Variobot varikabi Manual page 95

The Transistors
A transistor is a simple electronic amplifier with three connections:
base (B), emitter (E) and collector (C).
At a sufficiently high voltage of about U
base and emitter, the transistor reduces the resistance between
collector and emitter and - it is said - it switches through. The
collector current IC for ordinary transistors can be about 100 to 800
times larger than the base current I
varikabi uses Darlington transistors with a very high current amplification of
30 000. To supply the motor and LEDs with a current of 0.03 A (amps) = 30
mA (milliamperes), a base current of only 1 μA (microampere) is required:
30 mA / 30.000 = 0.001 mA = 1 µA
A Darlington transistor consists of two transistors connected in series and
requires about U =1.4 V instead of 0.7 V to switch through. To ensure that
BE
the motors react to the sensor signals in opposite directions, varikabi uses a
complementary pair of transistors:
one PNP transistor for T
= 0.7 V (V = Volt) between
BE
.
B
(BC516) and one NPN transistor for T
1
(BC517)
2
I
C
I
B
T
2
T
1
I
B
I
C