Page 48 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

Fig 1

• The transistor is normally off, without any output current unless forward voltage is applied in the base-emitter
circuit.
• The forward voltage controlling the base current determines the amount of output current.

In Fig 2 the control circuit of the input determines the base current. For the power circuit, the output is the
collector current. An NPN transistor is used for Q1. This type requires positive VBE forward voltage. The emitter
is common to both (a) the control circuit at the input and (b) the power output circuit.
The base emitter junction of Q1, in Fig 1 can be forward biased by the battery B1. Switch S1 must be closed to
apply the forward voltage. Reverse voltage for the collector of Q1 is supplied by B2.The reverse polarity means
that the N collector is more positive than the base. With switch S1 open, no current flows in the base-emitter (or
control) circuit.

Fig 2

The reason is that the forward voltage is not applied. Therefore, the resistance from the emitter to the collector of
the transistor is very high. No current flows in the power circuit, and the lamp does not light.
Next, assume that switch S1 is closed. This causes a small current to flow in the control circuit. R1 is a current
limiting resistor for the base circuit. Therefore, the resistance from the emitter to the collector of the transistor
drops. Consequently, a large current flows in the power circuit, causing the lamp to light.
Finally, the opening of the switch S1 in the control circuit cause the lamp in the power circuit to go out. This is
because the resistance from the emitter(E) to the collector (C) of Q1 has again increased to near infinity.
In summary, a small current in the control circuit causes a large current to flow in the power circuit. With no current
in the control circuit, the transistor acts like an open switch. With some current in the control circuit, the transistor
acts like a closed switch.
Operation of transistor switching circuit : The schematic circuit in Fig 2 shows the measured voltages and collector
current Ic in the ‘transistor off’ circuit. Note that only a tiny leakage current of 1micro amp flows from the emitter
to the collector. The resistance from E to C is calculated as

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