two transistors
connected in series. Therefore, they require
approximately U = 1.4 V to switch through.
BE
In order for varikabo's motors to respond in
opposite directions to the left and right
sensor signals, complementary transistors are
used. NPN transistors (T
voltage U and PNP transistors (T
BE
negative voltage U
BE
NPN (T )
2
I
C
C
I
B
B
E
) require a positive
2
) require a
1
to switch through.
PNP (T )
1
B
I
B
I
C
E
C