In the left side figure the connection effectively causes the phototransistor to be in the reverse bias situation, such that it now works like a photodiode. The light reacts with the exposed PN junction of the device, initiating the photoconductivity action.Ī phototransistor is mostly configured with its base pin unconnected as shown in the following two circuits. It works by allowing light to reach the PN junction of the device through the transparent opening. The phototransistor is generally in the form of a bipolar NPN silicon transistor encapsulated in a cover with a transparent opening. The following image shows the schematic symbol of a phototransistor There's another variant of photodiode which uses lead-sulfide (PbS) and there working characteristic is quite similar to LDRs but are designed to respond only to the infrared range lights. You will find photodiodes in systems such as infrared remote control circuits, beam interruption relays and intruder alarm circuits. On the other hand, photodiodes are specifically selected in applications that require fast detection of lights mostly in the infrared region. The schematic symbol of a photodiode can be see below.Ĭompared to cadmium-sulphide or cadmium-selenide photocells like LDRs, photodiodes are generally less sensitive to light, but their response to light changes is much faster.ĭue to this reason, photocells like LDRs are generally used in applications that involve visible light, and where the response time does not need to be quick. The working of all P-N junction based devices is similar and will exhibit photo-conductivity when exposed to light. This will produce a opposite response, resulting in a decreasing voltage across R1, when the photodiode is illuminated with external light. The photodiode in the above figure can be also connected on the ground side as shown below. This may result in a current as much as 1 mA through the diode, causing a rising voltage to develop across R1. This will expose the PN junction of the diode to light, and there will be an instant flow of current through it, in response to the incident light. However, suppose we have the same diode with its outer opaque cover scraped of or removed and connected with a reverse bias supply.
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