Here you will Learn About:
- What is Photo-transistor
- Symbol of Photo Transistor
- How it Works
- Applications of Photo-transistors in Light and Dark Activated Relays and Tachometer
What is Phototransistor?
Phototransistor is a type of transistor which is light sensitive. It means that It has different behaviors in presence and absence of light. It totally depends on light.
In a common type of phototransistor, the Base lead of a Bipolar Junction Transistor is replaced with a light dependent / sensitive surface. When this light sensitive surface and the base is placed in darkness, what happens is the collector emitter pair of the Bipolar Junction Transistor does not conduct and when the sensitive surface and base is kept to light, a small quantity of current does flow to emitter from base. This small current controls the large collector current. In below different figures you can see Phototransistors.
Symbol of Phototransistor:
In below diagram you can see the symbol used to represent the Phototransistor.
How Phototransistor Works
The above was all about the introduction of Phototransistor. Now here you will learn about the basic working method of this Phototransistor. The reference figure is shown with the explanation.
In below figure the configuration is so simple; here the bipolar phototransistor resembles a bipolar transistor that has extra large P-Type semiconductor region that is used for light exposure. That region is kept open as light source.
In presence of light energy the photons collide with electrons within the P-Type semiconductor, the get enough energy up to level that they jump across the PN-Junction energy barrier provided the photons are of the right frequency.
Due to jump of electrons from P-Region to N-Region, holes are created in the P-Type Semiconductor.
The extra electrons injected into the lower N-Type slab are drawn toward the positive (+ve) terminal of the battery, while electrons from the negative (-ve) terminal of the battery are draw into the upper N-Type semiconductor and across the NP junction, where they combine with the holes, the net result is an electrons current that flows from the emitter to the collector.
The most common usage of phototransistor is in
- Light Activated Relays
- Dark Activated Relays
Phototransistor Applications – Light Activated Relay
The most common application of Phototransistor is in Relays, where it is used to control the base current supplied to a power switching transistor that is used to supply current to relay (See below figure). As we know that the phototransistor is sensitive to light, so, in the circuit, light comes to it, it turns ON, and allows current to flow from supply to the base of Power Switching Transistor. This allows the power-switching transistor to turns on, and current flows through the relay, triggering it to switch states. The 100K pot is used to adjust the sensitivity of device by controlling current flow through the phototransistor.
Phototransistor Applications—Dark Activated Relay
The most common application of Phototransistor is in Relays, where it is used to control the base current supplied to a power switching transistor that is used to supply current to relay (See below figure). As we know that the phototransistor is sensitive to light, and when light is removed from the Phototransistor, it turns OFF, which allows more current to enter in the base of Power Switching Transistor. This allows the power-switching transistor to turns on, and current flows through the relay, triggering it to switch states. The 100K pot is used to adjust the sensitivity of device by controlling current flow through the phototransistor.
Phototransistor is also used in Tachometer. (See below figure) A rotating disk is connected to a rotating shaft. The rotating disk has one hole in it. For the given setup, the disk will allow light to pass through the hole once every Revolution. The light passing through the disk triggers the phototransistor into conduction. A frequency counter is used to count the number of electrical pulses generated.