Variobot varikabi Manual page 96

The adjacent figure shows a simple circuit with a battery, a
motor and an NPN transistor. Underneath, the corresponding
circuit is shown with a PNP transistor. The current flows in the
direction of the black arrow from plus to minus in both cases.
There are three different basic circuits with a transistor.
varikabi uses the so-called collector circuit.
It is called collector circuit because the collector (C) is
connected to a constant voltage (battery).
The red arrows symbolize the voltages. You can notice that
the voltage at the motor U
voltage U than the base-voltage U
BE
U = U – U = U – 1.4 V
E B BE B
Note:
Because the voltage at the emitter (E) follows the
voltage at the base (B) except for the difference
of U , this circuit is also called emitter follower.
BE
With the control voltage U
voltage U and thus the speed of the motor can be controlled.
E
is lower by the base-emitter
E
.
B
and a very low current I
B
U
BE
U
B
U
E
U
E
U
B
U
BE
, the
B