The transistor pair in the Darlington transistor can be formed with two separately connected BJTs. Darlington Transistor. This transistor is also called as a Darlington pair, contains of two BJTs that are connected to deliver a high current gain from a low base current. A Darlington transistor acts as a single transistor with high current gain, it means that a small amount of current is used from a microcontroller or a sensor to run a larger load.
For instance, the following circuit is explained below. The below Darlington circuit is built with two transistors shown in the circuit diagram. Current gain is the most important characteristic of a transistor and it is indicated with hFE. When the Darlington transistor is switched ON, then the current supplies through the load to the circuit.
The current gain of every transistor varies. For a normal transistor the current gain would be normally around So, a particular transistor cannot supply ample current to the load.
It is just enough to make LED B glow dimly. The transistor amplifies this small current to allow a larger current to flow through from its collector C to its emitter E. This collector current is large enough to make LED C light brightly. When the switch is open no base current flows, so the transistor switches off the collector current. Both LEDs are off. It is a good way to test a transistor and confirm it is working. A transistor amplifies current and can be used as a switch , as explained on this page.
With suitable resistors and capacitors for AC a transistor can amplify voltage such as an audio signal but this is not yet covered by this website.
This arrangement where the emitter E is in the controlling circuit base current and in the controlled circuit collector current is called common emitter mode. It is the most widely used arrangement for transistors so it is the one to learn first. The operation of a transistor is difficult to explain and understand in terms of its internal structure. It is more helpful to use this functional model. It must never be partly on with significant resistance between C and E because in this state the transistor is liable to overheat and be destroyed.
In the fully ON state the voltage V CE across the transistor is almost zero and the transistor is said to be saturated because it cannot pass any more collector current Ic. When choosing a transistor to use as a switch you need to consider its maximum collector current Ic max and its minimum current gain h FE min.
Transistor voltage ratings may be ignored for supply voltages less than 15V. Most suppliers provide data for transistors they sell, for example Rapid Electronics. Power developed in a transistor appears as heat and the transistor will be destroyed if it becomes too hot.
This should not be a problem for a transistor being used as a switch if it has been chosen and set up correctly because the power developed inside it will be very small. Relays are suitable for all these situations but note that a low power transistor may still be needed to switch the current for the relay's coil.
For more information, including the advantages and disadvantages, please see the relays page. If the transistor is switching a load with a coil such, as a motor or relay , a diode must be connected across the load to protect the transistor from the brief high voltage produced when the load is switched off.
The diagram shows how a protection diode is connected 'backwards' across the load, in this case a relay coil. Current flowing through a coil creates a magnetic field which collapses suddenly when the current is switched off. The sudden collapse of the magnetic field induces a brief high voltage across the coil which is very likely to damage transistors and ICs.
The protection diode allows the induced voltage to drive a brief current through the coil and diode so the magnetic field dies away quickly rather than instantly. This prevents the induced voltage becoming high enough to cause damage to transistors and ICs. Rapid Electronics: relays. Most ICs cannot supply large output currents so it may be necessary to use a transistor to switch the larger current required for devices such as lamps, motors and relays.
The timer IC is unusual in being able to supply a relatively large current of up to mA, sufficient for many relays and other loads without needing a transistor. A base resistor limits the current flowing into the base of the transistor to prevent it being damaged but it must also allow sufficient base current to flow to ensure that the transistor is fully saturated when switched on.
The next section explains how to choose a transistor and base resistor to ensure full saturation. A transistor can be used to enable an IC connected to a low voltage supply such as 5V to switch the current for a load with a separate DC supply such as 12V. The two power supplies must be linked. Transistor data is available from most suppliers, for example see Rapid Electronics. Darlington transistor uses two standard BJT Bi-polar junction transistor transistors which are connected together.
In the below images, two type of Darlington transistor are shown. In this configuration, the pair or the Darlington transistor produces much higher gain and large amplification capabilities. We need to provide current to the base which controls the collector current. When we provide enough current to the base, the BJT enters in saturation mode and the current flows from collector to emitter. This collector current is directly proportional with the base current.
In typical BJT transistor the current gain is limited depending on the transistor specification. The Darlington pair is perfect for the application where high current gain is needed. This type of cross configuration is called as Sziklai Darlington pair configuration or Push-Pull configuration. In the above image the Sziklai Darlington pairs are shown.
This configuration produces less heat and has advantages about response time. We will discuss about it later. It is used for class AB amplifier or where the Push-Pull topologies are needed. Here are few projects where we used the Darlington Transistors :.
In this situation, the current gain is unity or greater than one. IB2 is controlled by the emitter current of T1 , which is IE1. IE1 is directly connected across T2. So, IB2 and IE1 are same. So, the total collector current IC is a combinational gain of individual transistors gain. The first Transistor gain will be 30 and the second transistor gain will be We will calculate the base current for switching the load.
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