Page 49 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

The transistor has a resistance of 9 Megohm, which is like the open or off condition of a switch.
The schematic in Fig 3, shows the measured voltages and currents in the ‘transistor on’ circuit. First, the voltage
from the emitter to the base has been increased by adjusting B1. The forward-biased voltage of 0,86V at the
emitter-base junction of the transistor causes 1.8 mA to flow in the control circuit. This current in turn causes the
resistance of the transistor from E to C to drop. The effect is that a large current of 85mA flows from the collector
of the transistor. The resistance from E to C in Fig 4 is calculated as

Fig 3

The resistance of the transistor from E to C has dropped from its previous high value of 9 megohm to a low value
of 4.7 ohm. As a result, the transistor is acting like a closed switch.
The transistor in Fig 2 is said to be at cut off position. It has reached its maximum resistance from E to C and
has cut off the current. The very tiny current still flowing is due to minority current carriers in the transistor, which
is the leakage current.
The transistor in Fig 3 is said to be at saturation. It has reached its minimum resistance from E to C, which
produces the maximum collector current. When used as a switch, the transistor is driven to cut off or to saturation
by the base current caused by the emitter-base voltage.
Transistor switching times : Now let us pay attention to the behaviour of the transistor as it makes a transition from
one state to the other. Consider the transistor circuit in
Fig 4a, driven by the pulse wave-form in Fig 4b. This wave-form makes transitions between the voltage levels
V2 and V1. At V2 the transistor is at cut off, and at V1 is applied between the base and the emitter through a
resistor R1 which may be included explicitly in the circuit or may represent the output impedance of the source
in the wave-form Fig 4b.

Fig 4

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CITS : Power - Electrician & Wireman - Lesson 60-69