Page 1017 - Basic Electrical Engineering

junction, J is forward biased. The collector–base junction is constructed like
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a photodiode. In the absence of light falling on the junction, only a small

amount of reverse saturation current, also called leakage current, will flow
through the device. This current of very low value is due to the minority

charge carriers flowing at room temperature. When light energy falls on the
C–B junction, additional charge carriers are created to add to the reverse
saturation current. Thus, light energy increases the base current. The reverse

saturation current can be increased by increasing the illumination level of the
collector–base junction. This will increase the collector current, I . Collector
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current will increase when the illumination level of the C–B junction is
raised. The V–I characteristic is drawn between V and I at different
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C
illumination levels, L , L , L , L , etc., as shown in Fig. 14.54 (b). For an
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equal level of illumination, a phototransistor provides more output current
than a photodiode. A biased phototransistor starts working as soon as light
falls on the collector–base junction. An amplified current starts flowing
through the device, and consequently the output current increases. The main

difference between a phototransistor and a photodiode is in the current gain.
A phototransistor is packaged in a metal case with a lens filled on the top.

Plastic-encapsulated phototransistors are also available. Phototransistors can
be used in electronic control circuits like illumination control, emergency

lighting system, relays etc.
A simple phototransistor circuit has been shown in Fig. 14.55. An SCR is
being triggered with the help of a phototransistor. When the illumination

level is high, the phototransistor will allow sufficient current to flow through
it and resistance R . Resistance R and R are chosen so that when the
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phototransistor is on, very little current will flow through them and the
voltage V across the gate will not trigger the SCR. When the illumination
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level of the surrounding falls, the phototransistor will stop conducting and

current will flow through resistors R , R , and R . The value of these resistors
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are so chosen that, voltage across the gate, V , will trigger the SCR, and the
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SCR will start conducting. When the SCR is conducting, current will flow
through the load, which could be a lighting load.