AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |
Back to Blog
![]() This then makes darlington transistors ideal for interfacing with relays, lamps and motors to low power microcontroller, computer or logic controllers as shown. Then we can see that a darlington pair with a gain of 1,000:1, could switch an output current of 1 ampere in the collector-emitter circuit with an input base current of just 1mA. The advantage of using an arrangement such as this, is that the switching transistor is much more sensitive as only a tiny base current is required to switch a much larger load current as the typical gain of a Darlington configuration can be over 1,000 whereas normally a single transistor stage produces a gain of about 50 to 200. For example: the NPN TIP120 and its PNP equivalent the TIP125. Darlington transistor pairs with current gains of more than a thousand with maximum collector currents of several amperes are easily available. Then we can see that for two identical transistors, β 2 is used instead of β acting like one big transistor with a huge amount of gain. Then transistor TR1 is connected as an emitter follower and TR2 as a common emitter amplifier as shown below. In some cases where the current gain of a single transistor is too low to directly drive a load, one way to increase the gain is to use a Darlington pair.Ī Darlington Transistor configuration, also known as a “Darlington pair” or “super-alpha circuit”, consist of two NPN or PNP transistors connected together so that the emitter current of the first transistor TR 1 becomes the base current of the second transistor TR 2. A typical value of β for a standard bipolar transistor may be in the range of 50 to 200 and varies even between transistors of the same part number. The ratio of collector current to base current ( β ) is known as the current gain of the transistor. Then we can see that a smaller current flowing into the base terminal can cause a much larger current to flow between the collector and the emitter. ![]() A higher voltage causes an increased base current, Ib to flows into the device resulting in collector current Ic becoming large while the voltage drop across the colletor and emitter terminals, Vce becomes smaller. However, the transistors base terminal needs to be switched between zero and some positive value much greater than 0.7 volts for the transistor to fully conduct. If we operate the transistor between these two modes of cut-off and conduction, the transistor can be made to operate as an electronic switch. The transistor is now said to be switched “ON” (conducting). If we now forward biased the base terminal with respect to the emitter by using a voltage source greater than 0.7 volts, transistor action occurs causing in a much larger current to flow through the transistor between its collector and emitter terminals. As the base terminal is grounded, no current flows from the collector to the emitter terminals therefore the non-conducting NPN transistor is switched “OFF” (cut-off). when the transistor switches on, the collector voltage decreases).When the base terminal of the NPN transistor is grounded (0 volts), zero current flows into the base therfore Ib = 0. The transistor acts as a NOT gate (inverter) since a "high" at the base gives a "low" at the collector.If the max Ic value is exceeded, the transistor will probably "burn out".When choosing a transistor it is important to make sure that the gain is high enough for the required use and that the maximum current which it can handle is sufficient to drive the output device.The purpose of the IN4001 Diode shown is to restrict any Back EMF resulting from the Transistor being used to switch an inductive device (e.g. A base voltage of 1.2V is required to trigger the "pair". BFY51) with a larger signal than the original input and thus improves its switching action. The "first" transistor acts as a signal amplifier which triggers the "second" transistor (e.g. The switching action of transistors is improved by combining them in a Darlington Pair configuration. bCurrent handling capacity 300 mAmps max.Power handling capacity 100 mWatts max.Uses: As current amplifiers and electronic switches, for example LED Drive Amplifiers, Relay Drive Amplifiers, Audio Amplifiers.ĭata: Voltage between Collector and Emitter 3 - 20Volts max. The Emitter is connected to the negative (-ve) supply (0V). The input is connected to the Base via a limiting resistor (typically 1K). has coil windings such as a relay, solenoid or motor, then it is usual to connect a diode across it to prevent the Back EMF from damaging the transistor. The BC108 Collector is connected to the positive (+ve) supply via the load. 3 Dual Transistor Switch (Darlington Pair)Ī BC108 Transistor is an NPN (Negative-Positive-Negative) Bipolar Transistor: the most commonly used transistor configuration.
0 Comments
Read More
Leave a Reply. |