This article is based on the Photodiode working, operations and the Practical applications of Photodiode.
In our previous articles we have learned about diodes, working, and operations of diodes and also we have covered the applications of diode as a half wave rectifier, clipper, and clamper circuits, But this is not only the applications of Diode, numbers of diodes are manufactured for various applications in this series we have covered Zener Diode, Light Emitting Diode and now we are going to learn about Photodiodes.
What is Photodiode?
A Photodiode is a reverse-biased PN junction in which reverse current increases when the junction is exposed to light.
- It is sometimes known as Light-Detector, Photo-Sensor or Photo-Detector.
- The more the light falls on the PN junction the more reverse current it produces. It is directly proportional to the intensity of light. This means that greater the intensity of light on the PN junction of a Photodiode, the greater will be the reverse current in the Photodiode.
Working of Photodiode
When light (photons) falls on the PN junction, the energy from photons is imparted to the atoms in the junction. This will create more free electrons and holes in the junction. These additional free electrons will increase the reverse current. As the intensity of light on the PN junction increases, the reverse current also increases.
“In other words, as the incident light intensity increases, the resistance of the device (photo-diode) decreases”.
Symbol of Photodiode:
The symbol of a photodiode is very similar to the normal PN junction diode except that it contains arrow that is striking the diode to represent light or photons.
- A photodiode has two terminals:
Operation of Photodiode Based Circuit:
The circuit has reverse biased photo-diode connected, resistor R and D.C supply.
The operation of the photodiode is as under:
- When there is no light in the PN junction of Photodiode, the reverse current (IR) is very small. This is known as dark current. and the resistance due to no light on the PN junction of Photo-diode is known as Dark Resistance
- When light is exposed on the PN junction of the Photodiode, there is the transfer of energy from the photons to the junction of Photo-diode. This will create more free electrons (and more holes) as explained in working. These additional free electrons will increase the reverse current up to it becomes maximum and that is known as saturation current.
Advantages of Photo-Diode
They are used in many applications because they have numerous advantages:
- They can be used as variable resistance device.
- They are highly sensitive to the light.
- The speed of operation is very high. The switching of current and hence the resistance value from high to low or otherwise is very fast.
- They have the Linear response
- They have Small Size
- Less costly
- They generate very less noise.
Practical Applications of Photodiode:
There are various Practical applications of Photodiode, but here we will discuss two major applications of Photo-diode.
Alarm circuit using photo-diode.
The counter circuit using photo-diode.
(i) Alarm circuit using Photo-diode.
Given figure shows the use of photo-diode in an alarm system. The light source is used apply light to fall on a photo-diode fitted in the doorway. The reverse current is maximum till the light beam is not broken. If a person passes through the door, light beam gets broken and the reverse current becomes low as the dark current level, as the result alarm is sounded.
(ii) The counter circuit using Photo-diode.
A photodiode may be used to count items on a conveyor belt. It is the on of great application of photodiode, it is used in industries for counting. Given figure shows a photo-diode circuit used in a system that counts objects as they pass by on a conveyor.
- It is used in solar cell panels.
- They are used in Compact disc players as a Sensor.
- They are used in Smoke detectors to produce sound when smoke is present in an area.
- Photodiodes are used in medical applications for computed tomography, instruments and to analyze samples and pulse oximeters.
- Photodiodes are used for optical communications.
- They are used in infrared remote control devices used to control equipment from televisions to air conditioners
- They are used in Line Following Robots.
- They are used to count the People.
- Photodiodes are used to measure extremely low light intensities.
- It is used for exact measurement of the intensity of light in science & industry.
- It is used in demodulation.
- The photodiode is used in the logic circuit.